ext4: fix signed vs unsigned comparison in ext4_valid_extent()

Due to a signed vs unsigned comparison, an invalid extent where
ee_block (the logical block) is so large that lblk + len overflow
wasn't getting flagged as invalid.

As a result, we tripped the BUG_ON(end < lblk) in
ext4_es_cache_extent() when trying to mount a file system with a
corrupted journal inode was corrupted.

https://bugzilla.kernel.org/show_bug.cgi?id=205197

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org
---
 fs/ext4/extents.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On Tue, Oct 22, 2019 at 09:31:12PM -0400, Theodore Ts'o wrote:
> Due to a signed vs unsigned comparison, an invalid extent where
> ee_block (the logical block) is so large that lblk + len overflow
> wasn't getting flagged as invalid.
> 
> As a result, we tripped the BUG_ON(end < lblk) in
> ext4_es_cache_extent() when trying to mount a file system with a
> corrupted journal inode was corrupted.
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=205197
> 
> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
> Cc: stable@kernel.org
> ---
>  fs/ext4/extents.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index fb0f99dc8c22..d12bc287abdc 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -367,7 +367,7 @@ ext4_ext_max_entries(struct inode *inode, int depth)
>  static int ext4_valid_extent(struct inode *inode, struct ext4_extent *ext)
>  {
>  	ext4_fsblk_t block = ext4_ext_pblock(ext);
> -	int len = ext4_ext_get_actual_len(ext);
> +	unsigned int len = ext4_ext_get_actual_len(ext);
>  	ext4_lblk_t lblock = le32_to_cpu(ext->ee_block);
>  
>  	/*
> -- 
> 2.23.0
> 

This patch can't be fixing anything because the comparison is unsigned both
before and after this patch.

- Eric

On Tue, Oct 22, 2019 at 10:44:47PM -0700, Eric Biggers wrote:
> 
> This patch can't be fixing anything because the comparison is unsigned both
> before and after this patch.

Thanks, you're right; I had forgotten C's signed/unsigned rules for
addition.  The funny thing is the original reporter of BZ #205197
reported that the problem went away he tried a similar patch.

	      	  	       	    - Ted

On Wed, Oct 23, 2019 at 09:15:46AM -0400, Theodore Y. Ts'o wrote:
> On Tue, Oct 22, 2019 at 10:44:47PM -0700, Eric Biggers wrote:
> > 
> > This patch can't be fixing anything because the comparison is unsigned both
> > before and after this patch.
> 
> Thanks, you're right; I had forgotten C's signed/unsigned rules for
> addition.  The funny thing is the original reporter of BZ #205197
> reported that the problem went away he tried a similar patch.

Not trying to stick my nose in too much here but:

What does it mean if ext4_ext_get_actual_len() to return < 0?

Ira

> 
> 	      	  	       	    - Ted

On Wed, Oct 23, 2019 at 11:43:33AM -0700, Ira Weiny wrote:
> On Wed, Oct 23, 2019 at 09:15:46AM -0400, Theodore Y. Ts'o wrote:
> > On Tue, Oct 22, 2019 at 10:44:47PM -0700, Eric Biggers wrote:
> > > 
> > > This patch can't be fixing anything because the comparison is unsigned both
> > > before and after this patch.
> > 
> > Thanks, you're right; I had forgotten C's signed/unsigned rules for
> > addition.  The funny thing is the original reporter of BZ #205197
> > reported that the problem went away he tried a similar patch.
> 
> Not trying to stick my nose in too much here but:
> 
> What does it mean if ext4_ext_get_actual_len() to return < 0?

It's not possible for it to return < 0.  We probably should clean it
up to make it return an unsigned int, but that's a longer-term clean-up.

      	      	     		 - Ted