lib: Reduce user_access_begin() boundaries in strncpy_from_user() and strnlen_user()

The range passed to user_access_begin() by strncpy_from_user() and
strnlen_user() starts at 'src' and goes up to the limit of userspace
although reads will be limited by the 'count' param.

On 32 bits powerpc (book3s/32) access has to be granted for each
256Mbytes segment and the cost increases with the number of segments to
unlock.

Limit the range with 'count' param.

Fixes: 594cc251 ("make 'user_access_begin()' do 'access_ok()'")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/strncpy_from_user.c

@@ -30,13 +30,6 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src,
 	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
 	unsigned long res = 0;
 
-	/*
-	 * Truncate 'max' to the user-specified limit, so that
-	 * we only have one limit we need to check in the loop
-	 */
-	if (max > count)
-		max = count;
-
 	if (IS_UNALIGNED(src, dst))
 		goto byte_at_a_time;
 
@@ -114,6 +107,13 @@ long strncpy_from_user(char *dst, const char __user *src, long count)
 		unsigned long max = max_addr - src_addr;
 		long retval;
 
+		/*
+		 * Truncate 'max' to the user-specified limit, so that
+		 * we only have one limit we need to check in the loop
+		 */
+		if (max > count)
+			max = count;
+
 		kasan_check_write(dst, count);
 		check_object_size(dst, count, false);
 		if (user_access_begin(src, max)) {

lib/strnlen_user.c

@@ -26,13 +26,6 @@ static inline long do_strnlen_user(const char __user *src, unsigned long count,
 	unsigned long align, res = 0;
 	unsigned long c;
 
-	/*
-	 * Truncate 'max' to the user-specified limit, so that
-	 * we only have one limit we need to check in the loop
-	 */
-	if (max > count)
-		max = count;
-
 	/*
 	 * Do everything aligned. But that means that we
 	 * need to also expand the maximum..
@@ -109,6 +102,13 @@ long strnlen_user(const char __user *str, long count)
 		unsigned long max = max_addr - src_addr;
 		long retval;
 
+		/*
+		 * Truncate 'max' to the user-specified limit, so that
+		 * we only have one limit we need to check in the loop
+		 */
+		if (max > count)
+			max = count;
+
 		if (user_access_begin(str, max)) {
 			retval = do_strnlen_user(str, count, max);
 			user_access_end();