From: Jarkko Sakkinen <jarkko@kernel.org>
To: Cen Zhang <zzzccc427@gmail.com>
Cc: dhowells@redhat.com, paul@paul-moore.com, jmorris@namei.org,
serge@hallyn.com, sergeh@kernel.org, keyrings@vger.kernel.org,
linux-security-module@vger.kernel.org,
linux-kernel@vger.kernel.org, baijiaju1990@gmail.com,
jjzuming@gmail.com
Subject: Re: [PATCH] keys: Avoid the owner account dereference in named keyring lookup
Date: Thu, 8 Oct 2026 20:57:17 +0300 [thread overview]
Message-ID: <asfZfZcusHMRiUYY@kernel.org> (raw)
In-Reply-To: <pm-key-management-objects-candidate-0002-v2-dd6c20a1d92928b33ab7@gmail.com>
On Thu, Oct 08, 2026 at 02:30:46PM +0800, Cen Zhang wrote:
> Named keyring lookup must keep the storage containing the owner UID
> alive through the namespace mapping check. find_keyring_by_name()
> reads keyring->user->uid under keyring_name_lock, but that lock protects
> the keyring's name entry and allocation, not its separate key_user.
>
> A named session-keyring join can overlap a privileged KEYCTL_CHOWN on
> another CPU. If the keyring holds the last reference to its old dynamic
> key_user and the ownership transfer succeeds, the following ordering is
> possible:
>
> Named join Chown
> find_keyring_by_name() keyctl_chown_key()
> read_lock(keyring_name_lock) down_write(key->sem)
> load old keyring->user
> replace key->user and key->uid
> up_write(key->sem)
> key_put(key)
> key_user_put(old user): free
> read old user->uid
> read_unlock(keyring_name_lock)
>
> Chown neither takes keyring_name_lock nor key_session_mutex, so it can
> free the old account between the pointer load and the UID read. The
> lookup then reads freed memory even though the keyring itself is alive.
>
> Use keyring->uid for the mapping check. key_alloc() initializes this
> inline owner UID and keyctl_chown_key() updates it on chown.
> The existing name lock keeps its containing keyring allocated throughout
> the check, so lookup no longer depends on the account's lifetime. This
> also matches the owner UID used by the permission check.
>
> KASAN report as below:
>
> BUG: KASAN: slab-use-after-free in find_keyring_by_name+0x577/0x5d0
> Read of size 4 at addr ffff8881128ae2ec by task keycase/500
Similar nit. Not sure but I think even just having these two lines would
be sufficient or is there something that follows that would be
meaningful?
>
> CPU: 2 UID: 0 PID: 500 Comm: keycase Not tainted 7.2.0-rc5-pmb-bt-functional-v1+ #1 PREEMPT(lazy)
> Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> Call Trace:
> <TASK>
> dump_stack_lvl+0x93/0xd0
> print_report+0xce/0x630
> ? find_keyring_by_name+0x577/0x5d0
> ? srso_alias_return_thunk+0x5/0xfbef5
> ? __virt_addr_valid+0x20d/0x410
> ? find_keyring_by_name+0x577/0x5d0
> kasan_report+0xe0/0x110
> ? find_keyring_by_name+0x577/0x5d0
> find_keyring_by_name+0x577/0x5d0
> ? __pfx_find_keyring_by_name+0x10/0x10
> ? srso_alias_return_thunk+0x5/0xfbef5
> ? security_prepare_creds+0x4f/0xb0
> ? srso_alias_return_thunk+0x5/0xfbef5
> join_session_keyring+0x89/0x310
> keyctl_join_session_keyring+0x81/0xe0
> __do_sys_keyctl+0x3c6/0x460
> do_syscall_64+0x115/0x6a0
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f562a4817b9
> 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 8b 0d 27 66 0d 00 f7 d8 64 89 01 48
> RSP: 002b:00007ffcb80b34a8 EFLAGS: 00000246 ORIG_RAX: 00000000000000fa
> RAX: ffffffffffffffda RBX: 00007ffcb80b3638 RCX: 00007f562a4817b9
> RDX: 0000000000000000 RSI: 0000563212976004 RDI: 0000000000000001
> RBP: 00000000000001f5 R08: 0000000000000000 R09: 5200000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
> R13: 00007ffcb80b3658 R14: 00007f562a5ae000 R15: 0000563212977cf0
> </TASK>
>
> Allocated by task 501:
> kasan_save_stack+0x33/0x60
> kasan_save_track+0x14/0x30
> __kasan_kmalloc+0xaa/0xb0
> __kmalloc_cache_noprof+0x251/0x630
> key_user_lookup+0x181/0x530
> key_alloc+0x164/0x11e0
> keyring_alloc+0x49/0xa0
> join_session_keyring+0x296/0x310
> keyctl_join_session_keyring+0x81/0xe0
> __do_sys_keyctl+0x3c6/0x460
> do_syscall_64+0x115/0x6a0
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> Freed by task 502:
> kasan_save_stack+0x33/0x60
> kasan_save_track+0x14/0x30
> kasan_save_free_info+0x3b/0x60
> __kasan_slab_free+0x5f/0x80
> kfree+0x236/0x5a0
> key_user_put+0x57/0x60
> keyctl_chown_key+0x5a7/0xda0
> __do_sys_keyctl+0x1ba/0x460
> do_syscall_64+0x115/0x6a0
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> The buggy address belongs to the object at ffff8881128ae200
> which belongs to the cache kmalloc-256 of size 256
> The buggy address is located 236 bytes inside of
> freed 256-byte region [ffff8881128ae200, ffff8881128ae300)
>
> The buggy address belongs to the physical page:
> page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x1128ae
> head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
> flags: 0x200000000000040(head|node=0|zone=2)
> page_type: f5(slab)
> raw: 0200000000000040 ffff888100043400 dead000000000100 dead000000000122
> raw: 0000000000000000 0000000000100010 00000000f5000000 0000000000000000
> head: 0200000000000040 ffff888100043400 dead000000000100 dead000000000122
> head: 0000000000000000 0000000000100010 00000000f5000000 0000000000000000
> head: 0200000000000001 ffffffffffffff81 00000000ffffffff 00000000ffffffff
> head: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
> page dumped because: kasan: bad access detected
>
> Memory state around the buggy address:
> ffff8881128ae180: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8881128ae200: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >ffff8881128ae280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ^
> ffff8881128ae300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8881128ae380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ==================================================================
>
> Fixes: 2ea190d0a006 ("keys: skip keys from another user namespace")
> Assisted-by: LLM
> Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
> ---
>
> diff --git a/security/keys/keyring.c b/security/keys/keyring.c
> index 15bf4af8f28218ec3f12c97630d1c76939af7eca..46f774be72967a9bd16b0ddfdf323eda36edeff4 100644
> --- a/security/keys/keyring.c
> +++ b/security/keys/keyring.c
> @@ -1158,7 +1158,7 @@ struct key *find_keyring_by_name(const char *name, bool uid_keyring)
> * grants Search permission and that hasn't been revoked
> */
> list_for_each_entry(keyring, &ns->keyring_name_list, name_link) {
> - if (!kuid_has_mapping(ns, keyring->user->uid))
> + if (!kuid_has_mapping(ns, keyring->uid))
> continue;
>
> if (test_bit(KEY_FLAG_REVOKED, &keyring->flags))
Br, Jarkko
prev parent reply other threads:[~2026-10-08 17:57 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-08 6:30 Cen Zhang
2026-10-08 17:57 ` Jarkko Sakkinen [this message]
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