Re: [PATCH] kcsan: Fix 0-sized checks

From: Paul E. McKenney
Date: Wed Feb 05 2020 - 10:37:38 EST


On Wed, Feb 05, 2020 at 11:14:19AM +0100, Marco Elver wrote:
> 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@xxxxxxxxxx>

Queued, thank you!

Thanx, Paul

> ---
> 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;
> }
>
> --
> 2.25.0.341.g760bfbb309-goog
>