x86/sev: Evict cache lines during SNP memory validation

An SNP cache coherency vulnerability requires a cache line eviction
mitigation when validating memory after a page state change to private.
The specific mitigation is to touch the first and last byte of each 4K
page that is being validated. There is no need to perform the mitigation
when performing a page state change to shared and rescinding validation.

CPUID bit Fn8000001F_EBX[31] defines the COHERENCY_SFW_NO CPUID bit
that, when set, indicates that the software mitigation for this
vulnerability is not needed.

Implement the mitigation and invoke it when validating memory (making it
private) and the COHERENCY_SFW_NO bit is not set, indicating the SNP
guest is vulnerable.

Co-developed-by: Michael Roth <[email protected]>
Signed-off-by: Michael Roth <[email protected]>
Signed-off-by: Tom Lendacky <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Acked-by: Thomas Gleixner <[email protected]>

Author: tlendacky

arch/x86/boot/cpuflags.c

@@ -106,5 +106,18 @@ void get_cpuflags(void)
 			cpuid(0x80000001, &ignored, &ignored, &cpu.flags[6],
 			      &cpu.flags[1]);
 		}
+
+		if (max_amd_level >= 0x8000001f) {
+			u32 ebx;
+
+			/*
+			 * The X86_FEATURE_COHERENCY_SFW_NO feature bit is in
+			 * the virtualization flags entry (word 8) and set by
+			 * scattered.c, so the bit needs to be explicitly set.
+			 */
+			cpuid(0x8000001f, &ignored, &ebx, &ignored, &ignored);
+			if (ebx & BIT(31))
+				set_bit(X86_FEATURE_COHERENCY_SFW_NO, cpu.flags);
+		}
 	}
 }

arch/x86/boot/startup/sev-shared.c

@@ -810,6 +810,13 @@ static void __head pvalidate_4k_page(unsigned long vaddr, unsigned long paddr,
 		if (ret)
 			sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_PVALIDATE);
 	}
+
+	/*
+	 * If validating memory (making it private) and affected by the
+	 * cache-coherency vulnerability, perform the cache eviction mitigation.
+	 */
+	if (validate && !has_cpuflag(X86_FEATURE_COHERENCY_SFW_NO))
+		sev_evict_cache((void *)vaddr, 1);
 }
 
 /*

arch/x86/coco/sev/core.c

@@ -358,10 +358,31 @@ static void svsm_pval_pages(struct snp_psc_desc *desc)
 
 static void pvalidate_pages(struct snp_psc_desc *desc)
 {
+	struct psc_entry *e;
+	unsigned int i;
+
 	if (snp_vmpl)
 		svsm_pval_pages(desc);
 	else
 		pval_pages(desc);
+
+	/*
+	 * If not affected by the cache-coherency vulnerability there is no need
+	 * to perform the cache eviction mitigation.
+	 */
+	if (cpu_feature_enabled(X86_FEATURE_COHERENCY_SFW_NO))
+		return;
+
+	for (i = 0; i <= desc->hdr.end_entry; i++) {
+		e = &desc->entries[i];
+
+		/*
+		 * If validating memory (making it private) perform the cache
+		 * eviction mitigation.
+		 */
+		if (e->operation == SNP_PAGE_STATE_PRIVATE)
+			sev_evict_cache(pfn_to_kaddr(e->gfn), e->pagesize ? 512 : 1);
+	}
 }
 
 static int vmgexit_psc(struct ghcb *ghcb, struct snp_psc_desc *desc)

arch/x86/include/asm/cpufeatures.h

@@ -218,6 +218,7 @@
 #define X86_FEATURE_FLEXPRIORITY	( 8*32+ 1) /* "flexpriority" Intel FlexPriority */
 #define X86_FEATURE_EPT			( 8*32+ 2) /* "ept" Intel Extended Page Table */
 #define X86_FEATURE_VPID		( 8*32+ 3) /* "vpid" Intel Virtual Processor ID */
+#define X86_FEATURE_COHERENCY_SFW_NO	( 8*32+ 4) /* SNP cache coherency software work around not needed */
 
 #define X86_FEATURE_VMMCALL		( 8*32+15) /* "vmmcall" Prefer VMMCALL to VMCALL */
 #define X86_FEATURE_XENPV		( 8*32+16) /* Xen paravirtual guest */

arch/x86/include/asm/sev.h

@@ -619,6 +619,24 @@ int rmp_make_shared(u64 pfn, enum pg_level level);
 void snp_leak_pages(u64 pfn, unsigned int npages);
 void kdump_sev_callback(void);
 void snp_fixup_e820_tables(void);
+
+static inline void sev_evict_cache(void *va, int npages)
+{
+	volatile u8 val __always_unused;
+	u8 *bytes = va;
+	int page_idx;
+
+	/*
+	 * For SEV guests, a read from the first/last cache-lines of a 4K page
+	 * using the guest key is sufficient to cause a flush of all cache-lines
+	 * associated with that 4K page without incurring all the overhead of a
+	 * full CLFLUSH sequence.
+	 */
+	for (page_idx = 0; page_idx < npages; page_idx++) {
+		val = bytes[page_idx * PAGE_SIZE];
+		val = bytes[page_idx * PAGE_SIZE + PAGE_SIZE - 1];
+	}
+}
 #else
 static inline bool snp_probe_rmptable_info(void) { return false; }
 static inline int snp_rmptable_init(void) { return -ENOSYS; }
@@ -634,6 +652,7 @@ static inline int rmp_make_shared(u64 pfn, enum pg_level level) { return -ENODEV
 static inline void snp_leak_pages(u64 pfn, unsigned int npages) {}
 static inline void kdump_sev_callback(void) { }
 static inline void snp_fixup_e820_tables(void) {}
+static inline void sev_evict_cache(void *va, int npages) {}
 #endif
 
 #endif

arch/x86/kernel/cpu/scattered.c

@@ -48,6 +48,7 @@ static const struct cpuid_bit cpuid_bits[] = {
 	{ X86_FEATURE_PROC_FEEDBACK,		CPUID_EDX, 11, 0x80000007, 0 },
 	{ X86_FEATURE_AMD_FAST_CPPC,		CPUID_EDX, 15, 0x80000007, 0 },
 	{ X86_FEATURE_MBA,			CPUID_EBX,  6, 0x80000008, 0 },
+	{ X86_FEATURE_COHERENCY_SFW_NO,		CPUID_EBX, 31, 0x8000001f, 0 },
 	{ X86_FEATURE_SMBA,			CPUID_EBX,  2, 0x80000020, 0 },
 	{ X86_FEATURE_BMEC,			CPUID_EBX,  3, 0x80000020, 0 },
 	{ X86_FEATURE_TSA_SQ_NO,		CPUID_ECX,  1, 0x80000021, 0 },