linux-kernel - [PATCH 3.2 051/110] tracing/filter: Do not allow infix to exceed end of string

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Message-ID: <lsq.1439201551.243684745@decadent.org.uk>
Date:	Mon, 10 Aug 2015 12:12:31 +0200
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
Subject: [PATCH 3.2 051/110] tracing/filter: Do not allow infix to exceed
 end of string

3.2.71-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>

commit 6b88f44e161b9ee2a803e5b2b1fbcf4e20e8b980 upstream.

While debugging a WARN_ON() for filtering, I found that it is possible
for the filter string to be referenced after its end. With the filter:

 # echo '>' > /sys/kernel/debug/events/ext4/ext4_truncate_exit/filter

The filter_parse() function can call infix_get_op() which calls
infix_advance() that updates the infix filter pointers for the cnt
and tail without checking if the filter is already at the end, which
will put the cnt to zero and the tail beyond the end. The loop then calls
infix_next() that has

	ps->infix.cnt--;
	return ps->infix.string[ps->infix.tail++];

The cnt will now be below zero, and the tail that is returned is
already passed the end of the filter string. So far the allocation
of the filter string usually has some buffer that is zeroed out, but
if the filter string is of the exact size of the allocated buffer
there's no guarantee that the charater after the nul terminating
character will be zero.

Luckily, only root can write to the filter.

Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 kernel/trace/trace_events_filter.c | 6 ++++++
 1 file changed, 6 insertions(+)

--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -1027,6 +1027,9 @@ static void parse_init(struct filter_par

 static char infix_next(struct filter_parse_state *ps)
 {
+	if (!ps->infix.cnt)
+		return 0;
+
 	ps->infix.cnt--;

 	return ps->infix.string[ps->infix.tail++];
@@ -1042,6 +1045,9 @@ static char infix_peek(struct filter_par

 static void infix_advance(struct filter_parse_state *ps)
 {
+	if (!ps->infix.cnt)
+		return;
+
 	ps->infix.cnt--;
 	ps->infix.tail++;
 }

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