Re: [bug] pwritev02 hang on s390x with 4.8.0-rc7

[Jan Stancek]

----- Original Message -----
> From: "Al Viro" <viro@xxxxxxxxxxxxxxxxxx>
> To: "Jan Stancek" <jstancek@xxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Sent: Tuesday, 20 September, 2016 5:06:57 PM
> Subject: Re: [bug] pwritev02 hang on s390x with 4.8.0-rc7
> 
> On Tue, Sep 20, 2016 at 02:56:06PM +0200, Jan Stancek wrote:
> > Hi,
> > 
> > I'm hitting a regression with LTP's pwritev02 [1] on s390x with 4.8.0-rc7.
> > Specifically the EFAULT case, which is passing an iovec with invalid base
> > address:
> >   #define CHUNK 64
> >   static struct iovec wr_iovec3[] = {
> >   	{(char *)-1, CHUNK},
> >   };
> > hangs with 100% cpu usage and not very helpful stack trace:
> >   # cat /proc/28544/stack
> >   [<0000000000001000>] 0x1000
> >   [<ffffffffffffffff>] 0xffffffffffffffff
> > 
> > The problem starts with d4690f1e1cda "fix iov_iter_fault_in_readable()".
> > 
> > Before this commit fault_in_pages_readable() called __get_user() on start
> > address which presumably failed with -EFAULT immediately.
> > 
> > With this commit applied fault_in_multipages_readable() appears to return 0
> > for the case when "start" is invalid but "end" overflows. Then according to
> > my traces we keep spinning inside while loop in iomap_write_actor().
> 
> Cute.  Let me see if I understand what's going on there: we have a wraparound
> that would've been caught by most of access_ok(), but not on an architectures
> where access_ok() is a no-op; in that case the loop is skipped and we
> just check the last address, which passes and we get a false positive.
> Bug is real and it's definitely -stable fodder.
> 
> I'm not sure that the fix you propose is right, though.  Note that ERR_PTR()
> is not a valid address on any architecture, so any wraparound automatically
> means -EFAULT and we can simply check unlikely(uaddr > end) and bugger off
> if it holds.  while() turns into do-while(), of course, and the same is
> needed for the read side.
> 
> Could you check if the following works for you?

This fixes pwritev02 hang.

I ran all syscalls tests from LTP, and I see a change in behaviour
of couple other tests (writev01, writev03 and writev04 [1]) in 4.8.0-rc7.

These call writev() with partially invalid iovecs, and now fail with
EFAULT, while with previous -rc6 kernel they returned number of bytes
written before they encountered invalid iovec record.
This should be reproducible also on x86.

Regards,
Jan

[1] https://github.com/linux-test-project/ltp/tree/master/testcases/kernel/syscalls/writev

> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 66a1260..7e3d537 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -571,56 +571,56 @@ static inline int fault_in_pages_readable(const char
> __user *uaddr, int size)
>   */
>  static inline int fault_in_multipages_writeable(char __user *uaddr, int
>  size)
>  {
> -	int ret = 0;
>  	char __user *end = uaddr + size - 1;
>  
>  	if (unlikely(size == 0))
> -		return ret;
> +		return 0;
>  
> +	if (unlikely(uaddr > end))
> +		return -EFAULT;
>  	/*
>  	 * Writing zeroes into userspace here is OK, because we know that if
>  	 * the zero gets there, we'll be overwriting it.
>  	 */
> -	while (uaddr <= end) {
> -		ret = __put_user(0, uaddr);
> -		if (ret != 0)
> -			return ret;
> +	do {
> +		if (unlikely(__put_user(0, uaddr) != 0))
> +			return -EFAULT;
>  		uaddr += PAGE_SIZE;
> -	}
> +	} while (uaddr <= end);
>  
>  	/* Check whether the range spilled into the next page. */
>  	if (((unsigned long)uaddr & PAGE_MASK) ==
>  			((unsigned long)end & PAGE_MASK))
> -		ret = __put_user(0, end);
> +		return __put_user(0, end);
>  
> -	return ret;
> +	return 0;
>  }
>  
>  static inline int fault_in_multipages_readable(const char __user *uaddr,
>  					       int size)
>  {
>  	volatile char c;
> -	int ret = 0;
>  	const char __user *end = uaddr + size - 1;
>  
>  	if (unlikely(size == 0))
> -		return ret;
> +		return 0;
>  
> -	while (uaddr <= end) {
> -		ret = __get_user(c, uaddr);
> -		if (ret != 0)
> -			return ret;
> +	if (unlikely(uaddr > end))
> +		return -EFAULT;
> +
> +	do {
> +		if (unlikely(__get_user(c, uaddr) != 0))
> +			return -EFAULT;
>  		uaddr += PAGE_SIZE;
> -	}
> +	} while (uaddr <= end);
>  
>  	/* Check whether the range spilled into the next page. */
>  	if (((unsigned long)uaddr & PAGE_MASK) ==
>  			((unsigned long)end & PAGE_MASK)) {
> -		ret = __get_user(c, end);
> -		(void)c;
> +		return __get_user(c, end);
>  	}
>  
> -	return ret;
> +	return 0;
>  }
>  
>  int add_to_page_cache_locked(struct page *page, struct address_space
>  *mapping,
>