x86/efi: Restore IRQ state in EFI page fault handler

The kernel's softirq API does not permit re-enabling softirqs while IRQs
are disabled. The reason for this is that local_bh_enable() will not
only re-enable delivery of softirqs over the back of IRQs, it will also
handle any pending softirqs immediately, regardless of whether IRQs are
enabled at that point.

For this reason, commit

  d021985 ("x86/fpu: Improve crypto performance by making kernel-mode FPU reliably usable in softirqs")

disables softirqs only when IRQs are enabled, as it is not permitted
otherwise, but also unnecessary, given that asynchronous softirq
delivery never happens to begin with while IRQs are disabled.

However, this does mean that entering a kernel mode FPU section with
IRQs enabled and leaving it with IRQs disabled leads to problems, as
identified by Sashiko [0]: the EFI page fault handler is called from
page_fault_oops() with IRQs disabled, and thus ends the kernel mode FPU
section with IRQs disabled as well, regardless of whether IRQs were
enabled when it was started. This may result in schedule() being called
with a non-zero preempt_count, causing a BUG().

So take care to re-enable IRQs when handling any EFI page faults if they
were taken with IRQs enabled.

[0] https://sashiko.dev/#/patchset/20260430074107.27051-1-ivan.hu%40canonical.com

Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Ivan Hu <ivan.hu@canonical.com>
Cc: x86@kernel.org
Cc: <stable@vger.kernel.org>
Fixes: d021985 ("x86/fpu: Improve crypto performance by making kernel-mode FPU reliably usable in softirqs")
Reviewed-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Author: ardbiesheuvel

arch/x86/include/asm/efi.h

@@ -137,7 +137,8 @@ extern void __init efi_dump_pagetable(void);
 extern void __init efi_apply_memmap_quirks(void);
 extern int __init efi_reuse_config(u64 tables, int nr_tables);
 extern void efi_delete_dummy_variable(void);
-extern void efi_crash_gracefully_on_page_fault(unsigned long phys_addr);
+extern void efi_crash_gracefully_on_page_fault(unsigned long phys_addr,
+					       const struct pt_regs *regs);
 extern void efi_unmap_boot_services(void);
 
 void arch_efi_call_virt_setup(void);

arch/x86/mm/fault.c

@@ -686,7 +686,7 @@ page_fault_oops(struct pt_regs *regs, unsigned long error_code,
 	 * avoid hanging the system.
 	 */
 	if (IS_ENABLED(CONFIG_EFI))
-		efi_crash_gracefully_on_page_fault(address);
+		efi_crash_gracefully_on_page_fault(address, regs);
 
 	/* Only not-present faults should be handled by KFENCE. */
 	if (!(error_code & X86_PF_PROT) &&

arch/x86/platform/efi/quirks.c

@@ -761,7 +761,8 @@ int efi_capsule_setup_info(struct capsule_info *cap_info, void *kbuff,
  * @return: Returns, if the page fault is not handled. This function
  * will never return if the page fault is handled successfully.
  */
-void efi_crash_gracefully_on_page_fault(unsigned long phys_addr)
+void efi_crash_gracefully_on_page_fault(unsigned long phys_addr,
+					const struct pt_regs *regs)
 {
 	if (!IS_ENABLED(CONFIG_X86_64))
 		return;
@@ -810,6 +811,14 @@ void efi_crash_gracefully_on_page_fault(unsigned long phys_addr)
 		return;
 	}
 
+	/*
+	 * The API does not permit entering a kernel mode FPU section with
+	 * interrupts enabled and leaving it with interrupts disabled.  So
+	 * re-enable interrupts now if they were enabled when the page fault
+	 * occurred.
+	 */
+	local_irq_restore(regs->flags);
+
 	/*
 	 * Before calling EFI Runtime Service, the kernel has switched the
 	 * calling process to efi_mm. Hence, switch back to task_mm.