linux-ext4 - Re: [PATCH] ext4: fix invalid free tracking in ext4_xattr_move_to

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Date:   Mon, 8 May 2023 16:21:09 +0100
From:   Tudor Ambarus <tudor.ambarus@...aro.org>
To:     Theodore Ts'o <tytso@....edu>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>
Cc:     syzbot+64b645917ce07d89bde5@...kaller.appspotmail.com,
        syzbot+0d042627c4f2ad332195@...kaller.appspotmail.com,
        Lee Jones <joneslee@...gle.com>
Subject: Re: [PATCH] ext4: fix invalid free tracking in
 ext4_xattr_move_to_block()



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@...kaller.appspotmail.com
>> Reported-by: syzbot+0d042627c4f2ad332195@...kaller.appspotmail.com
>> Signed-off-by: Theodore Ts'o <tytso@....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@...r.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@...aro.org>
> 
> Cheers,
> ta
>>  		kvfree(buffer);
>>  	if (is)
>>  		brelse(is->iloc.bh);