Re: [PATCH] io_uring: fix provide_buffers sign extension

[Pavel Begunkov <asml.silence@gmail.com>]

On 19/03/2021 10:31, Colin Ian King wrote:
> On 19/03/2021 10:21, Pavel Begunkov wrote:
>> io_provide_buffers_prep()'s "p->len * p->nbufs" to sign extension
>> problems. Not a huge problem as it's only used for access_ok() and
>> increases the checked length, but better to keep typing right.
>>
>> Reported-by: Colin Ian King <colin.king@canonical.com>
>> Fixes: efe68c1ca8f49 ("io_uring: validate the full range of provided buffers for access")
>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>> ---
>>  fs/io_uring.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index c2489b463eb9..4f1c98502a09 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -3978,6 +3978,7 @@ static int io_remove_buffers(struct io_kiocb *req, unsigned int issue_flags)
>>  static int io_provide_buffers_prep(struct io_kiocb *req,
>>  				   const struct io_uring_sqe *sqe)
>>  {
>> +	unsigned long size;
>>  	struct io_provide_buf *p = &req->pbuf;
>>  	u64 tmp;
>>  
>> @@ -3991,7 +3992,8 @@ static int io_provide_buffers_prep(struct io_kiocb *req,
>>  	p->addr = READ_ONCE(sqe->addr);
>>  	p->len = READ_ONCE(sqe->len);
>>  
>> -	if (!access_ok(u64_to_user_ptr(p->addr), (p->len * p->nbufs)))
>> +	size = (unsigned long)p->len * p->nbufs;
>> +	if (!access_ok(u64_to_user_ptr(p->addr), size))
>>  		return -EFAULT;
>>  
>>  	p->bgid = READ_ONCE(sqe->buf_group);
>>
> 
> Does it make sense to make size a u64 and cast to a u64 rather than
> unsigned long?

static inline int __access_ok(unsigned long addr, unsigned long size)
{
	return 1;
}

Not sure. I was thinking about size_t, but ended up sticking
to access_ok types.

-- 
Pavel Begunkov