linux-kernel - [stable:PATCH v4.9.309 26/43] arm64: entry: Make the trampoline cleanup optional

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Message-Id: <20220406164546.1888528-26-james.morse@arm.com>
Date:   Wed,  6 Apr 2022 17:45:29 +0100
From:   James Morse <james.morse@....com>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     James Morse <james.morse@....com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: [stable:PATCH v4.9.309 26/43] arm64: entry: Make the trampoline cleanup optional

commit d739da1694a0eaef0358a42b76904b611539b77b upstream.

Subsequent patches will add additional sets of vectors that use
the same tricks as the kpti vectors to reach the full-fat vectors.
The full-fat vectors contain some cleanup for kpti that is patched
in by alternatives when kpti is in use. Once there are additional
vectors, the cleanup will be needed in more cases.

But on big/little systems, the cleanup would be harmful if no
trampoline vector were in use. Instead of forcing CPUs that don't
need a trampoline vector to use one, make the trampoline cleanup
optional.

Entry at the top of the vectors will skip the cleanup. The trampoline
vectors can then skip the first instruction, triggering the cleanup
to run.

Reviewed-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>
Reviewed-by: Catalin Marinas <catalin.marinas@....com>
Signed-off-by: James Morse <james.morse@....com>
---
 arch/arm64/kernel/entry.S | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 0414b0494dd3..160a3131a190 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -76,16 +76,20 @@
 	.align 7
 .Lventry_start\@:
 #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
-alternative_if ARM64_UNMAP_KERNEL_AT_EL0
 	.if	\el == 0
+	/*
+	 * This must be the first instruction of the EL0 vector entries. It is
+	 * skipped by the trampoline vectors, to trigger the cleanup.
+	 */
+	b	.Lskip_tramp_vectors_cleanup\@
 	.if	\regsize == 64
 	mrs	x30, tpidrro_el0
 	msr	tpidrro_el0, xzr
 	.else
 	mov	x30, xzr
 	.endif
+.Lskip_tramp_vectors_cleanup\@:
 	.endif
-alternative_else_nop_endif
 #endif
 
 	sub	sp, sp, #S_FRAME_SIZE
@@ -934,7 +938,7 @@ __ni_sys_trace:
 #endif
 	prfm	plil1strm, [x30, #(1b - tramp_vectors)]
 	msr	vbar_el1, x30
-	add	x30, x30, #(1b - tramp_vectors)
+	add	x30, x30, #(1b - tramp_vectors + 4)
 	isb
 	ret
 .org 1b + 128	// Did we overflow the ventry slot?
-- 
2.30.2