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

Tudor Ambarus
2023-05-08T15:21:15Z

On 5/8/23 11:20, Tudor Ambarus wrote:
>
>
> 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.
>

I meant that if buffer is null no operation is performed for
kvfree(buffer). Sent a patch on top of yours at:

https://lore.kernel.org/linux-ext4/20230508151337.79304-1-tudor.ambarus@linaro.org/T/#u

Cheers,
ta

> 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);