Re: [PATCH 1/2] x86: Use CPUID 0x8000001f to confirm SME availability
From: Limonciello, Mario
Date: Fri Feb 11 2022 - 14:52:01 EST
On 2/11/2022 08:55, Tom Lendacky wrote:
On 2/10/22 23:36, Mario Limonciello wrote:
An upcoming change will disable the X86 SME feature flag when the
kernel hasn't activated SME. Avoid relying upon that when determining
whether to call `native_wbinvd` by directly calling the CPUID that
indicates it.
Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
---
arch/x86/kernel/process.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
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))
Please test this by running kexec and alternating between mem_encrypt=on
boots of the kexec kernel and mem_encrypt=off boots of the kexec kernel
to ensure there is no memory corruption in the newly booted kernel. This
testing needs to be done on hardware that doesn't have the
X86_FEATURE_SME_COHERENT feature.
Or if you post the generated instructions in this area I should be able
to determine if the change is safe.
From objdump on process.o, here is the function with this patch
applied, built using gcc 11.2.0:
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)
Thanks,
Tom
native_wbinvd();
for (;;) {
/*