Re: [PATCH] ext4: validate readdir offset before accessing dirent

[Jan Kara]

On Fri 03-07-26 09:51:06, Yao Kai wrote:
> A corrupted directory can trigger the following KASAN report when
> ext4_readdir() resumes from an invalid position:
> 
>   BUG: KASAN: use-after-free in __ext4_check_dir_entry+0x5ef/0x820
>   Read of size 2 at addr ffff88810a646000 by task repro_linear/509
> 
>   Call Trace:
>    <TASK>
>    dump_stack_lvl+0x53/0x70
>    print_report+0xd0/0x630
>    kasan_report+0xce/0x100
>    __ext4_check_dir_entry+0x5ef/0x820
>    ext4_readdir+0xcde/0x2b70
>    iterate_dir+0x1a1/0x520
>    __x64_sys_getdents64+0x12b/0x220
>    do_syscall_64+0xf9/0x540
>    entry_SYSCALL_64_after_hwframe+0x77/0x7f
>    </TASK>
> 
> KASAN reports use-after-free because the out-of-bounds access lands in an
> adjacent freed page. The directory buffer itself is still referenced.
> 
> ext4_dir_llseek() invalidates the directory cookie so that ext4_readdir()
> rescans directory entries from the start of the block. The rescan checks
> only the lower bound of rec_len before advancing. A corrupted rec_len can
> therefore place the offset where the block has insufficient space for a
> complete directory entry. The rescan itself may dereference that truncated
> entry, or the main loop may pass it to __ext4_check_dir_entry(). The latter
> reads de->rec_len before validating the range. For example:
> 
>   block offset  0                           4092  4096
>                 |---- de1.rec_len = 4092 -----|----|
>                                                de2.inode
>                                                     | de2.rec_len
>                                                     ^ OOB, reported as UAF
> 
> de2 starts at offset 4092 in this 4 KiB block. Its four-byte inode fits in
> the block, but its rec_len starts at offset 4096 and crosses the boundary.
> 
> The minimum safe length is inode-dependent. Encrypted and casefolded
> directory entries need eight additional hash bytes, while a valid metadata
> checksum tail is only 12 bytes.
> 
> Cache the metadata checksum feature state and derive the minimum directory
> entry length from the on-disk format. Use it to bound both the rescan and
> the offset passed to the main loop. Report an offset in a truncated block
> tail and skip the remainder of the block, while continuing to accept an
> offset exactly at the block boundary.
> 
> Reported-by: syzbot+5322c5c260eb44d209ed@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=5322c5c260eb44d209ed
> Fixes: ac27a0ec112a ("[PATCH] ext4: initial copy of files from ext3")
> Signed-off-by: Yao Kai <yaokai34@xxxxxxxxxx>

Just one nit below. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

> @@ -257,6 +261,17 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
>  			info->cookie = inode_query_iversion(inode);
>  		}
>  
> +		if (unlikely(offset < sb->s_blocksize &&
> +			     offset > sb->s_blocksize -
> +			     ext4_dir_rec_len(1, has_csum ? NULL : inode))) {
> +			EXT4_ERROR_FILE(file, bh->b_blocknr,
> +					"bad entry in directory: %s - offset=%u, size=%lu",
> +					"directory entry too close to block end",
> +					offset, sb->s_blocksize);
> +			ctx->pos = (ctx->pos | (sb->s_blocksize - 1)) + 1;
				   ^^^ this is round_up(ctx->pos, sb->s_blocksize)


> +			goto next_block;
> +		}
> +
>  		while (ctx->pos < inode->i_size
>  		       && offset < sb->s_blocksize) {
>  			de = (struct ext4_dir_entry_2 *) (bh->b_data + offset);

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR