Re: [PATCH v4 18/45] instrumented.h: add KMSAN support

Marco Elver
2022-07-12T13:52:10Z

On Fri, 1 Jul 2022 at 16:24, Alexander Potapenko <glider@google.com> wrote:
>
> To avoid false positives, KMSAN needs to unpoison the data copied from
> the userspace. To detect infoleaks - check the memory buffer passed to
> copy_to_user().
>
> Signed-off-by: Alexander Potapenko <glider@google.com>

Reviewed-by: Marco Elver <elver@google.com>

With the code simplification below.

[...]
> --- a/mm/kmsan/hooks.c
> +++ b/mm/kmsan/hooks.c
> @@ -212,6 +212,44 @@ void kmsan_iounmap_page_range(unsigned long start, unsigned long end)
> }
> EXPORT_SYMBOL(kmsan_iounmap_page_range);
>
> +void kmsan_copy_to_user(void __user *to, const void *from, size_t to_copy,
> + size_t left)
> +{
> + unsigned long ua_flags;
> +
> + if (!kmsan_enabled || kmsan_in_runtime())
> + return;
> + /*
> + * At this point we've copied the memory already. It's hard to check it
> + * before copying, as the size of actually copied buffer is unknown.
> + */
> +
> + /* copy_to_user() may copy zero bytes. No need to check. */
> + if (!to_copy)
> + return;
> + /* Or maybe copy_to_user() failed to copy anything. */
> + if (to_copy <= left)
> + return;
> +
> + ua_flags = user_access_save();
> + if ((u64)to < TASK_SIZE) {
> + /* This is a user memory access, check it. */
> + kmsan_internal_check_memory((void *)from, to_copy - left, to,
> + REASON_COPY_TO_USER);

This could just do "} else {" and the stuff below, and would result in
simpler code with no explicit "return" and no duplicated
user_access_restore().

> + user_access_restore(ua_flags);
> + return;
> + }
> + /* Otherwise this is a kernel memory access. This happens when a compat
> + * syscall passes an argument allocated on the kernel stack to a real
> + * syscall.
> + * Don't check anything, just copy the shadow of the copied bytes.
> + */
> + kmsan_internal_memmove_metadata((void *)to, (void *)from,
> + to_copy - left);
> + user_access_restore(ua_flags);
> +}
> +EXPORT_SYMBOL(kmsan_copy_to_user);