linux-kernel - [tip:perf/urgent] perf/core: Fix Undefined behaviour in rb

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Date:	Mon, 21 Mar 2016 02:51:03 -0700
From:	tip-bot for Peter Zijlstra <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	vincent.weaver@...ne.edu, sasha.levin@...cle.com, acme@...hat.com,
	bp@...en8.de, acme@...nel.org, luto@...capital.net,
	namhyung@...nel.org, eranian@...gle.com, peterz@...radead.org,
	tglx@...utronix.de, jolsa@...hat.com, brgerst@...il.com,
	hpa@...or.com, alexander.shishkin@...ux.intel.com,
	dsahern@...il.com, linux-kernel@...r.kernel.org, mingo@...nel.org,
	dvlasenk@...hat.com, ryabinin.a.a@...il.com,
	torvalds@...ux-foundation.org
Subject: [tip:perf/urgent] perf/core: Fix Undefined behaviour in rb_alloc()

Commit-ID:  8184059e93c200757f5c0805dae0f14e880eab5d
Gitweb:     http://git.kernel.org/tip/8184059e93c200757f5c0805dae0f14e880eab5d
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Fri, 29 Jan 2016 15:17:51 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 21 Mar 2016 09:08:18 +0100

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 <sasha.levin@...cle.com>
Reported-by: Andrey Ryabinin <ryabinin.a.a@...il.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: David Ahern <dsahern@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Jiri Olsa <jolsa@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Stephane Eranian <eranian@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vince Weaver <vincent.weaver@...ne.edu>
Link: http://lkml.kernel.org/r/20160129141751.GA407@worktop
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/events/ring_buffer.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 1faad2c..c61f0cb 100644
--- 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);