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@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/bbb13848ff0100a76ee9ea95118058c30ae95f2c.1643613343.git.christophe.leroy@csgroup.eu

diff --git a/arch/powerpc/kernel/head_book3s_32.S b/arch/powerpc/kernel/head_book3s_32.S
index 7e27b44..d489b96 100644 (file)
--- a/arch/powerpc/kernel/head_book3s_32.S
+++ b/arch/powerpc/kernel/head_book3s_32.S
@@ -507,7 +507,9 @@ 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