[PATCH 02/21] arm64: entry: Only check for stack overflow on exceptions from EL1

[Will Deacon]

Checking for kernel stack overflow on entry from EL0 is largely
pointless and gets in the way of using SPSel to switch to the overflow
stack. If the kernel stack overflows on entry from userspace, that
implies that it was unbalanced on the last exception return which means
we can't sensibly reason about the state of the system given that we
could've returned to an arbitrary virtual address in an arbitrary mode.

Remove the stack overflow check from the EL0 exception entry path.

Cc: Mark Rutland <mark.rutland at arm.com>
Signed-off-by: Will Deacon <will at kernel.org>
---
 arch/arm64/kernel/entry.S | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 59f045e496ec..b5d8277f608a 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -54,6 +54,8 @@
 	.endif
 
 	sub	sp, sp, #PT_REGS_SIZE
+
+	.if	\el == 1
 	/*
 	 * Test whether the SP has overflowed, without corrupting a GPR.
 	 * Task and IRQ stacks are aligned so that SP & (1 << THREAD_SHIFT)
@@ -64,8 +66,11 @@
 	tbnz	x0, #THREAD_SHIFT, 0f
 	sub	x0, sp, x0			// x0'' = sp' - x0' = (sp + x0) - sp = x0
 	sub	sp, sp, x0			// sp'' = sp' - x0 = (sp + x0) - x0 = sp
+	.endif
+
 	b	el\el\ht\()_\regsize\()_\label
 
+	.if	\el == 1
 0:
 	/*
 	 * Either we've just detected an overflow, or we've taken an exception
@@ -96,6 +101,7 @@
 	sub	sp, sp, x0
 	mrs	x0, tpidrro_el0
 	b	el\el\ht\()_\regsize\()_\label
+	.endif
 .org .Lventry_start\@ + 128	// Did we overflow the ventry slot?
 	.endm
 
-- 
2.55.0.979.g7e5102b832-goog