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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>
This commit is contained in:
Ard Biesheuvel 2023-04-18 16:36:03 +02:00 committed by Catalin Marinas
parent 44c026a73b
commit 0936243cab
1 changed files with 0 additions and 10 deletions
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10
arch/arm64/kernel/entry.S
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@ -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 */