Re: [syzbot] [btrfs?] KMSAN: uninit-value in bcmp (2)

[Jan Kara]

On Sun 04-02-24 03:44:30, syzbot wrote:
> syzbot has found a reproducer for the following issue on:
> 
> HEAD commit:    9f8413c4a66f Merge tag 'cgroup-for-6.8' of git://git.kerne..
> git tree:       upstream
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=10fcfdc0180000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=656820e61b758b15
> dashboard link: https://syzkaller.appspot.com/bug?extid=3ce5dea5b1539ff36769
> 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=139dd53fe80000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12685aa8180000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/79d9f2f4b065/disk-9f8413c4.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/cbc68430d9c6/vmlinux-9f8413c4.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/9740ad9fc172/bzImage-9f8413c4.xz
> mounted in repro: https://storage.googleapis.com/syzbot-assets/25f4008bd752/mount_0.gz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+3ce5dea5b1539ff36769@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> =====================================================
> BUG: KMSAN: uninit-value in memcmp lib/string.c:692 [inline]
> BUG: KMSAN: uninit-value in bcmp+0x186/0x1c0 lib/string.c:713
>  memcmp lib/string.c:692 [inline]
>  bcmp+0x186/0x1c0 lib/string.c:713
>  fanotify_fh_equal fs/notify/fanotify/fanotify.c:51 [inline]

So I'm not sure how this is possible. In fanotify_encode_fh() we have:

        dwords = fh_len >> 2;
        type = exportfs_encode_fid(inode, buf, &dwords);
        err = -EINVAL;
        if (type <= 0 || type == FILEID_INVALID || fh_len != dwords << 2)
                goto out_err;

        fh->type = type;
        fh->len = fh_len;

So if the encoded file handle was different length than what we expected we
will fail the creation of the event...

Aha, the FAT filesystem is mounted in nfs=nostale_ro which means that we're
using fat_export_ops_nostale and thus fat_encode_fh_nostale for encoding
fh. And FAT_FID_SIZE_WITHOUT_PARENT is 3 (i.e. 12 bytes) but the function
initializes just the first 10 bytes of struct fat_fid. I'll send a fix for
FAT.

								Honza


>  fanotify_fid_event_equal fs/notify/fanotify/fanotify.c:72 [inline]
>  fanotify_should_merge fs/notify/fanotify/fanotify.c:168 [inline]
>  fanotify_merge+0x15f5/0x27e0 fs/notify/fanotify/fanotify.c:209
>  fsnotify_insert_event+0x1d0/0x600 fs/notify/notification.c:113
>  fanotify_handle_event+0x47f7/0x6140 fs/notify/fanotify/fanotify.c:966
>  send_to_group fs/notify/fsnotify.c:360 [inline]
>  fsnotify+0x2510/0x3530 fs/notify/fsnotify.c:570
>  fsnotify_parent include/linux/fsnotify.h:80 [inline]
>  fsnotify_file include/linux/fsnotify.h:100 [inline]
>  fsnotify_close include/linux/fsnotify.h:362 [inline]
>  __fput+0x578/0x10c0 fs/file_table.c:368
>  __fput_sync+0x74/0x90 fs/file_table.c:467
>  __do_sys_close fs/open.c:1554 [inline]
>  __se_sys_close+0x28a/0x4c0 fs/open.c:1539
>  __x64_sys_close+0x48/0x60 fs/open.c:1539
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0x6d/0x140 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x63/0x6b
> 
> Uninit was created at:
>  slab_post_alloc_hook+0x129/0xa70 mm/slab.h:768
>  slab_alloc_node mm/slub.c:3478 [inline]
>  slab_alloc mm/slub.c:3486 [inline]
>  __kmem_cache_alloc_lru mm/slub.c:3493 [inline]
>  kmem_cache_alloc+0x579/0xa90 mm/slub.c:3502
>  fanotify_alloc_fid_event fs/notify/fanotify/fanotify.c:584 [inline]
>  fanotify_alloc_event fs/notify/fanotify/fanotify.c:817 [inline]
>  fanotify_handle_event+0x2ff6/0x6140 fs/notify/fanotify/fanotify.c:952
>  send_to_group fs/notify/fsnotify.c:360 [inline]
>  fsnotify+0x2510/0x3530 fs/notify/fsnotify.c:570
>  fsnotify_parent include/linux/fsnotify.h:80 [inline]
>  fsnotify_file include/linux/fsnotify.h:100 [inline]
>  fsnotify_close include/linux/fsnotify.h:362 [inline]
>  __fput+0x578/0x10c0 fs/file_table.c:368
>  __fput_sync+0x74/0x90 fs/file_table.c:467
>  __do_sys_close fs/open.c:1554 [inline]
>  __se_sys_close+0x28a/0x4c0 fs/open.c:1539
>  __x64_sys_close+0x48/0x60 fs/open.c:1539
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0x6d/0x140 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x63/0x6b
> 
> CPU: 0 PID: 5010 Comm: syz-executor120 Not tainted 6.7.0-syzkaller-00562-g9f8413c4a66f #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/25/2024
> =====================================================
> 
> 
> ---
> 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.
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR