[RFC 2/2] efi/memattr: add efi_mem_attr_table as a reserved region in 820_table_firmware

[Usama Arif <usamaarif642@gmail.com>]

When this area is not reserved, it comes up as usable in
/sys/firmware/memmap. This means that kexec, which uses that memmap
to find usable memory regions, can select the region where
efi_mem_attr_table is and overwrite it and relocate_kernel.

Since the patch in [1] was merged, all boots after kexec
are producing the warning that it introduced.

Having a fix in firmware can be difficult to get through.
The next ideal place would be in libstub. However, it looks like
InstallMemoryAttributesTable [2] is not available as a boot service
call option [3], [4], and install_configuration_table does not
seem to work as a valid substitute.

As a last option for a fix, this patch marks that region as reserved in
e820_table_firmware if it is currently E820_TYPE_RAM so that kexec doesn't
use it for kernel segments.

[1] https://lore.kernel.org/all/20241031175822.2952471-2-ardb+git@google.com/
[2] https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/Dxe/Misc/MemoryAttributesTable.c#L100
[3] https://github.com/tianocore/edk2/blob/42a141800c0c26a09d2344e84a89ce4097a263ae/MdeModulePkg/Core/Dxe/DxeMain/DxeMain.c#L41
[4] https://elixir.bootlin.com/linux/v6.12.6/source/drivers/firmware/efi/libstub/efistub.h#L327

Reported-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Usama Arif <usamaarif642@gmail.com>
---
 arch/x86/include/asm/e820/api.h | 2 ++
 arch/x86/kernel/e820.c          | 6 ++++++
 arch/x86/platform/efi/efi.c     | 9 +++++++++
 drivers/firmware/efi/memattr.c  | 1 +
 include/linux/efi.h             | 7 +++++++
 5 files changed, 25 insertions(+)

diff --git a/arch/x86/include/asm/e820/api.h b/arch/x86/include/asm/e820/api.h
index 2e74a7f0e935..4e9aa24f03bd 100644
--- a/arch/x86/include/asm/e820/api.h
+++ b/arch/x86/include/asm/e820/api.h
@@ -16,6 +16,8 @@ extern bool e820__mapped_all(u64 start, u64 end, enum e820_type type);
 
 extern void e820__range_add   (u64 start, u64 size, enum e820_type type);
 extern u64  e820__range_update(u64 start, u64 size, enum e820_type old_type, enum e820_type new_type);
+extern u64  e820__range_update_firmware(u64 start, u64 size, enum e820_type old_type,
+					enum e820_type new_type);
 extern u64  e820__range_remove(u64 start, u64 size, enum e820_type old_type, bool check_type);
 extern u64  e820__range_update_table(struct e820_table *t, u64 start, u64 size, enum e820_type old_type, enum e820_type new_type);
 
diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index 82b96ed9890a..01d7d3c0d299 100644
--- a/arch/x86/kernel/e820.c
+++ b/arch/x86/kernel/e820.c
@@ -538,6 +538,12 @@ u64 __init e820__range_update_table(struct e820_table *t, u64 start, u64 size,
 	return __e820__range_update(t, start, size, old_type, new_type);
 }
 
+u64 __init e820__range_update_firmware(u64 start, u64 size, enum e820_type old_type,
+				       enum e820_type new_type)
+{
+	return __e820__range_update(e820_table_firmware, start, size, old_type, new_type);
+}
+
 /* Remove a range of memory from the E820 table: */
 u64 __init e820__range_remove(u64 start, u64 size, enum e820_type old_type, bool check_type)
 {
diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c
index a7ff189421c3..13684c5d7c05 100644
--- a/arch/x86/platform/efi/efi.c
+++ b/arch/x86/platform/efi/efi.c
@@ -168,6 +168,15 @@ static void __init do_add_efi_memmap(void)
 	e820__update_table(e820_table);
 }
 
+/* Reserve firmware area if it was marked as RAM */
+void arch_update_firmware_area(u64 addr, u64 size)
+{
+	if (e820__get_entry_type(addr, addr + size) == E820_TYPE_RAM) {
+		e820__range_update_firmware(addr, size, E820_TYPE_RAM, E820_TYPE_RESERVED);
+		e820__update_table(e820_table_firmware);
+	}
+}
+
 /*
  * Given add_efi_memmap defaults to 0 and there is no alternative
  * e820 mechanism for soft-reserved memory, import the full EFI memory
diff --git a/drivers/firmware/efi/memattr.c b/drivers/firmware/efi/memattr.c
index d3bc161361fb..d131781e2d7b 100644
--- a/drivers/firmware/efi/memattr.c
+++ b/drivers/firmware/efi/memattr.c
@@ -53,6 +53,7 @@ int __init efi_memattr_init(void)
 	size = tbl->num_entries * tbl->desc_size;
 	tbl_size = sizeof(*tbl) + size;
 	memblock_reserve(efi_mem_attr_table, tbl_size);
+	arch_update_firmware_area(efi_mem_attr_table, tbl_size);
 	set_bit(EFI_MEM_ATTR, &efi.flags);
 
 unmap:
diff --git a/include/linux/efi.h b/include/linux/efi.h
index e5815867aba9..8eb9698bd6a4 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -1358,4 +1358,11 @@ extern struct blocking_notifier_head efivar_ops_nh;
 void efivars_generic_ops_register(void);
 void efivars_generic_ops_unregister(void);
 
+#ifdef CONFIG_X86_64
+void __init arch_update_firmware_area(u64 addr, u64 size);
+#else
+static inline void __init arch_update_firmware_area(u64 addr, u64 size)
+{
+}
+#endif
 #endif /* _LINUX_EFI_H */
-- 
2.43.5