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Groups > linux.kernel > #1487565

[PATCH] fix fault_in_multipages_...() on architectures with no-op access_ok()

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Switching iov_iter fault-in to multipages variants has exposed an old
bug in underlying fault_in_multipages_...(); they break if the range
passed to them wraps around.  Normally access_ok() done by callers
will prevent such (and it's a guaranteed EFAULT - ERR_PTR() values
fall into such a range and they should not point to any valid objects).
However, on architectures where userland and kernel live in different
MMU contexts (e.g. s390) access_ok() is a no-op and on those a range
with a wraparound can reach fault_in_multipages_...().

Since any wraparound means EFAULT there, the fix is trivial - turn
those
    while (uaddr <= end)
	    ...
into
    if (unlikely(uaddr > end))
	    return -EFAULT;
    do
	    ...
    while (uaddr <= end);

Reported-by: Jan Stancek <jstancek@redhat.com>
Tested-by: Jan Stancek <jstancek@redhat.com>
Cc: stable@vger.kernel.org # v3.5+
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

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,

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Thread

[bug] pwritev02 hang on s390x with 4.8.0-rc7 Jan Stancek <jstancek@redhat.com> - 2016-09-20 15:00 +0200
  Re: [bug] pwritev02 hang on s390x with 4.8.0-rc7 Al Viro <viro@ZenIV.linux.org.uk> - 2016-09-20 17:10 +0200
    Re: [bug] pwritev02 hang on s390x with 4.8.0-rc7 Jan Stancek <jstancek@redhat.com> - 2016-09-20 19:20 +0200
      Re: [bug] pwritev02 hang on s390x with 4.8.0-rc7 Al Viro <viro@ZenIV.linux.org.uk> - 2016-09-20 19:40 +0200
      [PATCH] fix fault_in_multipages_...() on architectures with no-op  access_ok() Al Viro <viro@ZenIV.linux.org.uk> - 2016-09-20 21:10 +0200
        Re: [PATCH] fix fault_in_multipages_...() on architectures with no-op access_ok() Linus Torvalds <torvalds@linux-foundation.org> - 2016-09-20 22:30 +0200
          Re: [PATCH] fix fault_in_multipages_...() on architectures with  no-op access_ok() Al Viro <viro@ZenIV.linux.org.uk> - 2016-09-20 22:40 +0200
            Re: [PATCH] fix fault_in_multipages_...() on architectures with no-op access_ok() Linus Torvalds <torvalds@linux-foundation.org> - 2016-09-20 22:50 +0200
              Re: [PATCH] fix fault_in_multipages_...() on architectures with  no-op access_ok() Al Viro <viro@ZenIV.linux.org.uk> - 2016-09-20 23:10 +0200
                Re: [PATCH] fix fault_in_multipages_...() on architectures with  no-op access_ok() Al Viro <viro@ZenIV.linux.org.uk> - 2016-09-20 23:40 +0200
                Re: [PATCH] fix fault_in_multipages_...() on architectures with no-op access_ok() Linus Torvalds <torvalds@linux-foundation.org> - 2016-09-21 01:50 +0200
                Re: [PATCH] fix fault_in_multipages_...() on architectures with  no-op access_ok() Al Viro <viro@ZenIV.linux.org.uk> - 2016-09-21 02:40 +0200

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