RAMOOPS: Don't overflow over non-allocated regions

The current code mis-calculates the ramoops header size, leading to an
overflow over the next record at best, or over a non-allocated region at
worst.  Fix that calculation.
Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>
Acked-by: Marco Stornelli <marco.stornelli@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/ramoops.c

@@ -29,7 +29,6 @@
 #include <linux/ramoops.h>
 
 #define RAMOOPS_KERNMSG_HDR "===="
-#define RAMOOPS_HEADER_SIZE   (5 + sizeof(struct timeval))
 
 #define RECORD_SIZE 4096
 
@@ -65,8 +64,8 @@ static void ramoops_do_dump(struct kmsg_dumper *dumper,
 			struct ramoops_context, dump);
 	unsigned long s1_start, s2_start;
 	unsigned long l1_cpy, l2_cpy;
-	int res;
-	char *buf;
+	int res, hdr_size;
+	char *buf, *buf_orig;
 	struct timeval timestamp;
 
 	/* Only dump oopses if dump_oops is set */
@@ -74,6 +73,8 @@ static void ramoops_do_dump(struct kmsg_dumper *dumper,
 		return;
 
 	buf = (char *)(cxt->virt_addr + (cxt->count * RECORD_SIZE));
+	buf_orig = buf;
+
 	memset(buf, '\0', RECORD_SIZE);
 	res = sprintf(buf, "%s", RAMOOPS_KERNMSG_HDR);
 	buf += res;
@@ -81,8 +82,9 @@ static void ramoops_do_dump(struct kmsg_dumper *dumper,
 	res = sprintf(buf, "%lu.%lu\n", (long)timestamp.tv_sec, (long)timestamp.tv_usec);
 	buf += res;
 
-	l2_cpy = min(l2, (unsigned long)(RECORD_SIZE - RAMOOPS_HEADER_SIZE));
-	l1_cpy = min(l1, (unsigned long)(RECORD_SIZE - RAMOOPS_HEADER_SIZE) - l2_cpy);
+	hdr_size = buf - buf_orig;
+	l2_cpy = min(l2, (unsigned long)(RECORD_SIZE - hdr_size));
+	l1_cpy = min(l1, (unsigned long)(RECORD_SIZE - hdr_size) - l2_cpy);
 
 	s2_start = l2 - l2_cpy;
 	s1_start = l1 - l1_cpy;