[PATCH v3] audit: fix potential integer overflow in audit_log_n_hex()

[Ricardo Robaina <rrobaina@redhat.com>]

The function calculates new_len as len << 1 for hex encoding. This
has two overflow risks: the shift itself can overflow when len is
large, and the result can be truncated when assigned to new_len
(declared as int) from the size_t calculation.

Fix by using check_shl_overflow() to catch shift overflow and
changing new_len and loop counter i to size_t to prevent truncation.

Fixes: 168b7173959f ("AUDIT: Clean up logging of untrusted strings")
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Ricardo Robaina <rrobaina@redhat.com>
---
Changes in v2:
- Use check_shl_overflow() instead of manual overflow check.
Changes in v3:
- Log "?" before returning when overflow detected.

 kernel/audit.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/audit.c b/kernel/audit.c
index e1d489bc2dff..52eb3b511bad 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -62,6 +62,7 @@
 #include <net/ip.h>
 #include <net/ipv6.h>
 #include <linux/sctp.h>
+#include <linux/overflow.h>
 
 #include "audit.h"
 
@@ -2076,7 +2077,8 @@ void audit_log_format(struct audit_buffer *ab, const char *fmt, ...)
 void audit_log_n_hex(struct audit_buffer *ab, const unsigned char *buf,
 		size_t len)
 {
-	int i, avail, new_len;
+	int avail;
+	size_t i, new_len;
 	unsigned char *ptr;
 	struct sk_buff *skb;
 
@@ -2084,9 +2086,15 @@ void audit_log_n_hex(struct audit_buffer *ab, const unsigned char *buf,
 		return;
 
 	BUG_ON(!ab->skb);
+
 	skb = ab->skb;
 	avail = skb_tailroom(skb);
-	new_len = len<<1;
+
+	if (check_shl_overflow(len, 1, &new_len)) {
+		audit_log_format(ab, "?");
+		return;
+	}
+
 	if (new_len >= avail) {
 		/* Round the buffer request up to the next multiple */
 		new_len = AUDIT_BUFSIZ*(((new_len-avail)/AUDIT_BUFSIZ) + 1);
-- 
2.53.0