* [syzbot] [fs?] KASAN: slab-use-after-free Read in clone_mnt
@ 2026-02-14 10:01 syzbot
2026-02-14 15:59 ` Christian Brauner
0 siblings, 1 reply; 3+ messages in thread
From: syzbot @ 2026-02-14 10:01 UTC (permalink / raw)
To: brauner, jack, linux-fsdevel, linux-kernel, syzkaller-bugs, viro
Hello,
syzbot found the following issue on:
HEAD commit: c22e26bd0906 Merge tag 'landlock-7.0-rc1' of git://git.ker..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12c6a6e6580000
kernel config: https://syzkaller.appspot.com/x/.config?x=6428d17febdfb14e
dashboard link: https://syzkaller.appspot.com/bug?extid=a89f9434fb5a001ccd58
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
Unfortunately, I don't have any reproducer for this issue yet.
Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/b33c549157ca/disk-c22e26bd.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/34c7ded19553/vmlinux-c22e26bd.xz
kernel image: https://storage.googleapis.com/syzbot-assets/66faec2158ed/bzImage-c22e26bd.xz
IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+a89f9434fb5a001ccd58@syzkaller.appspotmail.com
==================================================================
BUG: KASAN: slab-use-after-free in __list_add_valid_or_report+0x4e/0x130 lib/list_debug.c:29
Read of size 8 at addr ffff8880341bcb40 by task syz.6.5169/22925
CPU: 1 UID: 0 PID: 22925 Comm: syz.6.5169 Tainted: G L syzkaller #0 PREEMPT_{RT,(full)}
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0xba/0x230 mm/kasan/report.c:482
kasan_report+0x117/0x150 mm/kasan/report.c:595
__list_add_valid_or_report+0x4e/0x130 lib/list_debug.c:29
__list_add_valid include/linux/list.h:96 [inline]
__list_add include/linux/list.h:158 [inline]
list_add include/linux/list.h:177 [inline]
clone_mnt+0x447/0x9a0 fs/namespace.c:1275
copy_tree+0xde/0x930 fs/namespace.c:2159
copy_mnt_ns+0x24d/0x990 fs/namespace.c:4246
create_new_namespaces+0xcf/0x6a0 kernel/nsproxy.c:98
unshare_nsproxy_namespaces+0x11a/0x160 kernel/nsproxy.c:226
ksys_unshare+0x4f4/0x900 kernel/fork.c:3174
__do_sys_unshare kernel/fork.c:3245 [inline]
__se_sys_unshare kernel/fork.c:3243 [inline]
__x64_sys_unshare+0x38/0x50 kernel/fork.c:3243
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7ff48d79bf79
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ff48b9f6028 EFLAGS: 00000246 ORIG_RAX: 0000000000000110
RAX: ffffffffffffffda RBX: 00007ff48da15fa0 RCX: 00007ff48d79bf79
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 000000002a020400
RBP: 00007ff48d8327e0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ff48da16038 R14: 00007ff48da15fa0 R15: 00007ffc92e1ac28
</TASK>
Allocated by task 22855:
kasan_save_stack mm/kasan/common.c:57 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
unpoison_slab_object mm/kasan/common.c:340 [inline]
__kasan_slab_alloc+0x6c/0x80 mm/kasan/common.c:366
kasan_slab_alloc include/linux/kasan.h:253 [inline]
slab_post_alloc_hook mm/slub.c:4459 [inline]
slab_alloc_node mm/slub.c:4788 [inline]
kmem_cache_alloc_noprof+0x33b/0x680 mm/slub.c:4795
alloc_vfsmnt+0x23/0x420 fs/namespace.c:287
clone_mnt+0x4b/0x9a0 fs/namespace.c:1246
create_new_namespace fs/namespace.c:3098 [inline]
open_new_namespace fs/namespace.c:3168 [inline]
vfs_open_tree+0x507/0x1040 fs/namespace.c:3217
__do_sys_open_tree fs/namespace.c:3227 [inline]
__se_sys_open_tree fs/namespace.c:3225 [inline]
__x64_sys_open_tree+0x96/0x110 fs/namespace.c:3225
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Freed by task 20:
kasan_save_stack mm/kasan/common.c:57 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:584
poison_slab_object mm/kasan/common.c:253 [inline]
__kasan_slab_free+0x5c/0x80 mm/kasan/common.c:285
kasan_slab_free include/linux/kasan.h:235 [inline]
slab_free_hook mm/slub.c:2670 [inline]
slab_free mm/slub.c:6082 [inline]
kmem_cache_free+0x185/0x690 mm/slub.c:6212
rcu_do_batch kernel/rcu/tree.c:2617 [inline]
rcu_core kernel/rcu/tree.c:2869 [inline]
rcu_cpu_kthread+0x99e/0x1470 kernel/rcu/tree.c:2957
smpboot_thread_fn+0x541/0xa50 kernel/smpboot.c:160
kthread+0x388/0x470 kernel/kthread.c:467
ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
Last potentially related work creation:
kasan_save_stack+0x3e/0x60 mm/kasan/common.c:57
kasan_record_aux_stack+0xbd/0xd0 mm/kasan/generic.c:556
__call_rcu_common kernel/rcu/tree.c:3131 [inline]
call_rcu+0xee/0x890 kernel/rcu/tree.c:3251
task_work_run+0x1d9/0x270 kernel/task_work.c:233
resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
__exit_to_user_mode_loop kernel/entry/common.c:67 [inline]
exit_to_user_mode_loop+0xed/0x480 kernel/entry/common.c:98
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:256 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:325 [inline]
do_syscall_64+0x32d/0xf80 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Second to last potentially related work creation:
kasan_save_stack+0x3e/0x60 mm/kasan/common.c:57
kasan_record_aux_stack+0xbd/0xd0 mm/kasan/generic.c:556
task_work_add+0xb6/0x440 kernel/task_work.c:70
mntput_no_expire_slowpath+0x70c/0xbd0 fs/namespace.c:1379
create_new_namespace fs/namespace.c:3162 [inline]
open_new_namespace fs/namespace.c:3168 [inline]
vfs_open_tree+0xe17/0x1040 fs/namespace.c:3217
__do_sys_open_tree fs/namespace.c:3227 [inline]
__se_sys_open_tree fs/namespace.c:3225 [inline]
__x64_sys_open_tree+0x96/0x110 fs/namespace.c:3225
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
The buggy address belongs to the object at ffff8880341bca80
which belongs to the cache mnt_cache of size 352
The buggy address is located 192 bytes inside of
freed 352-byte region [ffff8880341bca80, ffff8880341bcbe0)
The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff8880341bda40 pfn:0x341bc
head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
memcg:ffff8880341bc171
flags: 0x80000000000240(workingset|head|node=0|zone=1)
page_type: f5(slab)
raw: 0080000000000240 ffff888140412780 ffffea0001036010 ffffea000100bc10
raw: ffff8880341bda40 000001c00012000f 00000000f5000000 ffff8880341bc171
head: 0080000000000240 ffff888140412780 ffffea0001036010 ffffea000100bc10
head: ffff8880341bda40 000001c00012000f 00000000f5000000 ffff8880341bc171
head: 0080000000000001 ffffea0000d06f01 00000000ffffffff 00000000ffffffff
head: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000002
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 1, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 18378, tgid 18375 (syz.6.3692), ts 1535437491766, free_ts 1534179924298
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0x228/0x280 mm/page_alloc.c:1884
prep_new_page mm/page_alloc.c:1892 [inline]
get_page_from_freelist+0x28bb/0x2950 mm/page_alloc.c:3950
__alloc_frozen_pages_noprof+0x18d/0x380 mm/page_alloc.c:5245
alloc_slab_page mm/slub.c:3238 [inline]
allocate_slab+0x77/0x660 mm/slub.c:3411
new_slab mm/slub.c:3469 [inline]
refill_objects+0x334/0x3c0 mm/slub.c:7091
refill_sheaf mm/slub.c:2787 [inline]
__pcs_replace_empty_main+0x328/0x5f0 mm/slub.c:4536
alloc_from_pcs mm/slub.c:4639 [inline]
slab_alloc_node mm/slub.c:4773 [inline]
kmem_cache_alloc_noprof+0x433/0x680 mm/slub.c:4795
alloc_vfsmnt+0x23/0x420 fs/namespace.c:287
clone_mnt+0x4b/0x9a0 fs/namespace.c:1246
copy_tree+0x3d4/0x930 fs/namespace.c:2194
get_detached_copy fs/namespace.c:3044 [inline]
open_detached_copy+0x23e/0x5a0 fs/namespace.c:3060
vfs_open_tree+0x490/0x1040 fs/namespace.c:3220
__do_sys_open_tree fs/namespace.c:3227 [inline]
__se_sys_open_tree fs/namespace.c:3225 [inline]
__x64_sys_open_tree+0x96/0x110 fs/namespace.c:3225
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
page last free pid 5166 tgid 5166 stack trace:
reset_page_owner include/linux/page_owner.h:25 [inline]
free_pages_prepare mm/page_alloc.c:1433 [inline]
__free_frozen_pages+0xfd0/0x1160 mm/page_alloc.c:2973
__slab_free+0x24f/0x2a0 mm/slub.c:5490
qlink_free mm/kasan/quarantine.c:163 [inline]
qlist_free_all+0x97/0x100 mm/kasan/quarantine.c:179
kasan_quarantine_reduce+0x148/0x160 mm/kasan/quarantine.c:286
__kasan_slab_alloc+0x22/0x80 mm/kasan/common.c:350
kasan_slab_alloc include/linux/kasan.h:253 [inline]
slab_post_alloc_hook mm/slub.c:4459 [inline]
slab_alloc_node mm/slub.c:4788 [inline]
kmem_cache_alloc_noprof+0x33b/0x680 mm/slub.c:4795
alloc_filename fs/namei.c:142 [inline]
do_getname+0x2e/0x250 fs/namei.c:182
getname include/linux/fs.h:2512 [inline]
class_filename_constructor include/linux/fs.h:2539 [inline]
do_sys_openat2+0xca/0x200 fs/open.c:1365
do_sys_open fs/open.c:1372 [inline]
__do_sys_openat fs/open.c:1388 [inline]
__se_sys_openat fs/open.c:1383 [inline]
__x64_sys_openat+0x138/0x170 fs/open.c:1383
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Memory state around the buggy address:
ffff8880341bca00: 00 00 00 00 fc fc fc fc fc fc fc fc fc fc fc fc
ffff8880341bca80: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8880341bcb00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff8880341bcb80: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
ffff8880341bcc00: fc fc fc fc fc fc fc fc 00 00 00 00 00 00 00 00
==================================================================
---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.
syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title
If you want to overwrite report's subsystems, reply with:
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(See the list of subsystem names on the web dashboard)
If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report
If you want to undo deduplication, reply with:
#syz undup
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [syzbot] [fs?] KASAN: slab-use-after-free Read in clone_mnt
2026-02-14 10:01 [syzbot] [fs?] KASAN: slab-use-after-free Read in clone_mnt syzbot
@ 2026-02-14 15:59 ` Christian Brauner
2026-02-17 12:12 ` Christian Brauner
0 siblings, 1 reply; 3+ messages in thread
From: Christian Brauner @ 2026-02-14 15:59 UTC (permalink / raw)
To: syzbot; +Cc: jack, linux-fsdevel, linux-kernel, syzkaller-bugs, viro
[-- Attachment #1: Type: text/plain, Size: 1668 bytes --]
On Sat, Feb 14, 2026 at 02:01:26AM -0800, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: c22e26bd0906 Merge tag 'landlock-7.0-rc1' of git://git.ker..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=12c6a6e6580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=6428d17febdfb14e
> dashboard link: https://syzkaller.appspot.com/bug?extid=a89f9434fb5a001ccd58
> compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/b33c549157ca/disk-c22e26bd.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/34c7ded19553/vmlinux-c22e26bd.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/66faec2158ed/bzImage-c22e26bd.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+a89f9434fb5a001ccd58@syzkaller.appspotmail.com
Right, this should be fixed by the appended patch. If someone had the
bright idea to make the real rootfs a shared or dependent mount and it
is later copied the copy will become a peer of the old real rootfs mount
or a dependent mount of it. If locking the new rootfs mount for mounting
fails or the subsequent do_loopback() fails we rely on the copy of the
real root mount to be cleaned up by path_put(). The problem is that this
doesn't deal with mount propagation and will leave the mounts linked in
the propagation lists. Fix this by actually unmounting the copied tree.
[-- Attachment #2: 0001-mount-unmount-copied-tree.patch --]
[-- Type: text/x-diff, Size: 4131 bytes --]
From 3b3dc6a956af1afc6b95c68550b99083d47ce5fa Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Sat, 14 Feb 2026 16:22:13 +0100
Subject: [PATCH] mount: unmount copied tree
Fix an oversight when creating a new mount namespace. If someone had the
bright idea to make the real rootfs a shared or dependent mount and it
is later copied the copy will become a peer of the old real rootfs mount
or a dependent mount of it. The namespace semaphore is dropped and we
use mount lock exact to lock the new real root mount. If that fails or
the subsequent do_loopback() fails we rely on the copy of the real root
mount to be cleaned up by path_put(). The problem is that this doesn't
deal with mount propagation and will leave the mounts linked in the
propagation lists. Fix this by actually unmounting the copied tree.
Reported-by: syzbot+a89f9434fb5a001ccd58@syzkaller.appspotmail.com
Link: https:/lore.kernel.org/699047f6.050a0220.2757fb.0024.GAE@google.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
fs/namespace.c | 37 ++++++++++++++++++++++---------------
1 file changed, 22 insertions(+), 15 deletions(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index a67cbe42746d..89cac95e6c9b 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3073,16 +3073,13 @@ static struct file *open_detached_copy(struct path *path, unsigned int flags)
return file;
}
-DEFINE_FREE(put_empty_mnt_ns, struct mnt_namespace *,
- if (!IS_ERR_OR_NULL(_T)) free_mnt_ns(_T))
-
static struct mnt_namespace *create_new_namespace(struct path *path, unsigned int flags)
{
- struct mnt_namespace *new_ns __free(put_empty_mnt_ns) = NULL;
- struct path to_path __free(path_put) = {};
struct mnt_namespace *ns = current->nsproxy->mnt_ns;
struct user_namespace *user_ns = current_user_ns();
+ struct mnt_namespace *new_ns;
struct mount *new_ns_root;
+ struct path to_path;
struct mount *mnt;
unsigned int copy_flags = 0;
bool locked = false;
@@ -3096,8 +3093,10 @@ static struct mnt_namespace *create_new_namespace(struct path *path, unsigned in
scoped_guard(namespace_excl) {
new_ns_root = clone_mnt(ns->root, ns->root->mnt.mnt_root, copy_flags);
- if (IS_ERR(new_ns_root))
+ if (IS_ERR(new_ns_root)) {
+ emptied_ns = new_ns;
return ERR_CAST(new_ns_root);
+ }
/*
* If the real rootfs had a locked mount on top of it somewhere
@@ -3122,12 +3121,17 @@ static struct mnt_namespace *create_new_namespace(struct path *path, unsigned in
/* Borrow the reference from clone_mnt(). */
to_path.mnt = &new_ns_root->mnt;
- to_path.dentry = dget(new_ns_root->mnt.mnt_root);
+ to_path.dentry = new_ns_root->mnt.mnt_root;
/* Now lock for actual mounting. */
LOCK_MOUNT_EXACT(mp, &to_path);
- if (unlikely(IS_ERR(mp.parent)))
+ if (unlikely(IS_ERR(mp.parent))) {
+ guard(namespace_excl)();
+ emptied_ns = new_ns;
+ guard(mount_writer)();
+ umount_tree(new_ns_root, 0);
return ERR_CAST(mp.parent);
+ }
/*
* We don't emulate unshare()ing a mount namespace. We stick to the
@@ -3135,10 +3139,13 @@ static struct mnt_namespace *create_new_namespace(struct path *path, unsigned in
* saner and simpler semantics.
*/
mnt = __do_loopback(path, flags, copy_flags);
- if (IS_ERR(mnt))
- return ERR_CAST(mnt);
-
scoped_guard(mount_writer) {
+ if (IS_ERR(mnt)) {
+ emptied_ns = new_ns;
+ umount_tree(new_ns_root, 0);
+ return ERR_CAST(mnt);
+ }
+
if (locked)
mnt->mnt.mnt_flags |= MNT_LOCKED;
/*
@@ -3151,14 +3158,14 @@ static struct mnt_namespace *create_new_namespace(struct path *path, unsigned in
}
/* Add all mounts to the new namespace. */
- for (struct mount *p = new_ns_root; p; p = next_mnt(p, new_ns_root)) {
- mnt_add_to_ns(new_ns, p);
+ for (mnt = new_ns_root; mnt; mnt = next_mnt(mnt, new_ns_root)) {
+ mnt_add_to_ns(new_ns, mnt);
new_ns->nr_mounts++;
}
- new_ns->root = real_mount(no_free_ptr(to_path.mnt));
+ new_ns->root = real_mount(to_path.mnt);
ns_tree_add_raw(new_ns);
- return no_free_ptr(new_ns);
+ return new_ns;
}
static struct file *open_new_namespace(struct path *path, unsigned int flags)
--
2.47.3
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [syzbot] [fs?] KASAN: slab-use-after-free Read in clone_mnt
2026-02-14 15:59 ` Christian Brauner
@ 2026-02-17 12:12 ` Christian Brauner
0 siblings, 0 replies; 3+ messages in thread
From: Christian Brauner @ 2026-02-17 12:12 UTC (permalink / raw)
To: syzbot; +Cc: jack, linux-fsdevel, linux-kernel, syzkaller-bugs, viro
[-- Attachment #1: Type: text/plain, Size: 1923 bytes --]
On Sat, Feb 14, 2026 at 04:59:11PM +0100, Christian Brauner wrote:
> On Sat, Feb 14, 2026 at 02:01:26AM -0800, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit: c22e26bd0906 Merge tag 'landlock-7.0-rc1' of git://git.ker..
> > git tree: upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=12c6a6e6580000
> > kernel config: https://syzkaller.appspot.com/x/.config?x=6428d17febdfb14e
> > dashboard link: https://syzkaller.appspot.com/bug?extid=a89f9434fb5a001ccd58
> > compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/b33c549157ca/disk-c22e26bd.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/34c7ded19553/vmlinux-c22e26bd.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/66faec2158ed/bzImage-c22e26bd.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+a89f9434fb5a001ccd58@syzkaller.appspotmail.com
>
> Right, this should be fixed by the appended patch. If someone had the
> bright idea to make the real rootfs a shared or dependent mount and it
> is later copied the copy will become a peer of the old real rootfs mount
> or a dependent mount of it. If locking the new rootfs mount for mounting
> fails or the subsequent do_loopback() fails we rely on the copy of the
> real root mount to be cleaned up by path_put(). The problem is that this
> doesn't deal with mount propagation and will leave the mounts linked in
> the propagation lists. Fix this by actually unmounting the copied tree.
Actually lets just refactor this to be a bit cleaner as in the appended
patch. I plan on getting this upstream during this week.
[-- Attachment #2: 0001-mount-hold-namespace_sem-across-copy-in-create_new_n.patch --]
[-- Type: text/x-diff, Size: 8666 bytes --]
From 186af1baac0e03b0915287d31d8fcf237d9e5410 Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Sat, 14 Feb 2026 16:22:13 +0100
Subject: [PATCH] mount: hold namespace_sem across copy in
create_new_namespace()
Fix an oversight when creating a new mount namespace. If someone had the
bright idea to make the real rootfs a shared or dependent mount and it
is later copied the copy will become a peer of the old real rootfs mount
or a dependent mount of it. The namespace semaphore is dropped and we
use mount lock exact to lock the new real root mount. If that fails or
the subsequent do_loopback() fails we rely on the copy of the real root
mount to be cleaned up by path_put(). The problem is that this doesn't
deal with mount propagation and will leave the mounts linked in the
propagation lists.
When creating a new mount namespace create_new_namespace() first
acquires namespace_sem to clone the nullfs root, drops it, then
reacquires it via LOCK_MOUNT_EXACT which takes inode_lock first to
respect the inode_lock -> namespace_sem lock ordering. This
drop-and-reacquire pattern is fragile and was the source of the
propagation cleanup bug fixed in the preceding commit.
Extend lock_mount_exact() with a copy_mount mode that clones the mount
under the locks atomically. When copy_mount is true, path_overmounted()
is skipped since we're copying the mount, not mounting on top of it -
the nullfs root always has rootfs mounted on top so the check would
always fail. If clone_mnt() fails after get_mountpoint() has pinned the
mountpoint, __unlock_mount() is used to properly unpin the mountpoint
and release both locks.
This allows create_new_namespace() to use LOCK_MOUNT_EXACT_COPY which
takes inode_lock and namespace_sem once and holds them throughout the
clone and subsequent mount operations, eliminating the
drop-and-reacquire pattern entirely.
Reported-by: syzbot+a89f9434fb5a001ccd58@syzkaller.appspotmail.com
Fixes: 9b8a0ba68246 ("mount: add OPEN_TREE_NAMESPACE") # mainline only
Link: https://lore.kernel.org/699047f6.050a0220.2757fb.0024.GAE@google.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
fs/namespace.c | 112 +++++++++++++++++++++++++------------------------
1 file changed, 58 insertions(+), 54 deletions(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index 90700df65f0d..188be116f6e6 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2791,7 +2791,8 @@ static inline void unlock_mount(struct pinned_mountpoint *m)
}
static void lock_mount_exact(const struct path *path,
- struct pinned_mountpoint *mp);
+ struct pinned_mountpoint *mp, bool copy_mount,
+ unsigned int copy_flags);
#define LOCK_MOUNT_MAYBE_BENEATH(mp, path, beneath) \
struct pinned_mountpoint mp __cleanup(unlock_mount) = {}; \
@@ -2799,7 +2800,10 @@ static void lock_mount_exact(const struct path *path,
#define LOCK_MOUNT(mp, path) LOCK_MOUNT_MAYBE_BENEATH(mp, (path), false)
#define LOCK_MOUNT_EXACT(mp, path) \
struct pinned_mountpoint mp __cleanup(unlock_mount) = {}; \
- lock_mount_exact((path), &mp)
+ lock_mount_exact((path), &mp, false, 0)
+#define LOCK_MOUNT_EXACT_COPY(mp, path, copy_flags) \
+ struct pinned_mountpoint mp __cleanup(unlock_mount) = {}; \
+ lock_mount_exact((path), &mp, true, (copy_flags))
static int graft_tree(struct mount *mnt, const struct pinned_mountpoint *mp)
{
@@ -3073,16 +3077,13 @@ static struct file *open_detached_copy(struct path *path, unsigned int flags)
return file;
}
-DEFINE_FREE(put_empty_mnt_ns, struct mnt_namespace *,
- if (!IS_ERR_OR_NULL(_T)) free_mnt_ns(_T))
-
static struct mnt_namespace *create_new_namespace(struct path *path, unsigned int flags)
{
- struct mnt_namespace *new_ns __free(put_empty_mnt_ns) = NULL;
- struct path to_path __free(path_put) = {};
struct mnt_namespace *ns = current->nsproxy->mnt_ns;
struct user_namespace *user_ns = current_user_ns();
- struct mount *new_ns_root;
+ struct mnt_namespace *new_ns;
+ struct mount *new_ns_root, *old_ns_root;
+ struct path to_path;
struct mount *mnt;
unsigned int copy_flags = 0;
bool locked = false;
@@ -3094,71 +3095,64 @@ static struct mnt_namespace *create_new_namespace(struct path *path, unsigned in
if (IS_ERR(new_ns))
return ERR_CAST(new_ns);
- scoped_guard(namespace_excl) {
- new_ns_root = clone_mnt(ns->root, ns->root->mnt.mnt_root, copy_flags);
- if (IS_ERR(new_ns_root))
- return ERR_CAST(new_ns_root);
+ old_ns_root = ns->root;
+ to_path.mnt = &old_ns_root->mnt;
+ to_path.dentry = old_ns_root->mnt.mnt_root;
- /*
- * If the real rootfs had a locked mount on top of it somewhere
- * in the stack, lock the new mount tree as well so it can't be
- * exposed.
- */
- mnt = ns->root;
- while (mnt->overmount) {
- mnt = mnt->overmount;
- if (mnt->mnt.mnt_flags & MNT_LOCKED)
- locked = true;
- }
+ VFS_WARN_ON_ONCE(old_ns_root->mnt_id_unique != 1);
+ VFS_WARN_ON_ONCE(old_ns_root->mnt.mnt_sb->s_type != &nullfs_fs_type);
+
+ LOCK_MOUNT_EXACT_COPY(mp, &to_path, copy_flags);
+ if (IS_ERR(mp.parent)) {
+ free_mnt_ns(new_ns);
+ return ERR_CAST(mp.parent);
}
+ new_ns_root = mp.parent;
/*
- * We dropped the namespace semaphore so we can actually lock
- * the copy for mounting. The copied mount isn't attached to any
- * mount namespace and it is thus excluded from any propagation.
- * So realistically we're isolated and the mount can't be
- * overmounted.
+ * If the real rootfs had a locked mount on top of it somewhere
+ * in the stack, lock the new mount tree as well so it can't be
+ * exposed.
*/
-
- /* Borrow the reference from clone_mnt(). */
- to_path.mnt = &new_ns_root->mnt;
- to_path.dentry = dget(new_ns_root->mnt.mnt_root);
-
- /* Now lock for actual mounting. */
- LOCK_MOUNT_EXACT(mp, &to_path);
- if (unlikely(IS_ERR(mp.parent)))
- return ERR_CAST(mp.parent);
+ mnt = old_ns_root;
+ while (mnt->overmount) {
+ mnt = mnt->overmount;
+ if (mnt->mnt.mnt_flags & MNT_LOCKED)
+ locked = true;
+ }
/*
- * We don't emulate unshare()ing a mount namespace. We stick to the
- * restrictions of creating detached bind-mounts. It has a lot
- * saner and simpler semantics.
+ * We don't emulate unshare()ing a mount namespace. We stick
+ * to the restrictions of creating detached bind-mounts. It
+ * has a lot saner and simpler semantics.
*/
mnt = __do_loopback(path, flags, copy_flags);
- if (IS_ERR(mnt))
- return ERR_CAST(mnt);
-
scoped_guard(mount_writer) {
+ if (IS_ERR(mnt)) {
+ emptied_ns = new_ns;
+ umount_tree(new_ns_root, 0);
+ return ERR_CAST(mnt);
+ }
+
if (locked)
mnt->mnt.mnt_flags |= MNT_LOCKED;
/*
- * Now mount the detached tree on top of the copy of the
- * real rootfs we created.
+ * now mount the detached tree on top of the copy
+ * of the real rootfs we created.
*/
attach_mnt(mnt, new_ns_root, mp.mp);
if (user_ns != ns->user_ns)
lock_mnt_tree(new_ns_root);
}
- /* Add all mounts to the new namespace. */
- for (struct mount *p = new_ns_root; p; p = next_mnt(p, new_ns_root)) {
- mnt_add_to_ns(new_ns, p);
+ for (mnt = new_ns_root; mnt; mnt = next_mnt(mnt, new_ns_root)) {
+ mnt_add_to_ns(new_ns, mnt);
new_ns->nr_mounts++;
}
- new_ns->root = real_mount(no_free_ptr(to_path.mnt));
+ new_ns->root = new_ns_root;
ns_tree_add_raw(new_ns);
- return no_free_ptr(new_ns);
+ return new_ns;
}
static struct file *open_new_namespace(struct path *path, unsigned int flags)
@@ -3840,16 +3834,20 @@ static int do_new_mount(const struct path *path, const char *fstype,
}
static void lock_mount_exact(const struct path *path,
- struct pinned_mountpoint *mp)
+ struct pinned_mountpoint *mp, bool copy_mount,
+ unsigned int copy_flags)
{
struct dentry *dentry = path->dentry;
int err;
+ /* Assert that inode_lock() locked the correct inode. */
+ VFS_WARN_ON_ONCE(copy_mount && !path_mounted(path));
+
inode_lock(dentry->d_inode);
namespace_lock();
if (unlikely(cant_mount(dentry)))
err = -ENOENT;
- else if (path_overmounted(path))
+ else if (!copy_mount && path_overmounted(path))
err = -EBUSY;
else
err = get_mountpoint(dentry, mp);
@@ -3857,9 +3855,15 @@ static void lock_mount_exact(const struct path *path,
namespace_unlock();
inode_unlock(dentry->d_inode);
mp->parent = ERR_PTR(err);
- } else {
- mp->parent = real_mount(path->mnt);
+ return;
}
+
+ if (copy_mount)
+ mp->parent = clone_mnt(real_mount(path->mnt), dentry, copy_flags);
+ else
+ mp->parent = real_mount(path->mnt);
+ if (unlikely(IS_ERR(mp->parent)))
+ __unlock_mount(mp);
}
int finish_automount(struct vfsmount *__m, const struct path *path)
--
2.47.3
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2026-02-17 12:12 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
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2026-02-14 10:01 [syzbot] [fs?] KASAN: slab-use-after-free Read in clone_mnt syzbot
2026-02-14 15:59 ` Christian Brauner
2026-02-17 12:12 ` Christian Brauner
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