On Fri, 11 Jul 2025 at 23:42, Will Deacon [off-list ref] wrote:

On Wed, May 14, 2025 at 07:43:46PM +0200, Ard Biesheuvel wrote:

quoted

From: Ard Biesheuvel <ardb@kernel.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@kernel.org>
---
 arch/arm64/include/asm/efi.h | 3 ---
 arch/arm64/kernel/efi.c      | 3 +++
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/efi.h b/arch/arm64/include/asm/efi.h
index decf87777f57..abe9176a3a23 100644
--- a/arch/arm64/include/asm/efi.h
+++ b/arch/arm64/include/asm/efi.h

@@ -132,15 +132,12 @@ static inline void efi_set_pgd(struct mm_struct *mm)
                       * exception when invoking the EFI run-time services.
                       */
                      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
                       */
-                     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 d01ae156bb63..5d188c6c44d7 100644
--- a/arch/arm64/kernel/efi.c
+++ b/arch/arm64/kernel/efi.c

@@ -177,6 +177,8 @@ bool arch_efi_call_virt_setup(void)
              return false;

      efi_virtmap_load();
+     uaccess_ttbr0_enable();
+     post_ttbr_update_workaround();
      __efi_fpsimd_begin();
      return true;
 }

@@ -185,6 +187,7 @@ void arch_efi_call_virt_teardown(void)
 {
      __efi_fpsimd_end();
      efi_virtmap_unload();
+     uaccess_ttbr0_disable();

Moving this after updating the saved TTBR0 isn't great for SWPAN, as it
means that if we take an exception (e.g. an IRQ) before calling
uaccess_ttbr0_disable() then I think we'll end up running with the user
page-table installed briefly in TTBR0 which SWPAN is supposed to prevent
outside of genuine uaccess sections.

OK, I'll respin this to take that into account.