Subject: [PATCH v4 14/20] powerpc/64s: Clear/restore caller gprs in syscall interrupt/return

From: Rohan McLure <[email-masked]>
Date: Wed, 24 Aug 2022 12:05:42 +1000

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 <[email-masked]>
---
V1 -> V2: Update summary
V2 -> V3: Remove erroneous summary paragraph on syscall_exit_prepare
V3 -> V4: Use ZEROIZE instead of NULLIFY
---
 arch/powerpc/kernel/interrupt_64.S | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kernel/interrupt_64.S b/arch/powerpc/kernel/interrupt_64.S
index 0178aeba3820..a8065209dd8c 100644
--- a/arch/powerpc/kernel/interrupt_64.S
+++ b/arch/powerpc/kernel/interrupt_64.S
@@ -70,7 +70,7 @@ _ASM_NOKPROBE_SYMBOL(system_call_vectored_\name)
 	ld	r2,PACATOC(r13)
 	mfcr	r12
 	li	r11,0
-	/* Can we avoid saving r3-r8 in common case? */
+	/* Save syscall parameters in r3-r8 */
 	std	r3,GPR3(r1)
 	std	r4,GPR4(r1)
 	std	r5,GPR5(r1)
@@ -109,6 +109,13 @@ 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_GPRS(5, 12)
+	ZEROIZE_NVGPRS()
+
 	/* Calling convention has r3 = orig r0, r4 = regs */
 	mr	r3,r0
 	bl	system_call_exception
@@ -139,6 +146,7 @@ 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
 
@@ -181,7 +189,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
 	ld	r4,_LINK(r1)
 	ld	r5,_XER(r1)
 
-	REST_NVGPRS(r1)
 	ld	r0,GPR0(r1)
 	mtcr	r2
 	mtctr	r3
@@ -249,7 +256,7 @@ END_BTB_FLUSH_SECTION
 	ld	r2,PACATOC(r13)
 	mfcr	r12
 	li	r11,0
-	/* Can we avoid saving r3-r8 in common case? */
+	/* Save syscall parameters in r3-r8 */
 	std	r3,GPR3(r1)
 	std	r4,GPR4(r1)
 	std	r5,GPR5(r1)
@@ -300,6 +307,13 @@ END_BTB_FLUSH_SECTION
 	wrteei	1
 #endif
 
+	/*
+	 * Zero user registers to prevent influencing speculative execution
+	 * state of kernel code.
+	 */
+	ZEROIZE_GPRS(5, 12)
+	ZEROIZE_NVGPRS()
+
 	/* Calling convention has r3 = orig r0, r4 = regs */
 	mr	r3,r0
 	bl	system_call_exception
@@ -342,6 +356,7 @@ 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 */
@@ -377,7 +392,6 @@ 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
 	ld	r0,GPR0(r1)
-- 
2.34.1