[PATCH v3 3/5] efi: Apply allowlist to EFI configuration tables when running under Xen

[Demi Marie Obenour <demi@invisiblethingslab.com>]

As it turns out, Xen does not guarantee that EFI boot services data
regions in memory are preserved, which means that EFI configuration
tables pointing into such memory regions may be corrupted before the
dom0 OS has had a chance to inspect them.

This is causing problems for Qubes OS when it attempts to perform system
firmware updates, which requires that the contents of the EFI System
Resource Table are valid when the fwupd userspace program runs.

However, other configuration tables such as the memory attributes table
or the runtime properties table are equally affected, and so we need a
comprehensive workaround that works for any table type.

So when running under Xen, check the EFI memory descriptor covering the
start of the table, and disregard the table if it does not reside in
memory that is preserved by Xen.

Co-developed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
---
 drivers/firmware/efi/efi.c |  7 +++++++
 drivers/xen/efi.c          | 25 +++++++++++++++++++++++++
 include/linux/efi.h        |  2 ++
 3 files changed, 34 insertions(+)

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index bcb848e44e7b1350b10b7c0479c0b38d980fe37d..b49fcde06ca0ff5347047666f38b9309bd9cfe26 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -564,6 +564,13 @@ static __init int match_config_table(const efi_guid_t *guid,
 
 	for (i = 0; efi_guidcmp(table_types[i].guid, NULL_GUID); i++) {
 		if (!efi_guidcmp(*guid, table_types[i].guid)) {
+			if (IS_ENABLED(CONFIG_XEN_EFI) &&
+			    !xen_efi_config_table_is_usable(guid, table)) {
+				if (table_types[i].name[0])
+					pr_cont("(%s=0x%lx) may have been clobbered by Xen ",
+					        table_types[i].name, table);
+				return 1;
+			}
 			*(table_types[i].ptr) = table;
 			if (table_types[i].name[0])
 				pr_cont("%s=0x%lx ",
diff --git a/drivers/xen/efi.c b/drivers/xen/efi.c
index 3c792353b7308f9c2bf0a888eda9f827aa9177f8..fb321cd6415a40e8c4d0ad940611adcabe20ab97 100644
--- a/drivers/xen/efi.c
+++ b/drivers/xen/efi.c
@@ -328,3 +328,28 @@ int efi_mem_desc_lookup(u64 phys_addr, efi_memory_desc_t *out_md)
 
 	return 0;
 }
+
+bool __init xen_efi_config_table_is_usable(const efi_guid_t *guid,
+                                           unsigned long table)
+{
+	efi_memory_desc_t md;
+	int rc;
+
+	if (!efi_enabled(EFI_PARAVIRT))
+		return true;
+
+	rc = efi_mem_desc_lookup(table, &md);
+	if (rc)
+		return false;
+
+	switch (md.type) {
+	case EFI_RUNTIME_SERVICES_CODE:
+	case EFI_RUNTIME_SERVICES_DATA:
+	case EFI_ACPI_RECLAIM_MEMORY:
+	case EFI_ACPI_MEMORY_NVS:
+	case EFI_RESERVED_TYPE:
+		return true;
+	default:
+		return false;
+	}
+}
diff --git a/include/linux/efi.h b/include/linux/efi.h
index b407a302b730a6cc7481afa0f582360e59faf1e0..b210b50c4bdedaafcce6f63d44f57ff8329d1cfd 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -1322,4 +1322,6 @@ struct linux_efi_coco_secret_area {
 /* Header of a populated EFI secret area */
 #define EFI_SECRET_TABLE_HEADER_GUID	EFI_GUID(0x1e74f542, 0x71dd, 0x4d66,  0x96, 0x3e, 0xef, 0x42, 0x87, 0xff, 0x17, 0x3b)
 
+bool xen_efi_config_table_is_usable(const efi_guid_t *guid, unsigned long table);
+
 #endif /* _LINUX_EFI_H */
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab