perf/core: Fix Undefined behaviour in rb_alloc()

Sasha reported:

 [ 3494.030114] UBSAN: Undefined behaviour in kernel/events/ring_buffer.c:685:22
 [ 3494.030647] shift exponent -1 is negative

Andrey spotted that this is because:

  It happens if nr_pages = 0:
     rb->page_order = ilog2(nr_pages);

Fix it by making both assignments conditional on nr_pages; since
otherwise they should both be 0 anyway, and will be because of the
kzalloc() used to allocate the structure.

Reported-by: Sasha Levin <[email protected]>
Reported-by: Andrey Ryabinin <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vince Weaver <[email protected]>
Link: http://lkml.kernel.org/r/20160129141751.GA407@worktop
Signed-off-by: Ingo Molnar <[email protected]>

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 1faad2cfdb9e4572c91431e4b11941266e23a231..c61f0cbd308b5b4456e69c1539f9b334a34c0632 100644 (file)
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -746,8 +746,10 @@ struct ring_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
 
        rb->user_page = all_buf;
        rb->data_pages[0] = all_buf + PAGE_SIZE;
-       rb->page_order = ilog2(nr_pages);
-       rb->nr_pages = !!nr_pages;
+       if (nr_pages) {
+               rb->nr_pages = 1;
+               rb->page_order = ilog2(nr_pages);
+       }
 
        ring_buffer_init(rb, watermark, flags);