[PATCH 2/2] arm64: efi: Recover from synchronous exceptions occurring in firmware

Ard Biesheuvel
2022-12-05T20:12:27Z

Unlike x86, which has machinery to deal with page faults that occur
during the execution of EFI runtime services, arm64 has nothing like
that, and a synchronous exception raised by firmware code brings down
the whole system.

With more EFI based systems appearing that were not built to run Linux
(such as the Windows-on-ARM laptops based on Qualcomm SOCs), as well as
the introduction of PRM (platform specific firmware routines that are
callable just like EFI runtime services), we are more likely to run into
issues of this sort, and it is much more likely that we can identify and
work around such issues if they don't bring down the system entirely.

Since we already use a EFI runtime services call wrapper in assembler,
we can quite easily add some code that captures the execution state at
the point where the call is made, allowing us to revert to this state
and proceed execution if the call triggered a synchronous exception.

Given that the kernel and the firmware don't share any data structures
that could end up in an indeterminate state, we can happily continue
running, as long as we mark the EFI runtime services as unavailable from
that point on.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
---
arch/arm64/include/asm/efi.h | 8 +++++
arch/arm64/kernel/efi-rt-wrapper.S | 31 ++++++++++++++++----
arch/arm64/kernel/efi.c | 22 ++++++++++++++
arch/arm64/mm/fault.c | 4 +++
drivers/firmware/efi/runtime-wrappers.c | 1 +
5 files changed, 61 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/efi.h b/arch/arm64/include/asm/efi.h
index 1c408ec3c8b3a883..31d13a6001df49c4 100644
--- a/arch/arm64/include/asm/efi.h
+++ b/arch/arm64/include/asm/efi.h
@@ -14,8 +14,16 @@

#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);
diff --git a/arch/arm64/kernel/efi-rt-wrapper.S b/arch/arm64/kernel/efi-rt-wrapper.S
index b2786b968fee68dd..ee0c6c719ab573b7 100644
--- a/arch/arm64/kernel/efi-rt-wrapper.S
+++ b/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, #-32]!
+ stp x29, x30, [sp, #-112]!
mov x29, sp

/*
@@ -16,11 +16,20 @@ 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]
+
ldr_l x16, efi_rt_stack_top
mov sp, x16
-#ifdef CONFIG_SHADOW_CALL_STACK
- str x18, [sp, #-16]!
-#endif
+ stp x18, x29, [sp, #-16]!

/*
* We are lucky enough that no EFI runtime services take more than
@@ -38,7 +47,7 @@ SYM_FUNC_START(__efi_rt_asm_wrapper)
mov sp, x29
ldp x1, x2, [sp, #16]
cmp x2, x18
- ldp x29, x30, [sp], #32
+ ldp x29, x30, [sp], #112
b.ne 0f
ret
0:
@@ -56,3 +65,15 @@ SYM_FUNC_START(__efi_rt_asm_wrapper)

b efi_handle_corrupted_x18 // tail call
SYM_FUNC_END(__efi_rt_asm_wrapper)
+
+SYM_CODE_START(__efi_rt_asm_recover)
+ mov sp, x30
+
+ 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
+ ret
+SYM_CODE_END(__efi_rt_asm_recover)
diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c
index 8cb2e005f8aca589..0169a669fde7544f 100644
--- a/arch/arm64/kernel/efi.c
+++ b/arch/arm64/kernel/efi.c
@@ -149,6 +149,28 @@ DEFINE_SPINLOCK(efi_rt_lock);

asmlinkage u64 *efi_rt_stack_top __ro_after_init;

+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)
+ return false;
+
+ pr_err(FW_BUG "Unable to handle %s in EFI runtime service\n", msg);
+ add_taint(TAINT_FIRMWARE_WORKAROUND, LOCKDEP_STILL_OK);
+ clear_bit(EFI_RUNTIME_SERVICES, &efi.flags);
+
+ regs->regs[0] = EFI_ABORTED;
+ regs->regs[30] = efi_rt_stack_top[-1];
+ regs->pc = (u64)__efi_rt_asm_recover;
+
+ if (IS_ENABLED(CONFIG_SHADOW_CALL_STACK))
+ regs->regs[18] = efi_rt_stack_top[-2];
+
+ return true;
+}
+
/* required by the EFI spec */
static_assert(THREAD_SIZE >= SZ_8K);

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 5b391490e045be91..3e9cf9826417a434 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -30,6 +30,7 @@
#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>
@@ -391,6 +392,9 @@ 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);
}

diff --git a/drivers/firmware/efi/runtime-wrappers.c b/drivers/firmware/efi/runtime-wrappers.c
index f3e54f6616f02475..7feee3d9c2bfbeec 100644
--- a/drivers/firmware/efi/runtime-wrappers.c
+++ b/drivers/firmware/efi/runtime-wrappers.c
@@ -83,6 +83,7 @@ struct efi_runtime_work efi_rts_work;
else \
pr_err("Failed to queue work to efi_rts_wq.\n"); \
\
+ WARN_ON_ONCE(efi_rts_work.status == EFI_ABORTED); \
exit: \
efi_rts_work.efi_rts_id = EFI_NONE; \
efi_rts_work.status; \
--
2.35.1