[PATCH v3 1/2] x86/kexec: Add EFI config table identity mapping for kexec kernel

[Steve Wahl]

From: Tao Liu <ltao at redhat.com>

A kexec kernel boot failure is sometimes observed on AMD CPUs due to
an unmapped EFI config table array.  This can be seen when "nogbpages"
is on the kernel command line, and has been observed as a full BIOS
reboot rather than a successful kexec.

This was also the cause of reported regressions attributed to Commit
7143c5f4cf20 ("x86/mm/ident_map: Use gbpages only where full GB page
should be mapped.") which was subsequently reverted.

To avoid this page fault, explicitly include the EFI config table
array in the kexec identity map.

Further explanation:

The following 2 commits caused the EFI config table array to be
accessed when enabling sev at kernel startup.

    commit ec1c66af3a30 ("x86/compressed/64: Detect/setup SEV/SME features
                          earlier during boot")
    commit c01fce9cef84 ("x86/compressed: Add SEV-SNP feature
                          detection/setup")

This is in the code that examines whether SEV should be enabled or
not, so it can even affect systems that are not SEV capable.

This may result in a page fault if the EFI config table array's
address is unmapped. Since the page fault occurs before the new kernel
establishes its own identity map and page fault routines, it is
unrecoverable and kexec fails.

Most often, this problem is not seen because the EFI config table
array gets included in the map by the luck of being placed at a memory
address close enough to other memory areas that *are* included in the
map created by kexec.

Both the "nogbpages" command line option and the "use gpbages only
where full GB page should be mapped" patch greatly reduce the chance
of being included in the map by luck, which is why the problem
appears.

Signed-off-by: Tao Liu <ltao at redhat.com>
Signed-off-by: Steve Wahl <steve.wahl at hpe.com>
Tested-by: Pavin Joseph <me at pavinjoseph.com>
Tested-by: Sarah Brofeldt <srhb at dbc.dk>
Tested-by: Eric Hagberg <ehagberg at gmail.com>
---

Version 3: Do not rename map_efi_systab to map_efi_tables, and don't add
'config table' to the comments, per Ard Biesheuvel request.

 arch/x86/kernel/machine_kexec_64.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
index cc0f7f70b17b..9c9ac606893e 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -28,6 +28,7 @@
 #include <asm/setup.h>
 #include <asm/set_memory.h>
 #include <asm/cpu.h>
+#include <asm/efi.h>
 
 #ifdef CONFIG_ACPI
 /*
@@ -87,6 +88,8 @@ map_efi_systab(struct x86_mapping_info *info, pgd_t *level4p)
 {
 #ifdef CONFIG_EFI
 	unsigned long mstart, mend;
+	void *kaddr;
+	int ret;
 
 	if (!efi_enabled(EFI_BOOT))
 		return 0;
@@ -102,6 +105,30 @@ map_efi_systab(struct x86_mapping_info *info, pgd_t *level4p)
 	if (!mstart)
 		return 0;
 
+	ret = kernel_ident_mapping_init(info, level4p, mstart, mend);
+	if (ret)
+		return ret;
+
+	kaddr = memremap(mstart, mend - mstart, MEMREMAP_WB);
+	if (!kaddr) {
+		pr_err("Could not map UEFI system table\n");
+		return -ENOMEM;
+	}
+
+	mstart = efi_config_table;
+
+	if (efi_enabled(EFI_64BIT)) {
+		efi_system_table_64_t *stbl = (efi_system_table_64_t *)kaddr;
+
+		mend = mstart + sizeof(efi_config_table_64_t) * stbl->nr_tables;
+	} else {
+		efi_system_table_32_t *stbl = (efi_system_table_32_t *)kaddr;
+
+		mend = mstart + sizeof(efi_config_table_32_t) * stbl->nr_tables;
+	}
+
+	memunmap(kaddr);
+
 	return kernel_ident_mapping_init(info, level4p, mstart, mend);
 #endif
 	return 0;
-- 
2.26.2