linux-kernel - [PATCH v2 6/7] arm64/efi: Move uaccess en/disable out of efi_set

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Message-ID: <20250905133035.275517-15-ardb+git@google.com>
Date: Fri,  5 Sep 2025 15:30:42 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-efi@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	Ard Biesheuvel <ardb@...nel.org>, Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH v2 6/7] arm64/efi: Move uaccess en/disable out of efi_set_pgd()

From: Ard Biesheuvel <ardb@...nel.org>

efi_set_pgd() will no longer be called when invoking EFI runtime
services via the efi_rts_wq work queue, but the uaccess en/disable are
still needed when using PAN emulation using TTBR0 switching. So move
these into the callers.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/arm64/include/asm/efi.h | 13 +++----------
 arch/arm64/kernel/efi.c      | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/include/asm/efi.h b/arch/arm64/include/asm/efi.h
index decf87777f57..09650b2e15af 100644
--- a/arch/arm64/include/asm/efi.h
+++ b/arch/arm64/include/asm/efi.h
@@ -126,21 +126,14 @@ static inline void efi_set_pgd(struct mm_struct *mm)
 		if (mm != current->active_mm) {
 			/*
 			 * Update the current thread's saved ttbr0 since it is
-			 * restored as part of a return from exception. Enable
-			 * access to the valid TTBR0_EL1 and invoke the errata
-			 * workaround directly since there is no return from
-			 * exception when invoking the EFI run-time services.
+			 * restored as part of a return from exception.
 			 */
 			update_saved_ttbr0(current, mm);
-			uaccess_ttbr0_enable();
-			post_ttbr_update_workaround();
 		} else {
 			/*
-			 * Defer the switch to the current thread's TTBR0_EL1
-			 * until uaccess_enable(). Restore the current
-			 * thread's saved ttbr0 corresponding to its active_mm
+			 * Restore the current thread's saved ttbr0
+			 * corresponding to its active_mm
 			 */
-			uaccess_ttbr0_disable();
 			update_saved_ttbr0(current, current->active_mm);
 		}
 	}
diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c
index 8b999c07c7d1..ece046bcf0db 100644
--- a/arch/arm64/kernel/efi.c
+++ b/arch/arm64/kernel/efi.c
@@ -178,6 +178,15 @@ bool arch_efi_call_virt_setup(void)
 		return false;
 
 	efi_virtmap_load();
+
+	/*
+	 * Enable access to the valid TTBR0_EL1 and invoke the errata
+	 * workaround directly since there is no return from exception when
+	 * invoking the EFI run-time services.
+	 */
+	uaccess_ttbr0_enable();
+	post_ttbr_update_workaround();
+
 	__efi_fpsimd_begin();
 	return true;
 }
@@ -185,6 +194,15 @@ bool arch_efi_call_virt_setup(void)
 void arch_efi_call_virt_teardown(void)
 {
 	__efi_fpsimd_end();
+
+	/*
+	 * Defer the switch to the current thread's TTBR0_EL1 until
+	 * uaccess_enable(). Do so before efi_virtmap_unload() updates the
+	 * saved TTBR0 value, so the userland page tables are not activated
+	 * inadvertently over the back of an exception.
+	 */
+	uaccess_ttbr0_disable();
+
 	efi_virtmap_unload();
 	up(&efi_rt_lock);
 }
-- 
2.51.0.355.g5224444f11-goog