linux-ext4 - Re: [PATCH] ext4: verify extent header depth

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Date:	Wed, 6 Jul 2016 15:28:54 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Vegard Nossum <vegard.nossum@...cle.com>
Cc:	tytso@....edu, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: verify extent header depth

On Wed, Jul 06, 2016 at 11:27:11PM +0200, Vegard Nossum wrote:
> According to the wiki [1], eh_depth cannot be larger than 5:
> 
>     Depth of this extent node in the extent tree. 0 = this extent node
>     points to data blocks; otherwise, this extent node points to other
>     extent nodes. The extent tree can be at most 5 levels deep: a logical
>     block number can be at most 2^32, and the smallest n that satisfies
>     4*(((blocksize - 12)/12)^n) >= 2^32 is 5.
> 
> [1]: https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout#Extent_Tree
> 
> Without this, we can end up trying to reserve too much space for the
> transaction in case of malicious corruption (here, eh_depth = 65280):
> 
>     JBD2: ext4.exe wants too many credits credits:195849 rsv_credits:0 max:256
>     ------------[ cut here ]------------
>     WARNING: CPU: 0 PID: 50 at fs/jbd2/transaction.c:293 start_this_handle+0x569/0x580
>     CPU: 0 PID: 50 Comm: ext4.exe Not tainted 4.7.0-rc5+ #508
>     Stack:
>      604a8947 625badd8 0002fd09 00000000
>      60078643 00000000 62623910 601bf9bc
>      62623970 6002fc84 626239b0 900000125
>     Call Trace:
>      [<6001c2dc>] show_stack+0xdc/0x1a0
>      [<601bf9bc>] dump_stack+0x2a/0x2e
>      [<6002fc84>] __warn+0x114/0x140
>      [<6002fdff>] warn_slowpath_null+0x1f/0x30
>      [<60165829>] start_this_handle+0x569/0x580
>      [<60165d4e>] jbd2__journal_start+0x11e/0x220
>      [<60146690>] __ext4_journal_start_sb+0x60/0xa0
>      [<60120a81>] ext4_truncate+0x131/0x3a0
>      [<60123677>] ext4_setattr+0x757/0x840
>      [<600d5d0f>] notify_change+0x16f/0x2a0
>      [<600b2b16>] do_truncate+0x76/0xc0
>      [<600c3e56>] path_openat+0x806/0x1300
>      [<600c55c9>] do_filp_open+0x89/0xf0
>      [<600b4074>] do_sys_open+0x134/0x1e0
>      [<600b4140>] SyS_open+0x20/0x30
>      [<6001ea68>] handle_syscall+0x88/0x90
>      [<600295fd>] userspace+0x3fd/0x500
>      [<6001ac55>] fork_handler+0x85/0x90
> 
>     ---[ end trace 08b0b88b6387a244 ]---
> 
> Cc: Darrick J. Wong <darrick.wong@...cle.com>
> Signed-off-by: Vegard Nossum <vegard.nossum@...cle.com>
> ---
>  fs/ext4/extents.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 30de10a..b584ddd 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -476,6 +476,10 @@ static int __ext4_ext_check(const char *function, unsigned int line,
>  		error_msg = "invalid extent entries";
>  		goto corrupted;
>  	}
> +	if (unlikely(depth > 5)) {
> +		error_msg = "too large eh_depth";
> +		goto corrupted;
> +	}

Seems reasonable,
Reviewed-by: Darrick J. Wong <darrick.wong@...cle.com>

--D

>  	/* Verify checksum on non-root extent tree nodes */
>  	if (ext_depth(inode) != depth &&
>  	    !ext4_extent_block_csum_verify(inode, eh)) {
> -- 
> 1.9.1
> 
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