arm64: efi: Revert "Recover from synchronous exceptions ..."

This reverts commit 23715a26, which introduced some code in
assembler that manipulates both the ordinary and the shadow call stack
pointer in a way that could potentially be taken advantage of. So let's
revert it, and do a better job the next time around.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

arch/arm64/include/asm/efi.h

@@ -14,16 +14,8 @@
 
 #ifdef CONFIG_EFI
 extern void efi_init(void);
-
-bool efi_runtime_fixup_exception(struct pt_regs *regs, const char *msg);
 #else
 #define efi_init()
-
-static inline
-bool efi_runtime_fixup_exception(struct pt_regs *regs, const char *msg)
-{
-	return false;
-}
 #endif
 
 int efi_create_mapping(struct mm_struct *mm, efi_memory_desc_t *md);

arch/arm64/kernel/efi-rt-wrapper.S

@@ -6,7 +6,7 @@
 #include <linux/linkage.h>
 
 SYM_FUNC_START(__efi_rt_asm_wrapper)
-	stp	x29, x30, [sp, #-112]!
+	stp	x29, x30, [sp, #-32]!
 	mov	x29, sp
 
 	/*
@@ -16,20 +16,6 @@ SYM_FUNC_START(__efi_rt_asm_wrapper)
 	 */
 	stp	x1, x18, [sp, #16]
 
-	/*
-	 * Preserve all callee saved registers and record the stack pointer
-	 * value in a per-CPU variable so we can recover from synchronous
-	 * exceptions occurring while running the firmware routines.
-	 */
-	stp	x19, x20, [sp, #32]
-	stp	x21, x22, [sp, #48]
-	stp	x23, x24, [sp, #64]
-	stp	x25, x26, [sp, #80]
-	stp	x27, x28, [sp, #96]
-
-	adr_this_cpu	x8, __efi_rt_asm_recover_sp, x9
-	str		x29, [x8]
-
 	/*
 	 * We are lucky enough that no EFI runtime services take more than
 	 * 5 arguments, so all are passed in registers rather than via the
@@ -45,7 +31,7 @@ SYM_FUNC_START(__efi_rt_asm_wrapper)
 
 	ldp	x1, x2, [sp, #16]
 	cmp	x2, x18
-	ldp	x29, x30, [sp], #112
+	ldp	x29, x30, [sp], #32
 	b.ne	0f
 	ret
 0:
@@ -59,18 +45,3 @@ SYM_FUNC_START(__efi_rt_asm_wrapper)
 	mov	x18, x2
 	b	efi_handle_corrupted_x18	// tail call
 SYM_FUNC_END(__efi_rt_asm_wrapper)
-
-SYM_FUNC_START(__efi_rt_asm_recover)
-	ldr_this_cpu	x8, __efi_rt_asm_recover_sp, x9
-	mov		sp, x8
-
-	ldp	x0,  x18, [sp, #16]
-	ldp	x19, x20, [sp, #32]
-	ldp	x21, x22, [sp, #48]
-	ldp	x23, x24, [sp, #64]
-	ldp	x25, x26, [sp, #80]
-	ldp	x27, x28, [sp, #96]
-	ldp	x29, x30, [sp], #112
-
-	b	efi_handle_runtime_exception
-SYM_FUNC_END(__efi_rt_asm_recover)

arch/arm64/kernel/efi.c

@@ -9,7 +9,6 @@
 
 #include <linux/efi.h>
 #include <linux/init.h>
-#include <linux/percpu.h>
 
 #include <asm/efi.h>
 
@@ -145,28 +144,3 @@ asmlinkage efi_status_t efi_handle_corrupted_x18(efi_status_t s, const char *f)
 	pr_err_ratelimited(FW_BUG "register x18 corrupted by EFI %s\n", f);
 	return s;
 }
-
-asmlinkage DEFINE_PER_CPU(u64, __efi_rt_asm_recover_sp);
-
-asmlinkage efi_status_t __efi_rt_asm_recover(void);
-
-asmlinkage efi_status_t efi_handle_runtime_exception(const char *f)
-{
-	pr_err(FW_BUG "Synchronous exception occurred in EFI runtime service %s()\n", f);
-	clear_bit(EFI_RUNTIME_SERVICES, &efi.flags);
-	return EFI_ABORTED;
-}
-
-bool efi_runtime_fixup_exception(struct pt_regs *regs, const char *msg)
-{
-	 /* Check whether the exception occurred while running the firmware */
-	if (current_work() != &efi_rts_work.work || regs->pc >= TASK_SIZE_64)
-		return false;
-
-	pr_err(FW_BUG "Unable to handle %s in EFI runtime service\n", msg);
-	add_taint(TAINT_FIRMWARE_WORKAROUND, LOCKDEP_STILL_OK);
-	dump_stack();
-
-	regs->pc = (u64)__efi_rt_asm_recover;
-	return true;
-}

arch/arm64/mm/fault.c

@@ -30,7 +30,6 @@
 #include <asm/bug.h>
 #include <asm/cmpxchg.h>
 #include <asm/cpufeature.h>
-#include <asm/efi.h>
 #include <asm/exception.h>
 #include <asm/daifflags.h>
 #include <asm/debug-monitors.h>
@@ -392,9 +391,6 @@ static void __do_kernel_fault(unsigned long addr, unsigned long esr,
 		msg = "paging request";
 	}
 
-	if (efi_runtime_fixup_exception(regs, msg))
-		return;
-
 	die_kernel_fault(msg, addr, esr, regs);
 }