kcsan: Fix 0-sized checks

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@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index e3c7d8f34f2ffc47a7bb7f3fe985d29c7560a8c1..82c2bef827d42e87d61111d0ab892e8009914b14 100644 (file)
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -455,6 +455,13 @@ static __always_inline void check_access(const volatile void *ptr, size_t size,
        atomic_long_t *watchpoint;
        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

diff --git a/kernel/kcsan/test.c b/kernel/kcsan/test.c
index cc6000239dc0199ab2a3f1ccdd9474e7cd640066..d26a052d338383bc9e037393148897a097ec616c 100644 (file)
--- 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;
 }