[PATCH 3/4] powerpc/64e: Clear gprs on interrupt routine entry on Book3E

[Rohan McLure <rmclure@linux.ibm.com>]

Zero GPRS r14-r31 on entry into the kernel for interrupt sources to
limit influence of user-space values in potential speculation gadgets.
Prior to this commit, all other GPRS are reassigned during the common
prologue to interrupt handlers and so need not be zeroised explicitly.

This may be done safely, without loss of register state prior to the
interrupt, as the common prologue saves the initial values of
non-volatiles, which are unconditionally restored in interrupt_64.S.
Mitigation defaults to enabled by INTERRUPT_SANITIZE_REGISTERS.

Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
---
Resubmitting patches as their own series after v6 partially merged:
Link: https://lore.kernel.org/all/166488988686.779920.13794870102696416283.b4-ty@ellerman.id.au/t/

V4: New patch.
V5: Depend on Kconfig option. Remove ZEROIZE_NVGPRS on bad kernel
stack handler.
V6: Change name of zeroize macro to be consistent with Book3S.
---
 arch/powerpc/kernel/exceptions-64e.S | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/exceptions-64e.S b/arch/powerpc/kernel/exceptions-64e.S
index 2f68fb2ee4fc..91d8019123c2 100644
--- a/arch/powerpc/kernel/exceptions-64e.S
+++ b/arch/powerpc/kernel/exceptions-64e.S
@@ -358,6 +358,11 @@ ret_from_mc_except:
 	std	r14,PACA_EXMC+EX_R14(r13);				    \
 	std	r15,PACA_EXMC+EX_R15(r13)
 
+#ifdef CONFIG_INTERRUPT_SANITIZE_REGISTERS
+#define SANITIZE_ZEROIZE_NVGPRS()	ZEROIZE_NVGPRS()
+#else
+#define SANITIZE_ZEROIZE_NVGPRS()
+#endif
 
 /* Core exception code for all exceptions except TLB misses. */
 #define EXCEPTION_COMMON_LVL(n, scratch, excf)				    \
@@ -394,7 +399,8 @@ exc_##n##_common:							    \
 	std	r12,STACK_FRAME_OVERHEAD-16(r1); /* mark the frame */	    \
 	std	r3,_TRAP(r1);		/* set trap number		*/  \
 	std	r0,RESULT(r1);		/* clear regs->result */	    \
-	SAVE_NVGPRS(r1);
+	SAVE_NVGPRS(r1);						    \
+	SANITIZE_ZEROIZE_NVGPRS();	/* minimise speculation influence */
 
 #define EXCEPTION_COMMON(n) \
 	EXCEPTION_COMMON_LVL(n, SPRN_SPRG_GEN_SCRATCH, PACA_EXGEN)
-- 
2.34.1