[3.19.y-ckt stable] Patch "powerpc/mm: Fix pte_pagesize_index() crash on 4K w/64K hash" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    powerpc/mm: Fix pte_pagesize_index() crash on 4K w/64K hash

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt8.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 1f23d4ac8ec263f5b5a3584873c9daaedf659278 Mon Sep 17 00:00:00 2001
From: Michael Ellerman <mpe at ellerman.id.au>
Date: Fri, 7 Aug 2015 16:19:43 +1000
Subject: powerpc/mm: Fix pte_pagesize_index() crash on 4K w/64K hash

commit 74b5037baa2011a2799e2c43adde7d171b072f9e upstream.

The powerpc kernel can be built to have either a 4K PAGE_SIZE or a 64K
PAGE_SIZE.

However when built with a 4K PAGE_SIZE there is an additional config
option which can be enabled, PPC_HAS_HASH_64K, which means the kernel
also knows how to hash a 64K page even though the base PAGE_SIZE is 4K.

This is used in one obscure configuration, to support 64K pages for SPU
local store on the Cell processor when the rest of the kernel is using
4K pages.

In this configuration, pte_pagesize_index() is defined to just pass
through its arguments to get_slice_psize(). However pte_pagesize_index()
is called for both user and kernel addresses, whereas get_slice_psize()
only knows how to handle user addresses.

This has been broken forever, however until recently it happened to
work. That was because in get_slice_psize() the large kernel address
would cause the right shift of the slice mask to return zero.

However in commit 7aa0727f3302 ("powerpc/mm: Increase the slice range to
64TB"), the get_slice_psize() code was changed so that instead of a
right shift we do an array lookup based on the address. When passed a
kernel address this means we index way off the end of the slice array
and return random junk.

That is only fatal if we happen to hit something non-zero, but when we
do return a non-zero value we confuse the MMU code and eventually cause
a check stop.

This fix is ugly, but simple. When we're called for a kernel address we
return 4K, which is always correct in this configuration, otherwise we
use the slice mask.

Fixes: 7aa0727f3302 ("powerpc/mm: Increase the slice range to 64TB")
Reported-by: Cyril Bur <cyrilbur at gmail.com>
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar at linux.vnet.ibm.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/powerpc/include/asm/pgtable-ppc64.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/pgtable-ppc64.h b/arch/powerpc/include/asm/pgtable-ppc64.h
index b9dcc93..e741162 100644
--- a/arch/powerpc/include/asm/pgtable-ppc64.h
+++ b/arch/powerpc/include/asm/pgtable-ppc64.h
@@ -135,7 +135,19 @@
 #define pte_iterate_hashed_end() } while(0)

 #ifdef CONFIG_PPC_HAS_HASH_64K
-#define pte_pagesize_index(mm, addr, pte)	get_slice_psize(mm, addr)
+/*
+ * We expect this to be called only for user addresses or kernel virtual
+ * addresses other than the linear mapping.
+ */
+#define pte_pagesize_index(mm, addr, pte)			\
+	({							\
+		unsigned int psize;				\
+		if (is_kernel_addr(addr))			\
+			psize = MMU_PAGE_4K;			\
+		else						\
+			psize = get_slice_psize(mm, addr);	\
+		psize;						\
+	})
 #else
 #define pte_pagesize_index(mm, addr, pte)	MMU_PAGE_4K
 #endif
--
1.9.1