Re: [PATCH] 9P FS: Fix wild-memory-access write in v9fs_get_acl

[Ivan Orlov <ivan.orlov0322@gmail.com>]

On 3/11/23 09:16, Siddh Raman Pant wrote:
> On Sat, 11 Mar 2023 01:56:19 +0530, Ivan Orlov wrote:
>> KASAN reported the following issue:
>> [...]
>>
>> Calling '__v9fs_get_acl' method in 'v9fs_get_acl' creates the
>> following chain of function calls:
>>
>> __v9fs_get_acl
>>        v9fs_fid_get_acl
>>                v9fs_fid_xattr_get
>>                        p9_client_xattrwalk
>>
>> Function p9_client_xattrwalk accepts a pointer to u64-typed
>> variable attr_size and puts some u64 value into it. However,
>> after the executing the p9_client_xattrwalk, in some circumstances
>> we assign the value of u64-typed variable 'attr_size' to the
>> variable 'retval', which we will return. However, the type of
>> 'retval' is ssize_t, and if the value of attr_size is larger
>> than SSIZE_MAX, we will face the signed type overflow. If the
>> overflow occurs, the result of v9fs_fid_xattr_get may be
>> negative, but not classified as an error. When we try to allocate
>> an acl with 'broken' size we receive an error, but don't process
>> it. When we try to free this acl, we face the 'wild-memory-access'
>> error (because it wasn't allocated).
>>
>> This patch will modify the condition in the 'v9fs_fid_xattr_get'
>> function, so it will return an error if the 'attr_size' is larger
>> than SSIZE_MAX.
>>
>> Reported-by: syzbot+cb1d16facb3cc90de5fb@syzkaller.appspotmail.com
>> Link: https://syzkaller.appspot.com/bug?id=fbbef66d9e4d096242f3617de5d14d12705b4659
>> Signed-off-by: Ivan Orlov ivan.orlov0322@gmail.com>
> 
> You should also test with Syzkaller if it gave a reproducer.
> Check the following docs to know about it:
> https://github.com/google/syzkaller/blob/master/docs/syzbot.md#testing-patches
> 
>> ---
>>   fs/9p/xattr.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/9p/xattr.c b/fs/9p/xattr.c
>> index 50f7f3f6b55e..d6f7450107a8 100644
>> --- a/fs/9p/xattr.c
>> +++ b/fs/9p/xattr.c
>> @@ -35,7 +35,7 @@ ssize_t v9fs_fid_xattr_get(struct p9_fid *fid, const char *name,
>>   		return retval;
>>   	}
>>   	if (attr_size > buffer_size) {
>> -		if (!buffer_size) /* request to get the attr_size */
>> +		if (!buffer_size && attr_size <= (u64) SSIZE_MAX) /* request to get the attr_size */
>>   			retval = attr_size;
>>   		else
>>   			retval = -ERANGE;
> 
> You should use EOVERFLOW for overflow error. Make a new conditional
> instead of using AND. Also, the explicit u64 cast for SSIZE_MAX can
> be dropped for better readability.
> 
> Thanks,
> Siddh
> 
>> -- 
>> 2.34.1
>>
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Thank you for the review! I will modify the patch according to your 
comments, resend it as v2 version and test it via syzkaller.