[PATCH v6 22/25] powerpc/64s: Clear user GPRs in syscall interrupt entry

[Rohan McLure]

Clear user state in gprs (assign to zero) to reduce the influence of user
registers on speculation within kernel syscall handlers. Clears occur
at the very beginning of the sc and scv 0 interrupt handlers, with
restores occurring following the execution of the syscall handler.

Signed-off-by: Rohan McLure <rmclure at linux.ibm.com>
---
V2: Update summary
V3: Remove erroneous summary paragraph on syscall_exit_prepare
V4: Use ZEROIZE instead of NULLIFY. Clear r0 also.
V5: Move to end of patch series.
V6: Include clears which were previously in the syscall wrapper patch.
Move comment on r3-r8 register save to when we alter the calling
convention for system_call_exception.
---
 arch/powerpc/kernel/interrupt_64.S | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/interrupt_64.S b/arch/powerpc/kernel/interrupt_64.S
index a5dd78bdbe6d..40147558e1a6 100644
--- a/arch/powerpc/kernel/interrupt_64.S
+++ b/arch/powerpc/kernel/interrupt_64.S
@@ -106,6 +106,13 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
 	 * but this is the best we can do.
 	 */
 
+	/*
+	 * Zero user registers to prevent influencing speculative execution
+	 * state of kernel code.
+	 */
+	ZEROIZE_GPR(0)
+	ZEROIZE_GPRS(5, 12)
+	ZEROIZE_NVGPRS()
 	bl	system_call_exception
 
 .Lsyscall_vectored_\name\()_exit:
@@ -134,6 +141,7 @@ BEGIN_FTR_SECTION
 	HMT_MEDIUM_LOW
 END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
 
+	REST_NVGPRS(r1)
 	cmpdi	r3,0
 	bne	.Lsyscall_vectored_\name\()_restore_regs
 
@@ -285,6 +293,13 @@ END_BTB_FLUSH_SECTION
 	wrteei	1
 #endif
 
+	/*
+	 * Zero user registers to prevent influencing speculative execution
+	 * state of kernel code.
+	 */
+	ZEROIZE_GPR(0)
+	ZEROIZE_GPRS(5, 12)
+	ZEROIZE_NVGPRS()
 	bl	system_call_exception
 
 .Lsyscall_exit:
@@ -325,6 +340,7 @@ BEGIN_FTR_SECTION
 	stdcx.	r0,0,r1			/* to clear the reservation */
 END_FTR_SECTION_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
 
+	REST_NVGPRS(r1)
 	cmpdi	r3,0
 	bne	.Lsyscall_restore_regs
 	/* Zero volatile regs that may contain sensitive kernel data */
@@ -352,7 +368,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
 .Lsyscall_restore_regs:
 	ld	r3,_CTR(r1)
 	ld	r4,_XER(r1)
-	REST_NVGPRS(r1)
 	mtctr	r3
 	mtspr	SPRN_XER,r4
 	REST_GPR(0, r1)
-- 
2.34.1