Re: [PATCH v3 1/2] arm64: efi: Avoid workqueue to check whether EFI runtime is live

Mark Rutland
2023-01-09T13:26:54Z

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@kernel.org>

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

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
>