x86/fpu: Make the EFI FPU calling convention explicit

EFI uses kernel_fpu_begin() to conform to the UEFI calling convention.
This specifically requires initializing FCW (FPU Control Word), whereas
no sane 64-bit kernel code should use legacy 387 operations that
reference FCW.

This should allow to safely change the default semantics of
kernel_fpu_begin() to stop initializing FCW on 64-bit kernels.

 [ bp: Massage commit message a little. ]
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/25d392fff64680e0f4bb8cf0b1003314dc29eafe.1611205691.git.luto@kernel.org

arch/x86/include/asm/efi.h

@@ -68,17 +68,33 @@ extern unsigned long efi_fw_vendor, efi_config_table;
 		#f " called with too many arguments (" #p ">" #n ")");	\
 })
 
+static inline void efi_fpu_begin(void)
+{
+	/*
+	 * The UEFI calling convention (UEFI spec 2.3.2 and 2.3.4) requires
+	 * that FCW and MXCSR (64-bit) must be initialized prior to calling
+	 * UEFI code.  (Oddly the spec does not require that the FPU stack
+	 * be empty.)
+	 */
+	kernel_fpu_begin_mask(KFPU_387 | KFPU_MXCSR);
+}
+
+static inline void efi_fpu_end(void)
+{
+	kernel_fpu_end();
+}
+
 #ifdef CONFIG_X86_32
 #define arch_efi_call_virt_setup()					\
 ({									\
-	kernel_fpu_begin();						\
+	efi_fpu_begin();						\
 	firmware_restrict_branch_speculation_start();			\
 })
 
 #define arch_efi_call_virt_teardown()					\
 ({									\
 	firmware_restrict_branch_speculation_end();			\
-	kernel_fpu_end();						\
+	efi_fpu_end();							\
 })
 
 #define arch_efi_call_virt(p, f, args...)	p->f(args)
@@ -107,7 +123,7 @@ struct efi_scratch {
 #define arch_efi_call_virt_setup()					\
 ({									\
 	efi_sync_low_kernel_mappings();					\
-	kernel_fpu_begin();						\
+	efi_fpu_begin();						\
 	firmware_restrict_branch_speculation_start();			\
 	efi_switch_mm(&efi_mm);						\
 })
@@ -119,7 +135,7 @@ struct efi_scratch {
 ({									\
 	efi_switch_mm(efi_scratch.prev_mm);				\
 	firmware_restrict_branch_speculation_end();			\
-	kernel_fpu_end();						\
+	efi_fpu_end();							\
 })
 
 #ifdef CONFIG_KASAN

arch/x86/platform/efi/efi_64.c

@@ -850,7 +850,7 @@ efi_set_virtual_address_map(unsigned long memory_map_size,
 							 virtual_map);
 	efi_switch_mm(&efi_mm);
 
-	kernel_fpu_begin();
+	efi_fpu_begin();
 
 	/* Disable interrupts around EFI calls: */
 	local_irq_save(flags);
@@ -859,7 +859,7 @@ efi_set_virtual_address_map(unsigned long memory_map_size,
 			  descriptor_version, virtual_map);
 	local_irq_restore(flags);
 
-	kernel_fpu_end();
+	efi_fpu_end();
 
 	/* grab the virtually remapped EFI runtime services table pointer */
 	efi.runtime = READ_ONCE(systab->runtime);