linux-kernel - Re: [PATCH] efi: READ_ONCE rng seed size before munmap

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Message-ID: <CAKv+Gu83dOKGbYU1t3_KZevB_rn-ktoropFrjASjsv3DozrV1A@mail.gmail.com>
Date:   Mon, 17 Feb 2020 16:23:03 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] efi: READ_ONCE rng seed size before munmap

On Mon, 17 Feb 2020 at 13:34, Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> This function is consistent with using size instead of seed->size
> (except for one place that this patch fixes), but it reads seed->size
> without using READ_ONCE, which means the compiler might still do
> something unwanted. So, this commit simply adds the READ_ONCE
> wrapper.
>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> Cc: Ard Biesheuvel <ardb@...nel.org>
> Cc: stable@...r.kernel.org

Thanks Jason

I've queued this in efi/urgent with a fixes: tag rather than a cc:
stable, since it only applies clean to v5.4 and later. We'll need a
backport to 4.14 and 4.19 as well, which has a trivial conflict
(s/add_bootloader_randomness/add_device_randomness/) but we'll need to
wait for this patch to hit Linus's tree first.


> ---
>  drivers/firmware/efi/efi.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> index 621220ab3d0e..21ea99f65113 100644
> --- a/drivers/firmware/efi/efi.c
> +++ b/drivers/firmware/efi/efi.c
> @@ -552,7 +552,7 @@ int __init efi_config_parse_tables(void *config_tables, int count, int sz,
>
>                 seed = early_memremap(efi.rng_seed, sizeof(*seed));
>                 if (seed != NULL) {
> -                       size = seed->size;
> +                       size = READ_ONCE(seed->size);
>                         early_memunmap(seed, sizeof(*seed));
>                 } else {
>                         pr_err("Could not map UEFI random seed!\n");
> @@ -562,7 +562,7 @@ int __init efi_config_parse_tables(void *config_tables, int count, int sz,
>                                               sizeof(*seed) + size);
>                         if (seed != NULL) {
>                                 pr_notice("seeding entropy pool\n");
> -                               add_bootloader_randomness(seed->bits, seed->size);
> +                               add_bootloader_randomness(seed->bits, size);
>                                 early_memunmap(seed, sizeof(*seed) + size);
>                         } else {
>                                 pr_err("Could not map UEFI random seed!\n");
> --
> 2.25.0
>