diff mbox series

[RFC,v3,7/8] x86/sev-es: Include XSS value in GHCB CPUID request

Message ID 20230817181820.15315-8-john.allen@amd.com (mailing list archive)
State New, archived
Headers show
Series SVM guest shadow stack support | expand

Commit Message

John Allen Aug. 17, 2023, 6:18 p.m. UTC 
When a guest issues a cpuid instruction for Fn0000000D_x0B (CetUserOffset), the
hypervisor may intercept and access the guest XSS value. For SEV-ES, this is
encrypted and needs to be included in the GHCB to be visible to the hypervisor.
The rdmsr instruction needs to be called directly as the code may be used in
early boot in which case the rdmsr wrappers should be avoided as they are
incompatible with the decompression boot phase.

Signed-off-by: John Allen <john.allen@amd.com>
---
v3:
  - New in v3. Merged KVM support series and this single patch for guest
    kernel support.
---
 arch/x86/kernel/sev-shared.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
diff mbox series

Patch

diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index 2eabccde94fb..e38a1d049bc1 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -890,6 +890,21 @@  static enum es_result vc_handle_cpuid(struct ghcb *ghcb,
 		/* xgetbv will cause #GP - use reset value for xcr0 */
 		ghcb_set_xcr0(ghcb, 1);
 
+	if (has_cpuflag(X86_FEATURE_SHSTK) && regs->ax == 0xd && regs->cx <= 1) {
+		unsigned long lo, hi;
+		u64 xss;
+
+		/*
+		 * Since vc_handle_cpuid may be used during early boot, the
+		 * rdmsr wrappers are incompatible and should not be used.
+		 * Invoke the instruction directly.
+		 */
+		asm volatile("rdmsr" : "=a" (lo), "=d" (hi)
+			     : "c" (MSR_IA32_XSS));
+		xss = (hi << 32) | lo;
+		ghcb_set_xss(ghcb, xss);
+	}
+
 	ret = sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_CPUID, 0, 0);
 	if (ret != ES_OK)
 		return ret;