2022-02-01 15:34:42

by Christophe Leroy

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 <[email protected]>
---
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

2022-02-15 07:48:54

by Michael Ellerman

Subject: Re: [PATCH] powerpc/603: Clear C bit when PTE is read only

On Mon, 31 Jan 2022 07:17:57 +0000, Christophe Leroy wrote:
> 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.
>
> [...]

Applied to powerpc/next.

[1/1] powerpc/603: Clear C bit when PTE is read only
https://git.kernel.org/powerpc/c/4634bf4455fe26f07dabf97c3585c9ccb86353c4

cheers