x86/efistub: Clear BSS in EFI handover protocol entrypoint

The so-called EFI handover protocol is value-add from the distros that
permits a loader to simply copy a PE kernel image into memory and call
an alternative entrypoint that is described by an embedded boot_params
structure.

Most implementations of this protocol do not bother to check the PE
header for minimum alignment, section placement, etc, and therefore also
don't clear the image's BSS, or even allocate enough memory for it.

Allocating more memory on the fly is rather difficult, but at least
clear the BSS region explicitly when entering in this manner, so that
the EFI stub code does not get confused by global variables that were
not zero-initialized correctly.

When booting in mixed mode, this BSS clearing must occur before any
global state is created, so clear it in the 32-bit asm entry point.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230807162720.545787-7-ardb@kernel.org

diff --git a/arch/x86/boot/compressed/efi_mixed.S b/arch/x86/boot/compressed/efi_mixed.S
index 9308b595f6f0a5de2a9436ff82d9daac7a10091d..8a02a151806df14c123606a64e7dfef38fa3e445 100644 (file)
--- a/arch/x86/boot/compressed/efi_mixed.S
+++ b/arch/x86/boot/compressed/efi_mixed.S
@@ -142,6 +142,18 @@ SYM_FUNC_END(__efi64_thunk)
        .code32
 #ifdef CONFIG_EFI_HANDOVER_PROTOCOL
 SYM_FUNC_START(efi32_stub_entry)
+       call    1f
+1:     popl    %ecx
+
+       /* Clear BSS */
+       xorl    %eax, %eax
+       leal    (_bss - 1b)(%ecx), %edi
+       leal    (_ebss - 1b)(%ecx), %ecx
+       subl    %edi, %ecx
+       shrl    $2, %ecx
+       cld
+       rep     stosl
+
        add     $0x4, %esp              /* Discard return address */
        popl    %ecx
        popl    %edx
@@ -334,7 +346,7 @@ SYM_FUNC_END(efi32_pe_entry)
        .org    efi32_stub_entry + 0x200
        .code64
 SYM_FUNC_START_NOALIGN(efi64_stub_entry)
-       jmp     efi_stub_entry
+       jmp     efi_handover_entry
 SYM_FUNC_END(efi64_stub_entry)
 #endif
 

diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
index 3f3b3edf7a387d7c0c12c332b944c25c6e231ecf..9247dbc7dbbd12efb33845c0c409e89c45f21b22 100644 (file)
--- a/drivers/firmware/efi/libstub/x86-stub.c
+++ b/drivers/firmware/efi/libstub/x86-stub.c
@@ -970,12 +970,21 @@ fail:
 }
 
 #ifdef CONFIG_EFI_HANDOVER_PROTOCOL
+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);
+}
+
 #ifndef CONFIG_EFI_MIXED
-extern __alias(efi_stub_entry)
+extern __alias(efi_handover_entry)
 void efi32_stub_entry(efi_handle_t handle, efi_system_table_t *sys_table_arg,
                      struct boot_params *boot_params);
 
-extern __alias(efi_stub_entry)
+extern __alias(efi_handover_entry)
 void efi64_stub_entry(efi_handle_t handle, efi_system_table_t *sys_table_arg,
                      struct boot_params *boot_params);
 #endif