linux-kernel - Re: [PATCH] ext4: fix off-by-one error in do

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Message-ID: <odgkvml62unm4ux3sbnympgyzj22z7dwjgdvdmlbgtiybq4j7z@gnnaygdp7muw>
Date: Mon, 7 Apr 2025 15:02:58 +0200
From: Jan Kara <jack@...e.cz>
To: Artem Sadovnikov <a.sadovnikov@...ras.ru>
Cc: linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>, 
	Andreas Dilger <adilger.kernel@...ger.ca>, Eric Sandeen <sandeen@...hat.com>, Jan Kara <jack@...e.cz>, 
	linux-kernel@...r.kernel.org, lvc-project@...uxtesting.org, stable@...r.kernel.org
Subject: Re: [PATCH] ext4: fix off-by-one error in do_split

On Fri 04-04-25 08:28:05, Artem Sadovnikov wrote:
> Syzkaller detected a use-after-free issue in ext4_insert_dentry that was
> caused by out-of-bounds access due to incorrect splitting in do_split.
> 
> BUG: KASAN: use-after-free in ext4_insert_dentry+0x36a/0x6d0 fs/ext4/namei.c:2109
> Write of size 251 at addr ffff888074572f14 by task syz-executor335/5847
> 
> CPU: 0 UID: 0 PID: 5847 Comm: syz-executor335 Not tainted 6.12.0-rc6-syzkaller-00318-ga9cda7c0ffed #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/30/2024
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:94 [inline]
>  dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
>  print_address_description mm/kasan/report.c:377 [inline]
>  print_report+0x169/0x550 mm/kasan/report.c:488
>  kasan_report+0x143/0x180 mm/kasan/report.c:601
>  kasan_check_range+0x282/0x290 mm/kasan/generic.c:189
>  __asan_memcpy+0x40/0x70 mm/kasan/shadow.c:106
>  ext4_insert_dentry+0x36a/0x6d0 fs/ext4/namei.c:2109
>  add_dirent_to_buf+0x3d9/0x750 fs/ext4/namei.c:2154
>  make_indexed_dir+0xf98/0x1600 fs/ext4/namei.c:2351
>  ext4_add_entry+0x222a/0x25d0 fs/ext4/namei.c:2455
>  ext4_add_nondir+0x8d/0x290 fs/ext4/namei.c:2796
>  ext4_symlink+0x920/0xb50 fs/ext4/namei.c:3431
>  vfs_symlink+0x137/0x2e0 fs/namei.c:4615
>  do_symlinkat+0x222/0x3a0 fs/namei.c:4641
>  __do_sys_symlink fs/namei.c:4662 [inline]
>  __se_sys_symlink fs/namei.c:4660 [inline]
>  __x64_sys_symlink+0x7a/0x90 fs/namei.c:4660
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>  </TASK>
> 
> The following loop is located right above 'if' statement.
> 
> for (i = count-1; i >= 0; i--) {
> 	/* is more than half of this entry in 2nd half of the block? */
> 	if (size + map[i].size/2 > blocksize/2)
> 		break;
> 	size += map[i].size;
> 	move++;
> }
> 
> 'i' in this case could go down to -1, in which case sum of active entries
> wouldn't exceed half the block size, but previous behaviour would also do
> split in half if sum would exceed at the very last block, which in case of
> having too many long name files in a single block could lead to
> out-of-bounds access and following use-after-free.

Thanks for debugging this! The fix looks good, but I'm still failing to see
the use-after-free / end-of-buffer issue. If we wrongly split to two parts
count/2 each, then dx_move_dirents() and dx_pack_dirents() seem to still
work correctly. Just they will make too small amount of space in bh but
still at least one dir entry gets moved? Following add_dirent_to_buf() is
more likely to fail due to ENOSPC but still I don't see the buffer overrun
issue? Can you please tell me what I'm missing? Thanks!

Anyway, since the fix looks obviously correct feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>


								Honza
> 
> Found by Linux Verification Center (linuxtesting.org) with Syzkaller.
> 
> Cc: stable@...r.kernel.org
> Fixes: 5872331b3d91 ("ext4: fix potential negative array index in do_split()")
> Signed-off-by: Artem Sadovnikov <a.sadovnikov@...ras.ru>
> ---
>  fs/ext4/namei.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> index cb5cb33b1d91..e9712e64ec8f 100644
> --- a/fs/ext4/namei.c
> +++ b/fs/ext4/namei.c
> @@ -1971,7 +1971,7 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
>  	 * split it in half by count; each resulting block will have at least
>  	 * half the space free.
>  	 */
> -	if (i > 0)
> +	if (i >= 0)
>  		split = count - move;
>  	else
>  		split = count/2;
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR