From 0936243cabf0caf46f1a42606325ab93cfa05a6a Mon Sep 17 00:00:00 2001 From: Ard Biesheuvel Date: Tue, 18 Apr 2023 16:36:03 +0200 Subject: [PATCH] arm64: entry: Preserve/restore X29 even for compat tasks Currently, the KPTI trampoline code for returning to user space takes care to only preserve X29 into FAR_EL1 for native tasks, as compat tasks don't have access to this register anyway, and so preserving it is not necessary. It also means it does not need to be restored, and so we have two code paths for returning back to user space: the native one that restores X29 from FAR_EL1, and the compat one that leaves X29 clobbered, containing the value of TTBR1_EL1, which carries a physical address pointing somewhere into the kernel image. This is needlessly complex, and given that FAR_EL1 becomes UNKNOWN after an exception return anway, the only benefit of avoiding the preserve and restore is that we can skip the system register write and read. So let's simplify this, and collapse the two code paths into one that always preserves X29 into FAR_EL1, and always restores it again after the TTBR switch. Signed-off-by: Ard Biesheuvel Reviewed-by: Will Deacon Link: https://lore.kernel.org/r/20230418143604.1176437-2-ardb@kernel.org Signed-off-by: Catalin Marinas --- arch/arm64/kernel/entry.S | 10 ---------- 1 file changed, 10 deletions(-) diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S index ab2a6e3..16fbd0d 100644 --- a/arch/arm64/kernel/entry.S +++ b/arch/arm64/kernel/entry.S @@ -435,13 +435,9 @@ alternative_if_not ARM64_UNMAP_KERNEL_AT_EL0 eret alternative_else_nop_endif #ifdef CONFIG_UNMAP_KERNEL_AT_EL0 - bne 4f msr far_el1, x29 tramp_alias x30, tramp_exit_native, x29 br x30 -4: - tramp_alias x30, tramp_exit_compat, x29 - br x30 #endif .else ldr lr, [sp, #S_LR] @@ -740,9 +736,7 @@ alternative_else_nop_endif msr vbar_el1, x30 ldr lr, [sp, #S_LR] tramp_unmap_kernel x29 - .if \regsize == 64 mrs x29, far_el1 - .endif add sp, sp, #PT_REGS_SIZE // restore sp eret sb @@ -780,10 +774,6 @@ SYM_CODE_END(tramp_vectors) SYM_CODE_START(tramp_exit_native) tramp_exit SYM_CODE_END(tramp_exit_native) - -SYM_CODE_START(tramp_exit_compat) - tramp_exit 32 -SYM_CODE_END(tramp_exit_compat) .popsection // .entry.tramp.text #endif /* CONFIG_UNMAP_KERNEL_AT_EL0 */ -- 2.7.4