Re: [PATCH 1/2] kcov: use write memory barrier after memcpy() in kcov_move_area()

[Marco Elver <elver@google.com>]

On Mon, 28 Jul 2025 at 20:43, Soham Bagchi <soham.bagchi@utah.edu> wrote:
>
> KCOV Remote uses two separate memory buffers, one private to the kernel
> space (kcov_remote_areas) and the second one shared between user and
> kernel space (kcov->area). After every pair of kcov_remote_start() and
> kcov_remote_stop(), the coverage data collected in the
> kcov_remote_areas is copied to kcov->area so the user can read the
> collected coverage data. This memcpy() is located in kcov_move_area().
>
> The load/store pattern on the kernel-side [1] is:
>
> ```
> /* dst_area === kcov->area, dst_area[0] is where the count is stored */
> dst_len = READ_ONCE(*(unsigned long *)dst_area);
> ...
> memcpy(dst_entries, src_entries, ...);
> ...
> WRITE_ONCE(*(unsigned long *)dst_area, dst_len + entries_moved);
> ```
>
> And for the user [2]:
>
> ```
> /* cover is equivalent to kcov->area */
> n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
> ```
>
> Without a write-memory barrier, the atomic load for the user can
> potentially read fresh values of the count stored at cover[0],
> but continue to read stale coverage data from the buffer itself.
> Hence, we recommend adding a write-memory barrier between the
> memcpy() and the WRITE_ONCE() in kcov_move_area().
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/kernel/kcov.c?h=master#n978
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/Documentation/dev-tools/kcov.rst#n364
>
> Signed-off-by: Soham Bagchi <soham.bagchi@utah.edu>

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

> ---
>  kernel/kcov.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 187ba1b80bd..f6ee6d7dc2c 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -978,6 +978,15 @@ static void kcov_move_area(enum kcov_mode mode, void *dst_area,
>         memcpy(dst_entries, src_entries, bytes_to_move);
>         entries_moved = bytes_to_move >> entry_size_log;
>
> +       /*
> +        * A write memory barrier is required here, to ensure
> +        * that the writes from the memcpy() are visible before
> +        * the count is updated. Without this, it is possible for
> +        * a user to observe a new count value but stale
> +        * coverage data.
> +        */
> +       smp_wmb();
> +
>         switch (mode) {
>         case KCOV_MODE_TRACE_PC:
>                 WRITE_ONCE(*(unsigned long *)dst_area, dst_len + entries_moved);
> --
> 2.34.1
>
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