[PATCH v8 8/4] arm64: don't call C code with el0's fp register

[james.morse@arm.com (James Morse)]

On entry from el0, we save all the registers on the kernel stack, and
restore them before returning. x29 remains unchanged when we call out
to C code, which will store x29 as the frame-pointer on the stack.

Instead, write 0 into x29 after entry from el0, to avoid any risk of
tracing into user space.

Signed-off-by: James Morse <james.morse@arm.com>
---
 arch/arm64/kernel/entry.S | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 9a0450344335..8dd009b4cab2 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -93,6 +93,8 @@
 	and	tsk, tsk, #~(THREAD_SIZE - 1)	// Ensure MDSCR_EL1.SS is clear,
 	ldr	x19, [tsk, #TI_FLAGS]		// since we can unmask debug
 	disable_step_tsk x19, x20		// exceptions when scheduling.
+
+	mov	x29, xzr			// fp pointed to user-space
 	.else
 	add	x21, sp, #S_FRAME_SIZE
 	.endif
-- 
2.6.2