powerpc/40x: Introduce _PAGE_READ and remove _PAGE_USER

_PAGE_USER is used to select the zone. Today zone 0 is kernel
and zone 1 is user.

To implement _PAGE_NONE, _PAGE_USER is cleared, leading to no access
for user but kernel still has access to the page so it's possible for
a user application to write in that page by using a kernel function
as trampoline.

What is really wanted is to have user rights on pages below TASK_SIZE
and no user rights on pages above TASK_SIZE. Use zones for that.
There are 16 zones so lets use the 4 upper address bits to set the
zone and declare zone rights based on TASK_SIZE.

Then drop _PAGE_USER and reuse it as _PAGE_READ that will be checked
in Data TLB miss handler. That will properly handle PAGE_NONE for
both kernel and user.

In addition, it partially implements execute-only right. The
implementation won't be complete because once a TLB has been loaded
via the Instruction TLB miss handler, it will be possible to read
the page. But at least it can't be read unless it is executed first.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/2a13e3ba8a5dec43143cc1f9a91ec71ea1529f3c.1695659959.git.christophe.leroy@csgroup.eu

1 files changed, 12 insertions, 7 deletions

diff --git a/arch/powerpc/mm/nohash/40x.c b/arch/powerpc/mm/nohash/40x.c
index 3684d6e570fb..e835e80c09db 100644
--- a/arch/powerpc/mm/nohash/40x.c
+++ b/arch/powerpc/mm/nohash/40x.c
@@ -48,20 +48,25 @@
  */
 void __init MMU_init_hw(void)
 {
+	int i;
+	unsigned long zpr;
+
 	/*
 	 * The Zone Protection Register (ZPR) defines how protection will
-	 * be applied to every page which is a member of a given zone. At
-	 * present, we utilize only two of the 4xx's zones.
+	 * be applied to every page which is a member of a given zone.
 	 * The zone index bits (of ZSEL) in the PTE are used for software
-	 * indicators, except the LSB.  For user access, zone 1 is used,
-	 * for kernel access, zone 0 is used.  We set all but zone 1
-	 * to zero, allowing only kernel access as indicated in the PTE.
-	 * For zone 1, we set a 01 binary (a value of 10 will not work)
+	 * indicators. We use the 4 upper bits of virtual address to select
+	 * the zone. We set all zones above TASK_SIZE to zero, allowing
+	 * only kernel access as indicated in the PTE. For zones below
+	 * TASK_SIZE, we set a 01 binary (a value of 10 will not work)
 	 * to allow user access as indicated in the PTE.  This also allows
 	 * kernel access as indicated in the PTE.
 	 */
 
-        mtspr(SPRN_ZPR, 0x10000000);
+	for (i = 0, zpr = 0; i < TASK_SIZE >> 28; i++)
+		zpr |= 1 << (30 - i * 2);
+
+	mtspr(SPRN_ZPR, zpr);
 
 	flush_instruction_cache();