s390/uaccess: fix strncpy_from_user string length check

The "standard" and page table walk variants of strncpy_from_user() first
check the length of the to be copied string in userspace.
The string is then copied to kernel space and the length returned to the
caller.
However userspace can modify the string at any time while the kernel
checks for the length of the string or copies the string. In result the
returned length of the string is not necessarily correct.
Fix this by copying in a loop which mimics the mvcos variant of
strncpy_from_user(), which handles this correctly.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 18 insertions, 16 deletions

diff --git a/arch/s390/lib/uaccess_pt.c b/arch/s390/lib/uaccess_pt.c
index c1aaf22c326..304e07086ab 100644
--- a/arch/s390/lib/uaccess_pt.c
+++ b/arch/s390/lib/uaccess_pt.c
@@ -202,27 +202,29 @@ fault:
 static size_t strncpy_from_user_pt(size_t count, const char __user *src,
 				   char *dst)
 {
-	size_t n = strnlen_user_pt(count, src);
+	size_t done, len, offset, len_str;
 
 	if (unlikely(!count))
 		return 0;
-	if (!n)
-		return -EFAULT;
-	if (n > count)
-		n = count;
 	if (segment_eq(get_fs(), KERNEL_DS)) {
-		memcpy(dst, (const char __kernel __force *) src, n);
-		if (dst[n-1] == '\0')
-			return n-1;
-		else
-			return n;
+		len = strnlen((const char __kernel __force *) src, count) + 1;
+		if (len > count)
+			len = count;
+		memcpy(dst, (const char __kernel __force *) src, len);
+		return (dst[len - 1] == '\0') ? len - 1 : len;
 	}
-	if (__user_copy_pt((unsigned long) src, dst, n, 0))
-		return -EFAULT;
-	if (dst[n-1] == '\0')
-		return n-1;
-	else
-		return n;
+	done = 0;
+	do {
+		offset = (size_t)src & ~PAGE_MASK;
+		len = min(count - done, PAGE_SIZE - offset);
+		if (__user_copy_pt((unsigned long) src, dst, len, 0))
+			return -EFAULT;
+		len_str = strnlen(dst, len);
+		done += len_str;
+		src += len_str;
+		dst += len_str;
+	} while ((len_str == len) && (done < count));
+	return done;
 }
 
 static size_t copy_in_user_pt(size_t n, void __user *to,