On Fri, 2018-08-03 at 11:16 -0500, Seth Forshee wrote: > On Fri, Aug 03, 2018 at 10:54:59AM -0400, Mimi Zohar wrote: > > On Fri, 2018-08-03 at 08:11 -0500, Seth Forshee wrote: > > > On Wed, Jul 25, 2018 at 06:31:59PM -0500, Eric Richter wrote: > > > > IMA can verify the signature of kernel images loaded with kexec_file_load, > > > > but can not verify images loaded with the regular kexec_load syscall. > > > > Therefore, the appraisal will automatically fail during kexec_load when an > > > > appraise policy rule is set for func=KEXEC_KERNEL_CHECK. This can be used > > > > to effectively disable the kexec_load syscall, while still allowing the > > > > kexec_file_load to operate so long as the target kernel image is signed. > > > > > > > > However, this conflicts with CONFIG_KEXEC_VERIFY_SIG. If that option is > > > > enabled and there is an appraise rule set, then the target kernel would > > > > have to be verifiable by both IMA and the architecture specific kernel > > > > verification procedure. > > > > > > > > This patch adds a new func= for IMA appraisal specifically for the original > > > > kexec_load syscall. Therefore, the kexec_load syscall can be effectively > > > > disabled via IMA policy, leaving the kexec_file_load syscall able to do its > > > > own signature verification, and not require it to be signed via IMA. To > > > > retain compatibility, the existing func=KEXEC_KERNEL_CHECK flag is > > > > unchanged, and thus enables appraisal for both kexec syscalls. > > > > > > This seems like a roundabout way to disallow the kexec_load syscall. > > > Wouldn't it make more sense to simply disallow kexec_load any time > > > CONFIG_KEXEC_VERIFY_SIG is enabled, since it effectively renders that > > > option impotent? Or has that idea already been rejected? > > > > Agreed! We can modify the "case LOADING_KEXEC_IMAGE" in > > ima_load_data() to prevent the kexec_load based on > > CONFIG_KEXEC_VERIFY_SIG. > > > > The architecture specific policy would only include the IMA appraise > > rule if CONFIG_KEXEC_VERIFY_SIG was not defined. > > After looking at this some more I'm having second thoughts about my > suggestion. As a distro we produce a kernel that needs to be flexible > enough for a variety of scenarios, and if we completely close off the > ability to load an unsigned kernel for kexec that's almost certainly > going to end up breaking some use cases. > > So I think it is necessary to make this a run-time decision rather than > a compile-time decision. The patch as provided does this based on > whether or not the kernel was booted under secure boot. That might be > reasonable, though I still find this mechanism kind of awkward. Right, the above change is almost right. Instead of preventing the kexec_load syscall based on CONFIG_KEXEC_VERIFY_SIG it should be based on a runtime secure boot flag. Only if there is an arch specific secure boot function and the secure boot flag is enabled, would the kexec_load be disabled. Without an architecture specific secure boot function, the existing IMA code would fail the kexec_load syscall. > It seems > like ideally there would instead be some logic that would accept the > image if the KEXEC_VERIFY_SIG verification had passed, and otherwise > require IMA signature verification. True, but for now to coordinate between the two signature verification methods, only if CONFIG_KEXEC_VERIFY_SIG is not enabled would an IMA architecture specific rule be defined. Mimi -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
