Re: [PATCH] ext4: fix invalid free tracking in ext4_xattr_move_to_block()

[Tudor Ambarus <tudor.ambarus@linaro.org>]

On 4/30/23 17:04, Theodore Ts'o wrote:
> In ext4_xattr_move_to_block(), the value of the extended attribute
> which we need to move to an external block may be allocated by
> kvmalloc() if the value is stored in an external inode.  So at the end
> of the function the code tried to check if this was the case by
> testing entry->e_value_inum.
> 
> However, at this point, the pointer to the xattr entry is no longer
> valid, because it was removed from the original location where it had
> been stored.  So we could end up calling kvfree() on a pointer which
> was not allocated by kvmalloc(); or we could also potentially leak
> memory by not freeing the buffer when it should be freed.  Fix this by
> storing whether it should be freed in a separate variable.
> 
> Link: https://syzkaller.appspot.com/bug?id=5c2aee8256e30b55ccf57312c16d88417adbd5e1
> Link: https://syzkaller.appspot.com/bug?id=41a6b5d4917c0412eb3b3c3c604965bed7d7420b
> Reported-by: syzbot+64b645917ce07d89bde5@syzkaller.appspotmail.com
> Reported-by: syzbot+0d042627c4f2ad332195@syzkaller.appspotmail.com
> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
> ---
>  fs/ext4/xattr.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
> index 767454d74cd6..e33a323faf3c 100644
> --- a/fs/ext4/xattr.c
> +++ b/fs/ext4/xattr.c
> @@ -2615,6 +2615,7 @@ static int ext4_xattr_move_to_block(handle_t *handle, struct inode *inode,
>  		.in_inode = !!entry->e_value_inum,
>  	};
>  	struct ext4_xattr_ibody_header *header = IHDR(inode, raw_inode);
> +	int needs_kvfree = 0;
>  	int error;
>  
>  	is = kzalloc(sizeof(struct ext4_xattr_ibody_find), GFP_NOFS);
> @@ -2637,7 +2638,7 @@ static int ext4_xattr_move_to_block(handle_t *handle, struct inode *inode,
>  			error = -ENOMEM;
>  			goto out;
>  		}
> -
> +		needs_kvfree = 1;
>  		error = ext4_xattr_inode_get(inode, entry, buffer, value_size);
>  		if (error)
>  			goto out;
> @@ -2676,7 +2677,7 @@ static int ext4_xattr_move_to_block(handle_t *handle, struct inode *inode,
>  
>  out:
>  	kfree(b_entry_name);
> -	if (entry->e_value_inum && buffer)
> +	if (needs_kvfree && buffer)

If buffer is null, no operation should be performed, so we may get rid
of the buffer check.

We should also add the Fixes tag and Cc: stable@vger.kernel.org, as the
blamed commit fixes another bug and it was backported to 4.14+, thus
this one needs to be backported as well.

Fixes: 1e9d62d25281 ("ext4: optimize ea_inode block expansion")

Anyway:
Reviewed-by: Tudor Ambarus <tudor.ambarus@linaro.org>

Cheers,
ta
>  		kvfree(buffer);
>  	if (is)
>  		brelse(is->iloc.bh);