[syzbot] [block?] KMSAN: kernel-infoleak in filemap_read

[syzbot] [block?] KMSAN: kernel-infoleak in filemap_read

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    861deac3b092 Linux 6.7-rc7
git tree:       upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=14d12f76e80000
kernel config:  https://syzkaller.appspot.com/x/.config?x=e0c7078a6b901aa3
dashboard link: https://syzkaller.appspot.com/bug?extid=905d785c4923bea2c1db
compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15ef73d6e80000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15d27f76e80000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/0ea60ee8ed32/disk-861deac3.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/6d69fdc33021/vmlinux-861deac3.xz
kernel image: https://storage.googleapis.com/syzbot-assets/f0158750d452/bzImage-861deac3.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/52b4443ce279/mount_0.gz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+905d785c4923bea2c1db@syzkaller.appspotmail.com

=====================================================
BUG: KMSAN: kernel-infoleak in instrument_copy_to_user include/linux/instrumented.h:114 [inline]
BUG: KMSAN: kernel-infoleak in copy_to_user_iter lib/iov_iter.c:24 [inline]
BUG: KMSAN: kernel-infoleak in iterate_ubuf include/linux/iov_iter.h:29 [inline]
BUG: KMSAN: kernel-infoleak in iterate_and_advance2 include/linux/iov_iter.h:245 [inline]
BUG: KMSAN: kernel-infoleak in iterate_and_advance include/linux/iov_iter.h:271 [inline]
BUG: KMSAN: kernel-infoleak in _copy_to_iter+0x364/0x2520 lib/iov_iter.c:186
 instrument_copy_to_user include/linux/instrumented.h:114 [inline]
 copy_to_user_iter lib/iov_iter.c:24 [inline]
 iterate_ubuf include/linux/iov_iter.h:29 [inline]
 iterate_and_advance2 include/linux/iov_iter.h:245 [inline]
 iterate_and_advance include/linux/iov_iter.h:271 [inline]
 _copy_to_iter+0x364/0x2520 lib/iov_iter.c:186
 copy_page_to_iter+0x419/0x870 lib/iov_iter.c:381
 copy_folio_to_iter include/linux/uio.h:181 [inline]
 filemap_read+0xc00/0x14d0 mm/filemap.c:2644
 blkdev_read_iter+0x6c0/0x8e0 block/fops.c:742
 call_read_iter include/linux/fs.h:2014 [inline]
 new_sync_read fs/read_write.c:389 [inline]
 vfs_read+0x8f6/0xe00 fs/read_write.c:470
 ksys_read+0x20f/0x4c0 fs/read_write.c:613
 __do_sys_read fs/read_write.c:623 [inline]
 __se_sys_read fs/read_write.c:621 [inline]
 __x64_sys_read+0x93/0xd0 fs/read_write.c:621
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0x44/0x110 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

Uninit was stored to memory at:
 memcpy_to_iter lib/iov_iter.c:65 [inline]
 iterate_bvec include/linux/iov_iter.h:122 [inline]
 iterate_and_advance2 include/linux/iov_iter.h:249 [inline]
 iterate_and_advance include/linux/iov_iter.h:271 [inline]
 _copy_to_iter+0xe41/0x2520 lib/iov_iter.c:186
 copy_page_to_iter+0x419/0x870 lib/iov_iter.c:381
 shmem_file_read_iter+0xa0a/0x12b0 mm/shmem.c:2841
 do_iter_read+0xd21/0x1380 fs/read_write.c:795
 vfs_iter_read+0x8e/0xe0 fs/read_write.c:837
 lo_read_simple drivers/block/loop.c:290 [inline]
 do_req_filebacked drivers/block/loop.c:500 [inline]
 loop_handle_cmd drivers/block/loop.c:1915 [inline]
 loop_process_work+0x12e7/0x39d0 drivers/block/loop.c:1950
 loop_rootcg_workfn+0x2b/0x30 drivers/block/loop.c:1981
 process_one_work kernel/workqueue.c:2627 [inline]
 process_scheduled_works+0x104e/0x1e70 kernel/workqueue.c:2700
 worker_thread+0xf45/0x1490 kernel/workqueue.c:2781
 kthread+0x3ed/0x540 kernel/kthread.c:388
 ret_from_fork+0x66/0x80 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:242

Uninit was stored to memory at:
 memcpy_from_iter lib/iov_iter.c:73 [inline]
 iterate_bvec include/linux/iov_iter.h:122 [inline]
 iterate_and_advance2 include/linux/iov_iter.h:249 [inline]
 iterate_and_advance include/linux/iov_iter.h:271 [inline]
 __copy_from_iter lib/iov_iter.c:268 [inline]
 copy_page_from_iter_atomic+0x12f1/0x2b70 lib/iov_iter.c:500
 generic_perform_write+0x4ae/0xc40 mm/filemap.c:3926
 shmem_file_write_iter+0x2b5/0x2e0 mm/shmem.c:2895
 do_iter_write+0xceb/0x1340 fs/read_write.c:860
 vfs_iter_write+0x8e/0xe0 fs/read_write.c:901
 lo_write_bvec drivers/block/loop.c:249 [inline]
 lo_write_simple drivers/block/loop.c:271 [inline]
 do_req_filebacked drivers/block/loop.c:495 [inline]
 loop_handle_cmd drivers/block/loop.c:1915 [inline]
 loop_process_work+0x20c3/0x39d0 drivers/block/loop.c:1950
 loop_rootcg_workfn+0x2b/0x30 drivers/block/loop.c:1981
 process_one_work kernel/workqueue.c:2627 [inline]
 process_scheduled_works+0x104e/0x1e70 kernel/workqueue.c:2700
 worker_thread+0xf45/0x1490 kernel/workqueue.c:2781
 kthread+0x3ed/0x540 kernel/kthread.c:388
 ret_from_fork+0x66/0x80 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:242

Uninit was stored to memory at:
 memcpy_to_page include/linux/highmem.h:427 [inline]
 hfsplus_bnode_write+0xee/0x2e0 fs/hfsplus/bnode.c:83
 hfsplus_brec_insert+0xa9d/0x1400 fs/hfsplus/brec.c:133
 hfsplus_create_cat+0x1309/0x1810 fs/hfsplus/catalog.c:308
 hfsplus_link+0x7d8/0xbf0 fs/hfsplus/dir.c:342
 vfs_link+0x936/0xb60 fs/namei.c:4588
 do_linkat+0x6e1/0x1160 fs/namei.c:4659
 __do_sys_link fs/namei.c:4693 [inline]
 __se_sys_link fs/namei.c:4691 [inline]
 __x64_sys_link+0xec/0x140 fs/namei.c:4691
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0x44/0x110 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

Uninit was stored to memory at:
 hfsplus_cat_build_record fs/hfsplus/catalog.c:167 [inline]
 hfsplus_create_cat+0x11d1/0x1810 fs/hfsplus/catalog.c:300
 hfsplus_link+0x7d8/0xbf0 fs/hfsplus/dir.c:342
 vfs_link+0x936/0xb60 fs/namei.c:4588
 do_linkat+0x6e1/0x1160 fs/namei.c:4659
 __do_sys_link fs/namei.c:4693 [inline]
 __se_sys_link fs/namei.c:4691 [inline]
 __x64_sys_link+0xec/0x140 fs/namei.c:4691
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0x44/0x110 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

Uninit was created at:
 __alloc_pages+0x9a4/0xe00 mm/page_alloc.c:4591
 alloc_pages_mpol+0x62b/0x9d0 mm/mempolicy.c:2133
 alloc_pages+0x1be/0x1e0 mm/mempolicy.c:2204
 alloc_slab_page mm/slub.c:1870 [inline]
 allocate_slab mm/slub.c:2017 [inline]
 new_slab+0x421/0x1570 mm/slub.c:2070
 ___slab_alloc+0x13db/0x33d0 mm/slub.c:3223
 __slab_alloc mm/slub.c:3322 [inline]
 __slab_alloc_node mm/slub.c:3375 [inline]
 slab_alloc_node mm/slub.c:3468 [inline]
 slab_alloc mm/slub.c:3486 [inline]
 __kmem_cache_alloc_lru mm/slub.c:3493 [inline]
 kmem_cache_alloc_lru+0x552/0x970 mm/slub.c:3509
 alloc_inode_sb include/linux/fs.h:2937 [inline]
 hfsplus_alloc_inode+0x5a/0xc0 fs/hfsplus/super.c:627
 alloc_inode+0x83/0x440 fs/inode.c:261
 iget_locked+0x2dd/0xe80 fs/inode.c:1316
 hfsplus_iget+0x59/0xaf0 fs/hfsplus/super.c:64
 hfsplus_btree_open+0x13e/0x1d00 fs/hfsplus/btree.c:150
 hfsplus_fill_super+0x1113/0x26f0 fs/hfsplus/super.c:473
 mount_bdev+0x3d7/0x560 fs/super.c:1650
 hfsplus_mount+0x4d/0x60 fs/hfsplus/super.c:641
 legacy_get_tree+0x110/0x290 fs/fs_context.c:662
 vfs_get_tree+0xa5/0x520 fs/super.c:1771
 do_new_mount+0x68d/0x1550 fs/namespace.c:3337
 path_mount+0x73d/0x1f20 fs/namespace.c:3664
 do_mount fs/namespace.c:3677 [inline]
 __do_sys_mount fs/namespace.c:3886 [inline]
 __se_sys_mount+0x725/0x810 fs/namespace.c:3863
 __x64_sys_mount+0xe4/0x140 fs/namespace.c:3863
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0x44/0x110 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

Bytes 3258-3261 of 4096 are uninitialized
Memory access of size 4096 starts at ffff8881146b8000
Data copied to user address 000055ff3929d058

CPU: 1 PID: 5003 Comm: udevd Not tainted 6.7.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/17/2023
=====================================================


---
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If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
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If you want to overwrite report's subsystems, reply with:
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#syz undup

Re: [syzbot] [block?] KMSAN: kernel-infoleak in filemap_read

[shaurya]

[-- Attachment #1: Type: text/plain, Size: 83 bytes --]

#syz test:
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

[-- Attachment #2: 0001-hfsplus-fix-uninit-value-in-hfsplus_cat_build_record.patch --]
[-- Type: text/x-patch, Size: 2464 bytes --]

From 1336b22bb9f3ff4e144c4122a4b4b13c53e682a7 Mon Sep 17 00:00:00 2001
From: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
Date: Thu, 20 Nov 2025 00:44:21 +0530
Subject: [PATCH] hfsplus: fix uninit-value in hfsplus_cat_build_record

The root cause is in hfsplus_cat_build_record(), which builds catalog
entries using the union hfsplus_cat_entry. This union contains three
members with significantly different sizes:

  struct hfsplus_cat_folder folder;    (88 bytes)
  struct hfsplus_cat_file file;        (248 bytes)
  struct hfsplus_cat_thread thread;    (520 bytes)

The function was only zeroing the specific member being used (folder or
file), not the entire union. This left significant uninitialized data:

  For folders: 520 - 88  = 432 bytes uninitialized
  For files:   520 - 248 = 272 bytes uninitialized

This uninitialized data was then written to disk via hfs_brec_insert(),
read back through the loop device, and eventually copied to userspace
via filemap_read(), resulting in a leak of kernel stack memory.
Fix this by zeroing the entire union before initializing the specific
member. This ensures no uninitialized bytes remain.

Reported-by: syzbot+905d785c4923bea2c1db@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=905d785c4923bea2c1db
Fixes: 1da177e4c3f4

Signed-off-by: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
---
 fs/hfsplus/catalog.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/hfsplus/catalog.c b/fs/hfsplus/catalog.c
index 02c1eee4a4b8..4d42e7139f3b 100644
--- a/fs/hfsplus/catalog.c
+++ b/fs/hfsplus/catalog.c
@@ -111,7 +111,8 @@ static int hfsplus_cat_build_record(hfsplus_cat_entry *entry,
 		struct hfsplus_cat_folder *folder;
 
 		folder = &entry->folder;
-		memset(folder, 0, sizeof(*folder));
+		/* Zero the entire union to avoid leaking uninitialized data */
+		memset(entry, 0, sizeof(*entry));
 		folder->type = cpu_to_be16(HFSPLUS_FOLDER);
 		if (test_bit(HFSPLUS_SB_HFSX, &sbi->flags))
 			folder->flags |= cpu_to_be16(HFSPLUS_HAS_FOLDER_COUNT);
@@ -130,7 +131,8 @@ static int hfsplus_cat_build_record(hfsplus_cat_entry *entry,
 		struct hfsplus_cat_file *file;
 
 		file = &entry->file;
-		memset(file, 0, sizeof(*file));
+		/* Zero the entire union to avoid leaking uninitialized data */
+		memset(entry, 0, sizeof(*entry));
 		file->type = cpu_to_be16(HFSPLUS_FILE);
 		file->flags = cpu_to_be16(HFSPLUS_FILE_THREAD_EXISTS);
 		file->id = cpu_to_be32(cnid);
-- 
2.34.1

Re: [syzbot] [block?] KMSAN: kernel-infoleak in filemap_read

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+905d785c4923bea2c1db@syzkaller.appspotmail.com
Tested-by: syzbot+905d785c4923bea2c1db@syzkaller.appspotmail.com

Tested on:

commit:         23cb64fb Merge tag 'soc-fixes-6.18-3' of git://git.ker..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15de1b32580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f874b4f2cfd4b87
dashboard link: https://syzkaller.appspot.com/bug?extid=905d785c4923bea2c1db
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=15a25658580000

Note: testing is done by a robot and is best-effort only.