From e98a9d0146372b046d863164025a66ab4488b972 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Tue, 11 Aug 2026 15:04:29 +0100 Subject: [PATCH] arm64/efi: Avoid voluntary preemption with efi_mm installed Gus reports a bad kernel memory access when using software PAN (CONFIG_ARM64_SW_TTBR0_PAN=y) on a machine with support for EFI runtime services: Unable to handle kernel access to user memory outside uaccess routines at virtual address 00000000f322ff30 Mem abort info: ESR = 0x0000000096000004 FSC = 0x04: level 0 translation fault Internal error: Oops: 0000000096000004 [#1] SMP Workqueue: efi_rts_wq efi_call_rts pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) pc : efi_call_rts+0xd8/0x288 Call trace: efi_call_rts+0xd8/0x288 (P) process_one_work+0x178/0x4f8 worker_thread+0x194/0x328 This is because the fpsimd context management code called from __efi_fpsimd_begin() can preempt voluntarily, returning later to the EFI code with an incorrect value for TTBR0_EL1 thanks to the deferred mm switching used by the software PAN implementation. Since EFI runtime services cannot preempt voluntarily and because the fpsimd switching code does not rely on the TTBR0_EL1 mappings, simply reorder the fpsimd switch so that it occurs before we change the page-table. Cc: Ard Biesheuvel Reported-by: Gus Bourg Tested-by: Gus Bourg Fixes: a5baf582f4c0 ("arm64/efi: Call EFI runtime services without disabling preemption") Link: https://lore.kernel.org/all/20260806000144.3388823-1-gus@bourg.net/ Reviewed-by: Ard Biesheuvel Signed-off-by: Will Deacon --- arch/arm64/kernel/efi.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c index 30cd7f804398..0ec90fd1754e 100644 --- a/arch/arm64/kernel/efi.c +++ b/arch/arm64/kernel/efi.c @@ -184,6 +184,8 @@ void arch_efi_call_virt_setup(void) efi_virtmap_load(); } + __efi_fpsimd_begin(); + /* * Enable access to the valid TTBR0_EL1 and invoke the errata * workaround directly since there is no return from exception when @@ -191,8 +193,6 @@ void arch_efi_call_virt_setup(void) */ uaccess_ttbr0_enable(); post_ttbr_update_workaround(); - - __efi_fpsimd_begin(); } void arch_efi_call_virt_teardown(void)