x86/kexec: add a sanity check on previous kernel's ima kexec buffer

In the Linux kernel, the following vulnerability has been resolved:

x86/kexec: add a sanity check on previous kernel's ima kexec buffer

When the second-stage kernel is booted via kexec with a limiting command

line such as "mem=<size>", the physical range that contains the carried

over IMA measurement list may fall outside the truncated RAM leading to a

kernel panic.

BUG: unable to handle page fault for address: ffff97793ff47000

RIP: ima_restore_measurement_list+0xdc/0x45a

#PF: error_code(0x0000) – not-present page

Other architectures already validate the range with page_is_ram(), as done

in commit cbf9c4b9617b ("of: check previous kernel's ima-kexec-buffer

against memory bounds") do a similar check on x86.

Without carrying the measurement list across kexec, the attestation

would fail.