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 <ardb@kernel.org>
Reviewed-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20230418143604.1176437-2-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index ab2a6e3..16fbd0d 100644 (file)
--- 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 */