arm64: Fix TTBR + PAN + 52-bit PA logic in cpu_do_switch_mm

In cpu_do_switch_mm(.) with ARM64_SW_TTBR0_PAN=y we apply phys_to_ttbr
to a value that already has an ASID inserted into the upper bits. For
52-bit PA configurations this then can give us TTBR0_EL1 registers that
cause translation table walks to attempt to access non-zero PA[51:48]
spuriously. Ultimately leading to a Synchronous External Abort on level
1 translation.

This patch re-arranges the logic in cpu_do_switch_mm(.) such that
phys_to_ttbr is called before the ASID is inserted into the TTBR0 value.

Fixes: 6b88a32c ("arm64: kpti: Fix the interaction between ASID switching and software PAN")
Acked-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Tested-by: Kristina Martsenko <kristina.martsenko@arm.com>
Reviewed-by: Kristina Martsenko <kristina.martsenko@arm.com>
Signed-off-by: Steve Capper <steve.capper@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/proc.S

@@ -153,14 +153,14 @@ ENDPROC(cpu_do_resume)
 ENTRY(cpu_do_switch_mm)
 	mrs	x2, ttbr1_el1
 	mmid	x1, x1				// get mm->context.id
+	phys_to_ttbr x0, x3
 #ifdef CONFIG_ARM64_SW_TTBR0_PAN
-	bfi	x0, x1, #48, #16		// set the ASID field in TTBR0
+	bfi	x3, x1, #48, #16		// set the ASID field in TTBR0
 #endif
 	bfi	x2, x1, #48, #16		// set the ASID
 	msr	ttbr1_el1, x2			// in TTBR1 (since TCR.A1 is set)
 	isb
-	phys_to_ttbr x0, x2
-	msr	ttbr0_el1, x2			// now update TTBR0
+	msr	ttbr0_el1, x3			// now update TTBR0
 	isb
 	b	post_ttbr_update_workaround	// Back to C code...
 ENDPROC(cpu_do_switch_mm)