perf/core: Fix possible Spectre-v1 indexing for ->aux_pages[]

> kernel/events/ring_buffer.c:871 perf_mmap_to_page() warn: potential spectre issue 'rb->aux_pages'

Userspace controls @pgoff through the fault address. Sanitize the
array index before doing the array dereference.

Reported-by: Dan Carpenter <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vince Weaver <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>

Author: Peter Zijlstra

kernel/events/ring_buffer.c

@@ -14,6 +14,7 @@
 #include <linux/slab.h>
 #include <linux/circ_buf.h>
 #include <linux/poll.h>
+#include <linux/nospec.h>
 
 #include "internal.h"
 
@@ -867,8 +868,10 @@ perf_mmap_to_page(struct ring_buffer *rb, unsigned long pgoff)
 			return NULL;
 
 		/* AUX space */
-		if (pgoff >= rb->aux_pgoff)
-			return virt_to_page(rb->aux_pages[pgoff - rb->aux_pgoff]);
+		if (pgoff >= rb->aux_pgoff) {
+			int aux_pgoff = array_index_nospec(pgoff - rb->aux_pgoff, rb->aux_nr_pages);
+			return virt_to_page(rb->aux_pages[aux_pgoff]);
+		}
 	}
 
 	return __perf_mmap_to_page(rb, pgoff);