Re: [PATCH][v10] PM / hibernate: Verify the consistent of e820 memory map by md5 digest

[Denys Vlasenko]

On Fri, Sep 9, 2016 at 2:21 PM, Chen Yu <yu.c.chen@xxxxxxxxx> wrote:
> On some platforms, there is occasional panic triggered when trying to
> resume from hibernation, a typical panic looks like:
>
> "BUG: unable to handle kernel paging request at ffff880085894000
> IP: [<ffffffff810c5dc2>] load_image_lzo+0x8c2/0xe70"
>
> Investigation carried out by Lee Chun-Yi show that this is because
> e820 map has been changed by BIOS across hibernation, and one
> of the page frames from suspend kernel is right located in restore
> kernel's unmapped region, so panic comes out when accessing unmapped
> kernel address.
>
> In order to expose this issue earlier, the md5 hash of e820 map
> is passed from suspend kernel to restore kernel, and the restore
> kernel will terminate the resume process once it finds the md5
> hash are not the same.
>
> As the format of image header has been modified, the magic number
> should also be adjusted as kernels with the same RESTORE_MAGIC have
> to use the same header format and interpret all of the fields in
> it in the same way.
>
> If the suspend kernel is built without md5 support, and the restore
> kernel has md5 support, then the latter will bypass the check process.
> Vice versa the restore kernel will bypass the check if it does not
> support md5 operation.
>
> Note:
> 1. Without this patch applied, it is possible that BIOS has
>    provided an inconsistent memory map, but the resume kernel is still
>    able to restore the image anyway(e.g, E820_RAM region is the superset
>    of the previous one), although the system might be unstable. So this
>    patch tries to treat any inconsistent e820 as illegal.
>
> 2. Another case is, this patch replies on comparing the e820_saved, but
>    currently the e820_save might not be strictly the same across
>    hibernation, even if BIOS has provided consistent e820 map - In
>    theory mptable might modify the BIOS-provided e820_saved dynamically
>    in early_reserve_e820_mpc_new, which would allocate a buffer from
>    E820_RAM, and marks it from E820_RAM to E820_RESERVED).
>    This is a potential and rare case we need to deal with in OS in
>    the future.
>
> Suggested-by: Pavel Machek <pavel@xxxxxx>
> Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> Cc: Pavel Machek <pavel@xxxxxx>
> Cc: Lee Chun-Yi <jlee@xxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Acked-by: Pavel Machek <pavel@xxxxxx>
> Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> ---

> +static int get_e820_md5(struct e820map *map, void *buf)
> +{
> +       struct scatterlist sg;
> +       struct crypto_ahash *tfm;
> +       struct ahash_request *req;
> +       int ret = 0;
> +
> +       tfm = crypto_alloc_ahash("md5", 0, CRYPTO_ALG_ASYNC);
> +       if (IS_ERR(tfm))
> +               return -ENOMEM;
> +
> +       req = ahash_request_alloc(tfm, GFP_KERNEL);
> +       if (!req) {
> +               ret = -ENOMEM;
> +               goto free_ahash;
> +       }

I looked elsewhere in kernel, and there is this idiom for placing
struct ahash_request on stack. Instead of the ahash_request_alloc()
and never-actually-tirggering-error handling, you can do:

     {
          AHASH_REQUEST_ON_STACK(req, tfm);

> +
> +       sg_init_one(&sg, (u8 *)map, sizeof(struct e820map));
> +       ahash_request_set_callback(req, 0, NULL, NULL);
> +       ahash_request_set_crypt(req, &sg, buf, sizeof(struct e820map));
> +
> +       if (crypto_ahash_digest(req))
> +               ret = -EINVAL;
> +
> +       ahash_request_free(req);
> + free_ahash:

and, naturally, the free() and the label would not be needed too,
just close the one extra brace:

> +       crypto_free_ahash(tfm);
> +
> +       return ret;

      }

> +}