[PATCH v2] x86/speculation/srbds: do not try to turn mitigation off when not supported

[Ricardo Cañuelo]

When SRBDS is mitigated by TSX OFF, update_srbds_msr will still read and
write to MSR_IA32_MCU_OPT_CTRL even when that is not supported by the
microcode.

Checking for X86_FEATURE_SRBDS_CTRL as a CPU feature available makes more
sense than checking for SRBDS_MITIGATION_UCODE_NEEDED as the found
"mitigation".

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@xxxxxxxxxxxxx>
Tested-by: Ricardo Cañuelo <ricardo.canuelo@xxxxxxxxxxxxx>
---
Hi all,

This patch was originally posted here:

https://lore.kernel.org/all/20200609174313.2600320-1-cascardo@xxxxxxxxxxxxx/#t

by Boris, based on the original patch by Cascardo, I didn't make any
changes to it. I didn't see it merged or discussed further and I can
still reproduce the issue on a Samsung Galaxy Chromebook 2 (Intel
Cometlake-U). When booted without the proper CPU u-codes, TSX is
disabled (so the CPU isn't vulnerable to SRDBS) but this code still
tries to access an unavailable MSR register so I get these two warning
messages:

unchecked MSR access error: RDMSR from 0x123 at rIP: 0xffffffff8203707e (update_srbds_msr+0x2e/0xa0)
Call Trace:
 <TASK>
 check_bugs+0x994/0xa6e
 ? __get_locked_pte+0x8f/0x100
 start_kernel+0x630/0x664
 secondary_startup_64_no_verify+0xd5/0xdb
 </TASK>
unchecked MSR access error: WRMSR to 0x123 (tried to write 0x0000000000000001) at rIP: 0xffffffff820370a9 (update_srbds_msr+0x59/0xa0)
Call Trace:
 <TASK>
 check_bugs+0x994/0xa6e
 ? __get_locked_pte+0x8f/0x100
 start_kernel+0x630/0x664
 secondary_startup_64_no_verify+0xd5/0xdb
 </TASK>

This patch avoids them.

 arch/x86/kernel/cpu/bugs.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 6296e1ebed1d..9b14cb2ec693 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -443,14 +443,14 @@ void update_srbds_msr(void)
 	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
 		return;
 
-	if (srbds_mitigation == SRBDS_MITIGATION_UCODE_NEEDED)
+	if (srbds_mitigation == SRBDS_MITIGATION_UCODE_NEEDED ||
+	    srbds_mitigation == SRBDS_MITIGATION_TSX_OFF)
 		return;
 
 	rdmsrl(MSR_IA32_MCU_OPT_CTRL, mcu_ctrl);
 
 	switch (srbds_mitigation) {
 	case SRBDS_MITIGATION_OFF:
-	case SRBDS_MITIGATION_TSX_OFF:
 		mcu_ctrl |= RNGDS_MITG_DIS;
 		break;
 	case SRBDS_MITIGATION_FULL:
-- 
2.25.1