[CC'ing Paul Moore]
On Fri, 2023-07-14 at 09:46 +0800, Coiby Xu wrote:
> On Wed, Jul 12, 2023 at 02:31:43PM -0400, Mimi Zohar wrote:
> >[Cc'ing the LSM mailing list.]
> >
> >On Tue, 2023-07-11 at 11:16 +0800, Coiby Xu wrote:
> >> When IMA has verified the signature of the kernel image, kexec'ing this
> >> kernel should be allowed.
> >>
> >> Fixes: af16df54b89d ("ima: force signature verification when CONFIG_KEXEC_SIG is configured")
> >> Signed-off-by: Coiby Xu <coxu@xxxxxxxxxx>
> >
> >The original commit 29d3c1c8dfe7 ("kexec: Allow kexec_file() with
> >appropriate IMA policy when locked down") was not in lieu of the PE-
> >COFF signature, but allowed using the IMA signature on other
> >architectures.
> >
> >Currently on systems with both PE-COFF and IMA signatures, both
> >signatures are verified, assuming the file is in the IMA policy. If
> >either signature verification fails, the kexec fails.
> >
> >With this patch, only the IMA signature would be verified.
>
> Thanks for correcting me! I thought it's already a consensus that we could use
> either signature to verify a kernel image because that's what the code of
> commit 29d3c1c8dfe7 has done and the code comment seems to confirm it. But if
> we just read the commit message, it indeed didn't give an answer on whether x86
> and ARM are only allowed to use PE-COFF signature.
I'm not aware of any consensus one way or the other. Commit
29d3c1c8dfe7 continued to fail the kexec on failure, when
CONFIG_KEXEC_SIG_FORCE was enabled.
As there isn't a lockdown maintainer, Paul are you ok with this change?
>
> >
> >> ---
> >> kernel/kexec_file.c | 14 +++++++++-----
> >> 1 file changed, 9 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> >> index 881ba0d1714c..96fce001fbc0 100644
> >> --- a/kernel/kexec_file.c
> >> +++ b/kernel/kexec_file.c
> >> @@ -162,6 +162,13 @@ kimage_validate_signature(struct kimage *image)
> >> ret = kexec_image_verify_sig(image, image->kernel_buf,
> >> image->kernel_buf_len);
> >> if (ret) {
> >> + /*
> >> + * If the kernel image already has its IMA signature verified, permit it.
> >> + */
> >> + if (ima_appraise_signature(READING_KEXEC_IMAGE)) {
> >> + pr_notice("The kernel image already has its IMA signature verified.\n");
> >> + return 0;
> >> + }
> >>
> >> if (sig_enforce) {
> >> pr_notice("Enforced kernel signature verification failed (%d).\n", ret);
> >> @@ -169,12 +176,9 @@ kimage_validate_signature(struct kimage *image)
> >> }
> >>
> >> /*
> >> - * If IMA is guaranteed to appraise a signature on the kexec
> >> - * image, permit it even if the kernel is otherwise locked
> >> - * down.
> >> + * When both IMA and KEXEC_SIG fail in lockdown mode, reject it.
> >> */
> >> - if (!ima_appraise_signature(READING_KEXEC_IMAGE) &&
> >> - security_locked_down(LOCKDOWN_KEXEC))
> >> + if (security_locked_down(LOCKDOWN_KEXEC))
> >> return -EPERM;
> >>
> >> pr_debug("kernel signature verification failed (%d).\n", ret);
> >
> >
>
