linux-kernel - [PATCH kcsan 15/32] kcsan: Fix 0-sized checks

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Date:   Mon,  9 Mar 2020 12:04:03 -0700
From:   paulmck@...nel.org
To:     linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        kernel-team@...com, mingo@...nel.org
Cc:     elver@...gle.com, andreyknvl@...gle.com, glider@...gle.com,
        dvyukov@...gle.com, cai@....pw, boqun.feng@...il.com,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH kcsan 15/32] kcsan: Fix 0-sized checks

From: Marco Elver <elver@...gle.com>

Instrumentation of arbitrary memory-copy functions, such as user-copies,
may be called with size of 0, which could lead to false positives.

To avoid this, add a comparison in check_access() for size==0, which
will be optimized out for constant sized instrumentation
(__tsan_{read,write}N), and therefore not affect the common-case
fast-path.

Signed-off-by: Marco Elver <elver@...gle.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/kcsan/core.c |  7 +++++++
 kernel/kcsan/test.c | 10 ++++++++++
 2 files changed, 17 insertions(+)

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index e3c7d8f..82c2bef 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -456,6 +456,13 @@ static __always_inline void check_access(const volatile void *ptr, size_t size,
 	long encoded_watchpoint;
 
 	/*
+	 * Do nothing for 0 sized check; this comparison will be optimized out
+	 * for constant sized instrumentation (__tsan_{read,write}N).
+	 */
+	if (unlikely(size == 0))
+		return;
+
+	/*
 	 * Avoid user_access_save in fast-path: find_watchpoint is safe without
 	 * user_access_save, as the address that ptr points to is only used to
 	 * check if a watchpoint exists; ptr is never dereferenced.
diff --git a/kernel/kcsan/test.c b/kernel/kcsan/test.c
index cc60002..d26a052 100644
--- a/kernel/kcsan/test.c
+++ b/kernel/kcsan/test.c
@@ -92,6 +92,16 @@ static bool test_matching_access(void)
 		return false;
 	if (WARN_ON(matching_access(9, 1, 10, 1)))
 		return false;
+
+	/*
+	 * An access of size 0 could match another access, as demonstrated here.
+	 * Rather than add more comparisons to 'matching_access()', which would
+	 * end up in the fast-path for *all* checks, check_access() simply
+	 * returns for all accesses of size 0.
+	 */
+	if (WARN_ON(!matching_access(8, 8, 12, 0)))
+		return false;
+
 	return true;
 }
 
-- 
2.9.5