On Fri, Jan 06, 2023 at 06:47:02PM +0100, Ard Biesheuvel wrote: > Comparing current_work() against efi_rts_work.work is sufficient to > decide whether current is currently running EFI runtime services code at > any level in its call stack. > > However, there are other potential users of the EFI runtime stack, such > as the ACPI subsystem, which may invoke efi_call_virt_pointer() > directly, and so any sync exceptions occurring in firmware during those > calls are currently misidentified. > > So instead, let's check whether 'current' has preemption disabled, and > whether the stashed value of the thread stack pointer points into > current's thread stack. This can only be the case if current was > interrupted while running EFI runtime code. This implies that we should > clear the stashed value after switching back, to avoid false positives. > > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx> Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx> Thanks for this! Mark. > --- > arch/arm64/include/asm/efi.h | 9 +++++++++ > arch/arm64/kernel/efi-rt-wrapper.S | 6 ++++++ > arch/arm64/kernel/efi.c | 3 ++- > 3 files changed, 17 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/include/asm/efi.h b/arch/arm64/include/asm/efi.h > index 31d13a6001df49c4..f68d13c3a44e7bb2 100644 > --- a/arch/arm64/include/asm/efi.h > +++ b/arch/arm64/include/asm/efi.h > @@ -48,8 +48,17 @@ int efi_set_mapping_permissions(struct mm_struct *mm, efi_memory_desc_t *md); > }) > > extern spinlock_t efi_rt_lock; > +extern u64 *efi_rt_stack_top; > efi_status_t __efi_rt_asm_wrapper(void *, const char *, ...); > > +/* > + * efi_rt_stack_top[-1] contains the value the stack pointer had before > + * switching to the EFI runtime stack. > + */ > +#define current_in_efi() \ > + (!preemptible() && \ > + on_task_stack(current, READ_ONCE(efi_rt_stack_top[-1]), 1)) > + > #define ARCH_EFI_IRQ_FLAGS_MASK (PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT) > > /* > diff --git a/arch/arm64/kernel/efi-rt-wrapper.S b/arch/arm64/kernel/efi-rt-wrapper.S > index a00886410537d6a6..11f3ec9f09e86de6 100644 > --- a/arch/arm64/kernel/efi-rt-wrapper.S > +++ b/arch/arm64/kernel/efi-rt-wrapper.S > @@ -45,7 +45,10 @@ SYM_FUNC_START(__efi_rt_asm_wrapper) > mov x4, x6 > blr x8 > > + mov x16, sp > mov sp, x29 > + str xzr, [x16, #8] // clear recorded task SP value > + > ldp x1, x2, [sp, #16] > cmp x2, x18 > ldp x29, x30, [sp], #112 > @@ -70,6 +73,9 @@ SYM_FUNC_END(__efi_rt_asm_wrapper) > SYM_CODE_START(__efi_rt_asm_recover) > mov sp, x30 > > + ldr_l x16, efi_rt_stack_top // clear recorded task SP value > + str xzr, [x16, #-8] > + > ldp x19, x20, [sp, #32] > ldp x21, x22, [sp, #48] > ldp x23, x24, [sp, #64] > diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c > index fab05de2e12dd5d8..b273900f45668587 100644 > --- a/arch/arm64/kernel/efi.c > +++ b/arch/arm64/kernel/efi.c > @@ -11,6 +11,7 @@ > #include <linux/init.h> > > #include <asm/efi.h> > +#include <asm/stacktrace.h> > > static bool region_is_misaligned(const efi_memory_desc_t *md) > { > @@ -154,7 +155,7 @@ asmlinkage efi_status_t __efi_rt_asm_recover(void); > 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) > + if (!current_in_efi() || regs->pc >= TASK_SIZE_64) > return false; > > pr_err(FW_BUG "Unable to handle %s in EFI runtime service\n", msg); > -- > 2.39.0 >