[PATCH v2 3/4] x86/cpu: clear SME features when not in use
From: Mario Limonciello
Date: Fri Feb 11 2022 - 16:06:18 EST
Currently the SME CPU feature flag is reflective of whether the CPU
supports the features but not whether they have been activated by the
kernel.
Change this around to clear the features if the kernel is not using
them so userspace can determine if they are available and in use
from `/proc/cpuinfo`.
As the feature flag is now cleared on systems that SME isn't active
use CPUID 0x8000001f to confirm SME availability before calling
`native_wbinvd`.
Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
---
A concern was raised whether the assembly generated from stop_this_cpu
is still safe. Here is the assembly from compiling with this patch:
00000000000011d0 <stop_this_cpu>:
11d0: e8 00 00 00 00 call 11d5 <stop_this_cpu+0x5>
11d5: 55 push %rbp
11d6: 48 89 e5 mov %rsp,%rbp
11d9: 41 54 push %r12
11db: 53 push %rbx
11dc: 48 83 ec 18 sub $0x18,%rsp
11e0: 65 48 8b 04 25 28 00 mov %gs:0x28,%rax
11e7: 00 00
11e9: 48 89 45 e8 mov %rax,-0x18(%rbp)
11ed: 31 c0 xor %eax,%eax
11ef: ff 14 25 00 00 00 00 call *0x0
11f6: e8 00 00 00 00 call 11fb <stop_this_cpu+0x2b>
11fb: 31 f6 xor %esi,%esi
11fd: 48 c7 c3 00 00 00 00 mov $0x0,%rbx
1204: 89 c7 mov %eax,%edi
1206: e8 00 00 00 00 call 120b <stop_this_cpu+0x3b>
120b: e8 00 00 00 00 call 1210 <stop_this_cpu+0x40>
1210: e8 00 00 00 00 call 1215 <stop_this_cpu+0x45>
1215: 41 89 c4 mov %eax,%r12d
1218: 3d ff 1f 00 00 cmp $0x1fff,%eax
121d: 77 49 ja 1268 <stop_this_cpu+0x98>
121f: 4a 03 1c e5 00 00 00 add 0x0(,%r12,8),%rbx
1226: 00
1227: 48 89 df mov %rbx,%rdi
122a: e8 00 00 00 00 call 122f <stop_this_cpu+0x5f>
122f: c7 45 d8 1f 00 00 80 movl $0x8000001f,-0x28(%rbp)
1236: 48 8d 7d d8 lea -0x28(%rbp),%rdi
123a: 48 8d 75 dc lea -0x24(%rbp),%rsi
123e: c7 45 e0 00 00 00 00 movl $0x0,-0x20(%rbp)
1245: 48 8d 55 e0 lea -0x20(%rbp),%rdx
1249: 48 8d 4d e4 lea -0x1c(%rbp),%rcx
124d: ff 14 25 00 00 00 00 call *0x0
1254: f6 45 d8 01 testb $0x1,-0x28(%rbp)
1258: 74 02 je 125c <stop_this_cpu+0x8c>
125a: 0f 09 wbinvd
125c: eb 07 jmp 1265 <stop_this_cpu+0x95>
125e: 0f 00 2d 00 00 00 00 verw 0x0(%rip) # 1265 <stop_this_cpu+0x95>
1265: f4 hlt
1266: eb f4 jmp 125c <stop_this_cpu+0x8c>
1268: 4c 89 e6 mov %r12,%rsi
126b: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
1272: e8 00 00 00 00 call 1277 <stop_this_cpu+0xa7>
1277: eb a6 jmp 121f <stop_this_cpu+0x4f>
1279: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
arch/x86/kernel/cpu/amd.c | 3 +++
arch/x86/kernel/process.c | 5 ++++-
2 files changed, 7 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 4edb6f0f628c..9cea774344cb 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -607,6 +607,9 @@ static void early_detect_mem_encrypt(struct cpuinfo_x86 *c)
if (IS_ENABLED(CONFIG_X86_32))
goto clear_all;
+ if (!sme_me_mask)
+ goto clear_all;
+
rdmsrl(MSR_K7_HWCR, msr);
if (!(msr & MSR_K7_HWCR_SMMLOCK))
goto clear_sev;
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 81d8ef036637..e131d71b3cae 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -765,8 +765,11 @@ void stop_this_cpu(void *dummy)
* without the encryption bit, they don't race each other when flushed
* and potentially end up with the wrong entry being committed to
* memory.
+ *
+ * Test the CPUID bit directly because the machine might've cleared
+ * X86_FEATURE_SME due to cmdline options.
*/
- if (boot_cpu_has(X86_FEATURE_SME))
+ if (cpuid_eax(0x8000001f) & BIT(0))
native_wbinvd();
for (;;) {
/*
--
2.34.1