From 95af254e116802490481c6293ae752f5b61e561b Mon Sep 17 00:00:00 2001 From: Arthur Heymans Date: Wed, 27 Nov 2019 17:34:37 +0100 Subject: [PATCH] cpu/x86/smm: Add sinkhole mitigation to relocatable smmstub The sinkhole exploit exists in placing the lapic base such that it messes with GDT. This can be mitigated by checking the lapic MSR against the current program counter. Change-Id: I49927c4f4218552b732bac8aae551d845ad7f079 Signed-off-by: Arthur Heymans Reviewed-on: https://review.coreboot.org/c/coreboot/+/37289 Tested-by: build bot (Jenkins) Reviewed-by: Angel Pons --- src/cpu/x86/smm/smm_stub.S | 39 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 39 insertions(+) diff --git a/src/cpu/x86/smm/smm_stub.S b/src/cpu/x86/smm/smm_stub.S index c83839cc01..02532a4d9f 100644 --- a/src/cpu/x86/smm/smm_stub.S +++ b/src/cpu/x86/smm/smm_stub.S @@ -45,10 +45,49 @@ fallback_stack_top: (CR0_CD | CR0_NW | CR0_PG | CR0_AM | CR0_WP | \ CR0_NE | CR0_TS | CR0_EM | CR0_MP) +#define SMM_DEFAULT_SIZE 0x10000 + .text .code16 .global _start _start: +smm_handler_start: +#if CONFIG(SMM_LAPIC_REMAP_MITIGATION) + /* Check if the LAPIC register block overlaps with the stub. + * This block needs to work without data accesses because they + * may be routed into the LAPIC register block. + * Code accesses, on the other hand, are never routed to LAPIC, + * which is what makes this work in the first place. + */ + mov $LAPIC_BASE_MSR, %ecx + rdmsr + and $(~0xfff), %eax + call 1f + /* Get the current program counter */ +1: + pop %ebx + sub %ebx, %eax + cmp $(SMM_DEFAULT_SIZE), %eax + ja untampered_lapic +1: +#if CONFIG(CONSOLE_SERIAL) + /* emit "Crash" on serial */ + mov $(CONFIG_TTYS0_BASE), %dx + mov $'C', %al + out %al, (%dx) + mov $'r', %al + out %al, (%dx) + mov $'a', %al + out %al, (%dx) + mov $'s', %al + out %al, (%dx) + mov $'h', %al + out %al, (%dx) +#endif /* CONFIG_CONSOLE_SERIAL */ + /* now crash for real */ + ud2 +untampered_lapic: +#endif movl $(smm_relocate_gdt), %ebx lgdtl (%ebx)