On Thu, Jul 13, 2023 at 05:59:38PM +0000, Eric Snowberg wrote:
On Jul 12, 2023, at 12:31 PM, Mimi Zohar <zohar@xxxxxxxxxxxxx> 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.--- 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; + }The issue I see here is ret could be many things, for example it could be -EKEYREJECTED, meaning it was contained on a revocation list. With this patch the revocation could be overruled if the image was IMA signed with a different key. Do we really want to add the ability to overrule a revocation?
Thanks for raising the concern! I haven't thought about this case of the key being in a revocation list. If the IMA signature comes from a distribution, the distribution should also blocklist the IMA key when the PE-COFF signature key is added to the revocation list. If the IMA signature is from an end-user, I think the user wants to pass the verification in this case. Or how about only allowing IMA signature when EFI runtime services are disabled? Another reason I am yet to add to the commit message is a real-time kernel currently disables EFI runtime services for latency issues so it can't access the MOK keys to verify the PECOFF signature.
