Felix Held has submitted this change. ( https://review.coreboot.org/c/coreboot/+/37289 )
Change subject: cpu/x86/smm: Add sinkhole mitigation to relocatable smmstub ......................................................................
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 arthur@aheymans.xyz Reviewed-on: https://review.coreboot.org/c/coreboot/+/37289 Tested-by: build bot (Jenkins) no-reply@coreboot.org Reviewed-by: Angel Pons th3fanbus@gmail.com --- M src/cpu/x86/smm/smm_stub.S 1 file changed, 39 insertions(+), 0 deletions(-)
Approvals: build bot (Jenkins): Verified Angel Pons: Looks good to me, approved
diff --git a/src/cpu/x86/smm/smm_stub.S b/src/cpu/x86/smm/smm_stub.S index c83839c..02532a4 100644 --- a/src/cpu/x86/smm/smm_stub.S +++ b/src/cpu/x86/smm/smm_stub.S @@ -45,10 +45,49 @@ (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)