Re: [syzbot] [fuse?] KMSAN: uninit-value in fuse_dentry_revalidate (2)

[Luis Henriques <luis@igalia.com>]

On Sun, Feb 15 2026, syzbot wrote:

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    770aaedb461a Merge tag 'bootconfig-v7.0' of git://git.kern..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=158f7e5a580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=50148b563a4d5941
> dashboard link: https://syzkaller.appspot.com/bug?extid=fdebb2dc960aa56c600a
> compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=138f7e5a580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13a5c15a580000
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/24ba89b61208/disk-770aaedb.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/b38352aa3489/vmlinux-770aaedb.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/c388a7a46371/bzImage-770aaedb.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+fdebb2dc960aa56c600a@syzkaller.appspotmail.com
>
> =====================================================
> BUG: KMSAN: uninit-value in fuse_dentry_revalidate+0x150/0x13d0 fs/fuse/dir.c:394

This seems to point here:

	if (entry->d_time < atomic_read(&fc->epoch))
		goto invalid;

which means that the following initialisation is missing:

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index f25ee47822ad..2ce306e35be3 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -481,6 +481,7 @@ static int fuse_dentry_init(struct dentry *dentry)
 	fd->dentry = dentry;
 	RB_CLEAR_NODE(&fd->node);
 	dentry->d_fsdata = fd;
+	dentry->d_time = 0;
 
 	return 0;
 }

Or maybe should that be done in __d_alloc() instead?

Cheers,
-- 
Luís

>  fuse_dentry_revalidate+0x150/0x13d0 fs/fuse/dir.c:394
>  d_revalidate fs/namei.c:1030 [inline]
>  lookup_open fs/namei.c:4405 [inline]
>  open_last_lookups fs/namei.c:4583 [inline]
>  path_openat+0x1614/0x64c0 fs/namei.c:4827
>  do_file_open+0x2aa/0x680 fs/namei.c:4859
>  do_sys_openat2+0x163/0x380 fs/open.c:1366
>  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+0x240/0x300 fs/open.c:1383
>  x64_sys_call+0x2445/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:258
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0x134/0xf80 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> Uninit was created at:
>  slab_post_alloc_hook mm/slub.c:4466 [inline]
>  slab_alloc_node mm/slub.c:4788 [inline]
>  kmem_cache_alloc_lru_noprof+0x382/0x1280 mm/slub.c:4807
>  __d_alloc+0x55/0xa00 fs/dcache.c:1740
>  d_alloc_parallel+0x99/0x2740 fs/dcache.c:2604
>  lookup_open fs/namei.c:4398 [inline]
>  open_last_lookups fs/namei.c:4583 [inline]
>  path_openat+0x135f/0x64c0 fs/namei.c:4827
>  do_file_open+0x2aa/0x680 fs/namei.c:4859
>  do_sys_openat2+0x163/0x380 fs/open.c:1366
>  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+0x240/0x300 fs/open.c:1383
>  x64_sys_call+0x2445/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:258
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0x134/0xf80 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> CPU: 1 UID: 0 PID: 6074 Comm: syz.0.20 Not tainted syzkaller #0 PREEMPT(full) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
> =====================================================
>
>
> ---
> 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 syzbot to run the reproducer, reply with:
> #syz test: git://repo/address.git branch-or-commit-hash
> If you attach or paste a git patch, syzbot will apply it before testing.
>
> If you want to overwrite report's subsystems, reply with:
> #syz set subsystems: new-subsystem
> (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
>