[PATCH] Fix off by one in tools/perf strncpy size argument

From: Dominik 'disconnect3d' Czarnota <dominik.b.czarnota@gmail.com>
To: unlisted-recipients:; (no To-header on input)
Cc: dominik.b.czarnota@gmail.com,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Song Liu <songliubraving@fb.com>,
	John Keeping <john@metanate.com>,
	Changbin Du <changbin.du@intel.com>,
	linux-kernel@vger.kernel.org
Subject: [PATCH] Fix off by one in tools/perf strncpy size argument
Date: Mon,  9 Mar 2020 11:48:53 +0100	[thread overview]
Message-ID: <20200309104855.3775-1-dominik.b.czarnota@gmail.com> (raw)

From: disconnect3d <dominik.b.czarnota@gmail.com>

This patch fixes an off-by-one error in strncpy size argument in
tools/perf/util/map.c. The issue is that in:

        strncmp(filename, "/system/lib/", 11)

the passed string literal: "/system/lib/" has 12 bytes (without the NULL
byte) and the passed size argument is 11. As a result, the logic won't
match the ending "/" byte and will pass filepaths that are stored in
other directories e.g. "/system/libmalicious/bin" or just
"/system/libmalicious".

This functionality seems to be present only on Android. I assume the
/system/ directory is only writable by the root user, so I don't
think this bug has much (or any) security impact.

Signed-off-by: disconnect3d <dominik.b.czarnota@gmail.com>
---

Notes:
    I can't test this patch, so if someone can, please, do so.
    
    The bug could also be fixed by changing the size argument to `sizeof("string literal")-1` but I am not proposing this change as that would have to be changed in other places.
    
    There are also more cases like this in kernel sources which I am going to report soon.
    
    Also please note that other path comparisons in this file lack the "/" at the end and it seems they may imply similar issue. I haven't analysed the code deeply to see if that is a real issue.
    
    This bug has been found by running a massive grep-like search using Google's BigQuery on GitHub repositories data. I am also going to work on a CodeQL/Semmle query to be able to find more sophisticated cases like this that can't be found via grepping.

 tools/perf/util/map.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/util/map.c b/tools/perf/util/map.c
index a08ca276098e..addd7edb0486 100644
--- a/tools/perf/util/map.c
+++ b/tools/perf/util/map.c
@@ -89,7 +89,7 @@ static inline bool replace_android_lib(const char *filename, char *newfilename)
 		return true;
 	}
 
-	if (!strncmp(filename, "/system/lib/", 11)) {
+	if (!strncmp(filename, "/system/lib/", 12)) {
 		char *ndk, *app;
 		const char *arch;
 		size_t ndk_length;
-- 
2.25.1

