linux-kernel - [PATCH] powerpc/603: Clear C bit when PTE is read only

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Date:   Mon, 31 Jan 2022 07:17:57 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     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" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: [PATCH] powerpc/603: Clear C bit when PTE is read only

On book3s/32 MMU, PP bits don't offer kernel RO protection,
kernel pages are always RW.

However, on the 603 a page fault is always generated when the
C bit (change bit = dirty bit) is not set.

Enforce kernel RO protection by clearing C bit in TLB miss
handler when the page doesn't have _PAGE_RW flag.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 arch/powerpc/kernel/head_book3s_32.S | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/powerpc/kernel/head_book3s_32.S b/arch/powerpc/kernel/head_book3s_32.S
index 7f4f3a52b730..6c739beb938c 100644
--- a/arch/powerpc/kernel/head_book3s_32.S
+++ b/arch/powerpc/kernel/head_book3s_32.S
@@ -504,7 +504,9 @@ DataLoadTLBMiss:
 	/* Convert linux-style PTE to low word of PPC-style PTE */
 	rlwinm	r1,r0,32-9,30,30	/* _PAGE_RW -> PP msb */
 	rlwimi	r0,r0,32-1,30,30	/* _PAGE_USER -> PP msb */
+	rlwimi	r1,r0,32-3,24,24	/* _PAGE_RW -> _PAGE_DIRTY */
 	rlwimi	r0,r0,32-1,31,31	/* _PAGE_USER -> PP lsb */
+	xori	r1,r1,_PAGE_DIRTY	/* clear dirty when not rw */
 	ori	r1,r1,0xe04		/* clear out reserved bits */
 	andc	r1,r0,r1		/* PP = user? rw? 1: 3: 0 */
 BEGIN_FTR_SECTION
-- 
2.33.1