[syzbot] [bcachefs?] KMSAN: uninit-value in bch2_extent_update_i_size_sectors

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: dccb07f2914c Merge tag 'for-6.9-rc7-tag' of git://git.kern..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14c66e24980000
kernel config: https://syzkaller.appspot.com/x/.config?x=617171361dd3cd47
dashboard link: https://syzkaller.appspot.com/bug?extid=eceea46e8838baeeba67
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/fdbc7be30633/disk-dccb07f2.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/a9e4c11aa835/vmlinux-dccb07f2.xz
kernel image: https://storage.googleapis.com/syzbot-assets/43c3a343ea93/bzImage-dccb07f2.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+eceea4...@syzkaller.appspotmail.com

bcachefs (loop0): snapshots_read... done
bcachefs (loop0): journal_replay... done
bcachefs (loop0): resume_logged_ops... done
bcachefs (loop0): going read-write
bcachefs (loop0): done starting filesystem
=====================================================
BUG: KMSAN: uninit-value in bch2_extent_update_i_size_sectors+0x140f/0x17d0 fs/bcachefs/io_write.c:237
bch2_extent_update_i_size_sectors+0x140f/0x17d0 fs/bcachefs/io_write.c:237
bch2_extent_update+0x4f5/0xac0 fs/bcachefs/io_write.c:314
bch2_write_index_default fs/bcachefs/io_write.c:366 [inline]
__bch2_write_index+0x1653/0x2bd0 fs/bcachefs/io_write.c:520
bch2_write_data_inline fs/bcachefs/io_write.c:1538 [inline]
bch2_write+0x1a13/0x1b40 fs/bcachefs/io_write.c:1606
closure_queue include/linux/closure.h:257 [inline]
closure_call include/linux/closure.h:390 [inline]
bch2_writepage_do_io fs/bcachefs/fs-io-buffered.c:468 [inline]
bch2_writepages+0x24a/0x3c0 fs/bcachefs/fs-io-buffered.c:660
do_writepages+0x427/0xc30 mm/page-writeback.c:2612
filemap_fdatawrite_wbc+0x1d8/0x270 mm/filemap.c:397
__filemap_fdatawrite_range+0xe3/0x120 mm/filemap.c:430
sync_file_range+0x316/0x450 fs/sync.c:292
ksys_sync_file_range fs/sync.c:364 [inline]
__do_sys_sync_file_range fs/sync.c:373 [inline]
__se_sys_sync_file_range fs/sync.c:370 [inline]
__x64_sys_sync_file_range+0x15a/0x2a0 fs/sync.c:370
x64_sys_call+0xc90/0x3b50 arch/x86/include/generated/asm/syscalls_64.h:278
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Uninit was stored to memory at:
memcpy_u64s_small fs/bcachefs/util.h:511 [inline]
bkey_reassemble fs/bcachefs/bkey.h:505 [inline]
__bch2_bkey_make_mut_noupdate fs/bcachefs/btree_update.h:225 [inline]
__bch2_bkey_get_mut_noupdate fs/bcachefs/btree_update.h:282 [inline]
bch2_bkey_get_mut_noupdate fs/bcachefs/btree_update.h:293 [inline]
bch2_extent_update_i_size_sectors+0x9a9/0x17d0 fs/bcachefs/io_write.c:219
bch2_extent_update+0x4f5/0xac0 fs/bcachefs/io_write.c:314
bch2_write_index_default fs/bcachefs/io_write.c:366 [inline]
__bch2_write_index+0x1653/0x2bd0 fs/bcachefs/io_write.c:520
bch2_write_data_inline fs/bcachefs/io_write.c:1538 [inline]
bch2_write+0x1a13/0x1b40 fs/bcachefs/io_write.c:1606
closure_queue include/linux/closure.h:257 [inline]
closure_call include/linux/closure.h:390 [inline]
bch2_writepage_do_io fs/bcachefs/fs-io-buffered.c:468 [inline]
bch2_writepages+0x24a/0x3c0 fs/bcachefs/fs-io-buffered.c:660
do_writepages+0x427/0xc30 mm/page-writeback.c:2612
filemap_fdatawrite_wbc+0x1d8/0x270 mm/filemap.c:397
__filemap_fdatawrite_range+0xe3/0x120 mm/filemap.c:430
sync_file_range+0x316/0x450 fs/sync.c:292
ksys_sync_file_range fs/sync.c:364 [inline]
__do_sys_sync_file_range fs/sync.c:373 [inline]
__se_sys_sync_file_range fs/sync.c:370 [inline]
__x64_sys_sync_file_range+0x15a/0x2a0 fs/sync.c:370
x64_sys_call+0xc90/0x3b50 arch/x86/include/generated/asm/syscalls_64.h:278
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Uninit was stored to memory at:
memcpy_u64s_small fs/bcachefs/util.h:511 [inline]
bkey_reassemble fs/bcachefs/bkey.h:505 [inline]
btree_key_cache_fill fs/bcachefs/btree_key_cache.c:454 [inline]
bch2_btree_path_traverse_cached_slowpath+0x5f02/0x79f0 fs/bcachefs/btree_key_cache.c:530
bch2_btree_path_traverse_cached+0xd1a/0x1140
bch2_btree_path_traverse_one+0x737/0x5290 fs/bcachefs/btree_iter.c:1155
bch2_btree_path_traverse fs/bcachefs/btree_iter.h:225 [inline]
bch2_btree_iter_peek_slot+0x128c/0x3840 fs/bcachefs/btree_iter.c:2473
__bch2_bkey_get_iter fs/bcachefs/btree_iter.h:549 [inline]
__bch2_bkey_get_mut_noupdate fs/bcachefs/btree_update.h:278 [inline]
bch2_bkey_get_mut_noupdate fs/bcachefs/btree_update.h:293 [inline]
bch2_extent_update_i_size_sectors+0x404/0x17d0 fs/bcachefs/io_write.c:219
bch2_extent_update+0x4f5/0xac0 fs/bcachefs/io_write.c:314
bch2_write_index_default fs/bcachefs/io_write.c:366 [inline]
__bch2_write_index+0x1653/0x2bd0 fs/bcachefs/io_write.c:520
bch2_write_data_inline fs/bcachefs/io_write.c:1538 [inline]
bch2_write+0x1a13/0x1b40 fs/bcachefs/io_write.c:1606
closure_queue include/linux/closure.h:257 [inline]
closure_call include/linux/closure.h:390 [inline]
bch2_writepage_do_io fs/bcachefs/fs-io-buffered.c:468 [inline]
bch2_writepages+0x24a/0x3c0 fs/bcachefs/fs-io-buffered.c:660
do_writepages+0x427/0xc30 mm/page-writeback.c:2612
filemap_fdatawrite_wbc+0x1d8/0x270 mm/filemap.c:397
__filemap_fdatawrite_range+0xe3/0x120 mm/filemap.c:430
sync_file_range+0x316/0x450 fs/sync.c:292
ksys_sync_file_range fs/sync.c:364 [inline]
__do_sys_sync_file_range fs/sync.c:373 [inline]
__se_sys_sync_file_range fs/sync.c:370 [inline]
__x64_sys_sync_file_range+0x15a/0x2a0 fs/sync.c:370
x64_sys_call+0xc90/0x3b50 arch/x86/include/generated/asm/syscalls_64.h:278
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Uninit was created at:
__kmalloc_large_node+0x231/0x370 mm/slub.c:3927
__do_kmalloc_node mm/slub.c:3960 [inline]
__kmalloc_node+0xb10/0x10c0 mm/slub.c:3979
kmalloc_node include/linux/slab.h:648 [inline]
kvmalloc_node+0xc0/0x2d0 mm/util.c:634
kvmalloc include/linux/slab.h:766 [inline]
btree_bounce_alloc fs/bcachefs/btree_io.c:118 [inline]
bch2_btree_node_read_done+0x4e68/0x75e0 fs/bcachefs/btree_io.c:1185
btree_node_read_work+0x8a5/0x1eb0 fs/bcachefs/btree_io.c:1324
bch2_btree_node_read+0x3d42/0x4b50
__bch2_btree_root_read fs/bcachefs/btree_io.c:1748 [inline]
bch2_btree_root_read+0xa6c/0x13d0 fs/bcachefs/btree_io.c:1772
read_btree_roots+0x454/0xee0 fs/bcachefs/recovery.c:457
bch2_fs_recovery+0x7adb/0x9310 fs/bcachefs/recovery.c:785
bch2_fs_start+0x7b2/0xbd0 fs/bcachefs/super.c:1043
bch2_fs_open+0x135f/0x1670 fs/bcachefs/super.c:2102
bch2_mount+0x90d/0x1d90 fs/bcachefs/fs.c:1903
legacy_get_tree+0x114/0x290 fs/fs_context.c:662
vfs_get_tree+0xa7/0x570 fs/super.c:1779
do_new_mount+0x71f/0x15e0 fs/namespace.c:3352
path_mount+0x742/0x1f20 fs/namespace.c:3679
do_mount fs/namespace.c:3692 [inline]
__do_sys_mount fs/namespace.c:3898 [inline]
__se_sys_mount+0x725/0x810 fs/namespace.c:3875
__x64_sys_mount+0xe4/0x150 fs/namespace.c:3875
x64_sys_call+0x2bf4/0x3b50 arch/x86/include/generated/asm/syscalls_64.h:166
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

CPU: 1 PID: 6218 Comm: syz-executor.0 Not tainted 6.9.0-rc7-syzkaller-00012-gdccb07f2914c #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
=====================================================


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[Kent Overstreet]

This (and a few others I've looked at) look more likely to be bugs in
KMSAN, not bcachefs.

here, the supposedly uninitialized key was read in from disk, so the key
couldn't possibly have been legitimately marked uninitialized.

All the suspicious KMSAN splats I'm looking at involve bkeys, but I'm
not seeing any other common factors besides that.

[Alexander Potapenko]

On Thu, May 23, 2024 at 1:49 AM Kent Overstreet
<kent.ov...@linux.dev> wrote:
>
> This (and a few others I've looked at) look more likely to be bugs in
> KMSAN, not bcachefs.

Yeah, this is possible if we are missing some initialization.

>
> here, the supposedly uninitialized key was read in from disk, so the key
> couldn't possibly have been legitimately marked uninitialized.

Am I right that the key is supposed to be initialized before the first
call of memcpy_u64s_small() (the _lower_ "Uninit was stored to memory
at:" stack trace)?
Do you have an idea what code performed the actual load of the data from disk?
If that load was done by some assembly routine, we could've easily
missed that - in that case we'll need to annotate it to fix the false
positives.

--
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

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