x86/efistub: Clear decompressor BSS in native EFI entrypoint

The EFI stub on x86 no longer invokes the decompressor as a subsequent
boot stage, but calls into the decompression code directly while running
in the context of the EFI boot services.

This means that when using the native EFI entrypoint (as opposed to the
EFI handover protocol, which clears BSS explicitly), the firmware PE
image loader is being relied upon to ensure that BSS is zeroed before
the EFI stub is entered from the firmware.

As Radek's report proves, this is a bad idea. Not all loaders do this
correctly, which means some global variables that should be statically
initialized to 0x0 may have junk in them.

So clear BSS explicitly when entering via efi_pe_entry(). Note that
zeroing BSS from C code is not generally safe, but in this case, the
following assignment and dereference of a global pointer variable
ensures that the memset() cannot be deferred or reordered.

Cc: <stable@kernel.org> # v6.1+
Reported-by: Radek Podgorny <radek@podgorny.cz>
Closes: https://lore.kernel.org/all/a99a831a-8ad5-4cb0-bff9-be637311f771@podgorny.cz
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/libstub/x86-stub.c

@@ -21,6 +21,8 @@
 #include "efistub.h"
 #include "x86-stub.h"
 
+extern char _bss[], _ebss[];
+
 const efi_system_table_t *efi_system_table;
 const efi_dxe_services_table_t *efi_dxe_table;
 static efi_loaded_image_t *image = NULL;
@@ -474,6 +476,8 @@ efi_status_t __efiapi efi_pe_entry(efi_handle_t handle,
 	efi_status_t status;
 	char *cmdline_ptr;
 
+	memset(_bss, 0, _ebss - _bss);
+
 	efi_system_table = sys_table_arg;
 
 	/* Check if we were booted by the EFI firmware */
@@ -967,8 +971,6 @@ void __noreturn efi_stub_entry(efi_handle_t handle,
 void efi_handover_entry(efi_handle_t handle, efi_system_table_t *sys_table_arg,
 			struct boot_params *boot_params)
 {
-	extern char _bss[], _ebss[];
-
 	memset(_bss, 0, _ebss - _bss);
 	efi_stub_entry(handle, sys_table_arg, boot_params);
 }