Re: [PATCH v2] kexec: Do not verify the signature without the lockdown or mandatory signature

Baoquan He
2020-06-08T07:41:02Z

On 06/02/20 at 12:59pm, Lianbo Jiang wrote:
> Signature verification is an important security feature, to protect
> system from being attacked with a kernel of unknown origin. Kexec
> rebooting is a way to replace the running kernel, hence need be
> secured carefully.
>
> In the current code of handling signature verification of kexec kernel,
> the logic is very twisted. It mixes signature verification, IMA signature
> appraising and kexec lockdown.
>
> If there is no KEXEC_SIG_FORCE, kexec kernel image doesn't have one of
> signature, the supported crypto, and key, we don't think this is wrong,
> Unless kexec lockdown is executed. IMA is considered as another kind of
> signature appraising method.
>
> If kexec kernel image has signature/crypto/key, it has to go through the
> signature verification and pass. Otherwise it's seen as verification
> failure, and won't be loaded.
>
> Seems kexec kernel image with an unqualified signature is even worse than
> those w/o signature at all, this sounds very unreasonable. E.g. If people
> get a unsigned kernel to load, or a kernel signed with expired key, which
> one is more dangerous?
>
> So, here, let's simplify the logic to improve code readability. If the
> KEXEC_SIG_FORCE enabled or kexec lockdown enabled, signature verification
> is mandated. Otherwise, we lift the bar for any kernel image.
>
> Signed-off-by: Lianbo Jiang <lijiang@redhat.com>

Looks good to me.

Acked-by: Baoquan He <bhe@redhat.com>

> ---
> Changes since v1:
> [1] Modify the log level(suggested by Jiri Bohac)
>
> kernel/kexec_file.c | 34 ++++++----------------------------
> 1 file changed, 6 insertions(+), 28 deletions(-)
>
> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> index faa74d5f6941..fae496958a68 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -181,34 +181,19 @@ void kimage_file_post_load_cleanup(struct kimage *image)
> static int
> kimage_validate_signature(struct kimage *image)
> {
> - const char *reason;
> int ret;
>
> ret = arch_kexec_kernel_verify_sig(image, image->kernel_buf,
> image->kernel_buf_len);
> - switch (ret) {
> - case 0:
> - break;
> + if (ret) {
>
> - /* Certain verification errors are non-fatal if we're not
> - * checking errors, provided we aren't mandating that there
> - * must be a valid signature.
> - */
> - case -ENODATA:
> - reason = "kexec of unsigned image";
> - goto decide;
> - case -ENOPKG:
> - reason = "kexec of image with unsupported crypto";
> - goto decide;
> - case -ENOKEY:
> - reason = "kexec of image with unavailable key";
> - decide:
> if (IS_ENABLED(CONFIG_KEXEC_SIG_FORCE)) {
> - pr_notice("%s rejected\n", reason);
> + pr_notice("Enforced kernel signature verification failed (%d).\n", ret);
> return ret;
> }
>
> - /* If IMA is guaranteed to appraise a signature on the kexec
> + /*
> + * If IMA is guaranteed to appraise a signature on the kexec
> * image, permit it even if the kernel is otherwise locked
> * down.
> */
> @@ -216,17 +201,10 @@ kimage_validate_signature(struct kimage *image)
> security_locked_down(LOCKDOWN_KEXEC))
> return -EPERM;
>
> - return 0;
> -
> - /* All other errors are fatal, including nomem, unparseable
> - * signatures and signature check failures - even if signatures
> - * aren't required.
> - */
> - default:
> - pr_notice("kernel signature verification failed (%d).\n", ret);
> + pr_debug("kernel signature verification failed (%d).\n", ret);
> }
>
> - return ret;
> + return 0;
> }
> #endif
>
> --
> 2.17.1
>