[PATCH] lib/string: avoid reading beyond src buffer in strscpy

[Eryu Guan <eguan@redhat.com>]

strscpy() tries to copy sizeof(unsigned long) bytes a time from src
to dest when possible, and stops the loop when 'max' is less than
sizeof(unsigned long). But it doesn't check if (src+res) goes beyond
src buffer and does out-of-bound access to the underlying memory.

KASAN reported global-out-of-bound bug when reading seccomp
actions_logged file in procfs:

  cat /proc/sys/kernel/seccomp/actions_logged

Because seccomp_names_from_actions_logged() is copying short strings
(less than sizeof(unsigned long)) to buffer 'names'. e.g.

  ret = strscpy(names, " ", size);

Fixed by capping the 'max' value according to the src buffer size,
to make sure we won't go beyond src buffer.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Eryu Guan <eguan@redhat.com>
---
 lib/string.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/lib/string.c b/lib/string.c
index 64a9e33f1daa..13a0147eea00 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -179,6 +179,7 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
 {
 	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
 	size_t max = count;
+	size_t src_sz = strlen(src) + 1;
 	long res = 0;
 
 	if (count == 0)
@@ -200,6 +201,10 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
 		max = 0;
 #endif
 
+	/* avoid reading beyond src buffer */
+	if (max > src_sz)
+		max = src_sz;
+
 	while (max >= sizeof(unsigned long)) {
 		unsigned long c, data;
 
-- 
2.14.3