linux-kernel - [PATCH] powerpc/32: Clear volatile regs on syscall exit

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Date:   Wed, 23 Feb 2022 18:11:36 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Kees Cook <keescook@...omium.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH] powerpc/32: Clear volatile regs on syscall exit

Commit a82adfd5c7cb ("hardening: Introduce CONFIG_ZERO_CALL_USED_REGS")
added zeroing of used registers at function exit.

At the time being, PPC64 clears volatile registers on syscall exit but
PPC32 doesn't do it for performance reason.

Add that clearing in PPC32 syscall exit as well, but only when
CONFIG_ZERO_CALL_USED_REGS is selected.

On an 8xx, the null_syscall selftest gives:
- Without CONFIG_ZERO_CALL_USED_REGS		: 288 cycles
- With CONFIG_ZERO_CALL_USED_REGS		: 305 cycles
- With CONFIG_ZERO_CALL_USED_REGS + this patch	: 319 cycles

Note that (independent of this patch), with pmac32_defconfig,
vmlinux size is as follows with/without CONFIG_ZERO_CALL_USED_REGS:

   text	   	data	    bss	    dec	    hex		filename
9578869		2525210	 194400	12298479	bba8ef	vmlinux.without
10318045	2525210  194400	13037655	c6f057	vmlinux.with

That is a 7.7% increase on text size, 6.0% on overall size.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 arch/powerpc/kernel/entry_32.S | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index 7748c278d13c..199f23092c02 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S
@@ -151,6 +151,21 @@ syscall_exit_finish:
 	bne	3f
 	mtcr	r5
 
+#ifdef CONFIG_ZERO_CALL_USED_REGS
+	/* Zero volatile regs that may contain sensitive kernel data */
+	li	r0,0
+	li	r4,0
+	li	r5,0
+	li	r6,0
+	li	r7,0
+	li	r8,0
+	li	r9,0
+	li	r10,0
+	li	r11,0
+	li	r12,0
+	mtctr	r0
+	mtxer	r0
+#endif
 1:	lwz	r2,GPR2(r1)
 	lwz	r1,GPR1(r1)
 	rfi
-- 
2.34.1