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

[Marco Elver]

On Fri, 1 Jul 2022 at 16:24, Alexander Potapenko <glider@xxxxxxxxxx> 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@xxxxxxxxxx>

Reviewed-by: Marco Elver <elver@xxxxxxxxxx>

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);