From ed95f95c86cd53621103d865d62b5e1f96e60edb Mon Sep 17 00:00:00 2001 From: Marco Elver Date: Wed, 5 Feb 2020 11:14:19 +0100 Subject: [PATCH] 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 Signed-off-by: Paul E. McKenney Signed-off-by: Ingo Molnar --- 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 e3c7d8f34f2ff..82c2bef827d42 100644 --- 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 cc6000239dc01..d26a052d33838 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; } -- 2.30.2