Re: [PATCH] Carry forward IMA measurement log on kexec on x86_64

Mimi Zohar
2022-04-26T13:50:57Z

On Tue, 2022-04-26 at 12:08 +0000, Jonathan McDowell wrote:
> On Mon, Apr 25, 2022 at 12:29:17PM -0400, Mimi Zohar wrote:
> > Hi Jonathan,
> >
> > On Fri, 2022-04-22 at 13:50 +0000, Jonathan McDowell wrote:
> > > On kexec file load Integrity Measurement Architecture (IMA) subsystem
> > > may verify the IMA signature of the kernel and initramfs, and measure
> > > it. The command line parameters passed to the kernel in the kexec call
> > > may also be measured by IMA. A remote attestation service can verify
> > > a TPM quote based on the TPM event log, the IMA measurement list, and
> > > the TPM PCR data. This can be achieved only if the IMA measurement log
> > > is carried over from the current kernel to the next kernel across
> > > the kexec call.
> > >
> > > powerpc and ARM64 both achieve this using device tree with a
> > > "linux,ima-kexec-buffer" node. x86 platforms generally don't make use of
> > > device tree, so the IMA infrastructure is extended to allow non device
> > > tree platforms to provide a log buffer. x86 then passes the IMA buffer
> > > to the new kernel via the setup_data mechanism.
> > >
> > > Signed-off-by: Jonathan McDowell <noodles@fb.com>
> >
> > FYI, after applying, building, and booting a kernel with this patch,
> > "kexec -s -l /boot/vmlinuz-5.18.0-rc4+ --reuse-cmdline --
> > initrd=/boot/initramfs-5.18.0-rc4+.img" properly loads the kernel, but
> > "kexec -s -e" fails to reboot, at least on a test laptop even with only
> > the "boot_aggregate" measurement record.
> >
> > Without enabling CONFIG_IMA_KEXEC, kexec boots properly.
>
> Thanks for giving it a try. At a guess your laptop is booting with
> EFI, whereas for my testing I was using qemu with legacy BIOS. I've
> managed to reproduce the issue with qemu+OVMF and isolated the mistake
> in the setup data calculation I made when EFI is involved. If you have
> time can you try with the below on top of the original patch?

Thank you! With the change, as expected there are two "boot_aggregate"
records in the measurement list. With a custom policy, the measurement
list verifies.

# grep boot_aggregate
/sys/kernel/security/ima/ascii_runtime_measurements
10 fe0b821290b1bd229e0d34c5571f48eeff403119 ima-sig
sha1:a87d47e560d148cd1f4c8da677a84ddbe27f12f8 boot_aggregate
10 fe0b821290b1bd229e0d34c5571f48eeff403119 ima-sig
sha1:a87d47e560d148cd1f4c8da677a84ddbe27f12f8 boot_aggregate

# cat /sys/kernel/security/ima/runtime_measurements_count
5597

# evmctl ima_measurement
/sys/kernel/security/ima/binary_runtime_measurements
Matched per TPM
bank calculated digest(s).

FYI, the builtin "ima_policy=tcb" results in measurement violations.
Normally, the measurement list can still be verified using the evmctl
"--ignore-violations" option. For some reason with the "tcb" policy,
the measurement list doesn't verify even with the "--ignore-violations"
option after kexec. I assume this is a result of additional
measurements being added after the kexec load, which aren't being
carried across kexec.

thanks,

Mimi

>
> diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
> index 07625da33075..cdc73e081585 100644
> --- a/arch/x86/kernel/kexec-bzimage64.c
> +++ b/arch/x86/kernel/kexec-bzimage64.c
> @@ -277,7 +277,9 @@ setup_boot_parameters(struct kimage *image, struct boot_params *params,
> #ifdef CONFIG_IMA_KEXEC
> /* Setup IMA log buffer state */
> setup_ima_state(image, params, params_load_addr,
> - efi_setup_data_offset + ALIGN(efi_map_sz, 16) + sizeof(struct setup_data));
> + efi_setup_data_offset +
> + sizeof(struct setup_data) +
> + sizeof(struct efi_setup_data));
> #endif
>
> /* Setup EDD info */