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

[Mark Rutland <mark.rutland@arm.com>]

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
> 

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