[PATCH RFC v8 15/56] x86/sev: Invalidate pages from the direct map when adding them to the RMP table

[Michael Roth <michael.roth@amd.com>]

From: Brijesh Singh <brijesh.singh@amd.com>

The integrity guarantee of SEV-SNP is enforced through the RMP table.
The RMP is used with standard x86 and IOMMU page tables to enforce
memory restrictions and page access rights. The RMP check is enforced as
soon as SEV-SNP is enabled globally in the system. When hardware
encounters an RMP-check failure, it raises a page-fault exception.

The rmp_make_private() and rmp_make_shared() helpers are used to add
or remove the pages from the RMP table. Improve the rmp_make_private()
to invalidate state so that pages cannot be used in the direct-map after
they are added the RMP table, and restored to their default valid
permission after the pages are removed from the RMP table.

Co-developed-by: Ashish Kalra <ashish.kalra@amd.com>
Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
---
 arch/x86/kernel/sev.c | 57 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index a49f30c10dc1..3e5ff5934e83 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -2595,6 +2595,37 @@ int psmash(u64 pfn)
 }
 EXPORT_SYMBOL_GPL(psmash);
 
+static int restore_direct_map(u64 pfn, int npages)
+{
+	int i, ret = 0;
+
+	for (i = 0; i < npages; i++) {
+		ret = set_direct_map_default_noflush(pfn_to_page(pfn + i));
+		if (ret)
+			goto cleanup;
+	}
+
+cleanup:
+	WARN(ret > 0, "Failed to restore direct map for pfn 0x%llx\n", pfn + i);
+	return ret;
+}
+
+static int invalidate_direct_map(u64 pfn, int npages)
+{
+	int i, ret = 0;
+
+	for (i = 0; i < npages; i++) {
+		ret = set_direct_map_invalid_noflush(pfn_to_page(pfn + i));
+		if (ret)
+			goto cleanup;
+	}
+
+cleanup:
+	WARN(ret > 0, "Failed to invalidate direct map for pfn 0x%llx\n", pfn + i);
+	restore_direct_map(pfn, i);
+	return ret;
+}
+
 static int rmpupdate(u64 pfn, struct rmp_state *val)
 {
 	int max_attempts = 4 * num_present_cpus();
@@ -2605,6 +2636,21 @@ static int rmpupdate(u64 pfn, struct rmp_state *val)
 	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
 		return -ENXIO;
 
+	level = RMP_TO_X86_PG_LEVEL(val->pagesize);
+	npages = page_level_size(level) / PAGE_SIZE;
+
+	/*
+	 * If page is getting assigned in the RMP table then unmap it from the
+	 * direct map.
+	 */
+	if (val->assigned) {
+		if (invalidate_direct_map(pfn, npages)) {
+			pr_err("Failed to unmap %d pages at pfn 0x%llx from the direct_map\n",
+			       npages, pfn);
+			return -EFAULT;
+		}
+	}
+
 	do {
 		/* Binutils version 2.36 supports the RMPUPDATE mnemonic. */
 		asm volatile(".byte 0xF2, 0x0F, 0x01, 0xFE"
@@ -2630,6 +2676,17 @@ static int rmpupdate(u64 pfn, struct rmp_state *val)
 			 attempts, val->asid, ret, pfn, npages);
 	}
 
+	/*
+	 * Restore the direct map after the page is removed from the RMP table.
+	 */
+	if (!val->assigned) {
+		if (restore_direct_map(pfn, npages)) {
+			pr_err("Failed to map %d pages at pfn 0x%llx into the direct_map\n",
+			       npages, pfn);
+			return -EFAULT;
+		}
+	}
+
 	return 0;
 }
 
-- 
2.25.1