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

[Al Viro <viro@ZenIV.linux.org.uk>]

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?

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,