[tip:x86/urgent] x86/mm: Check if PUD is large when validating a kernel address

From: tip-bot for Mel Gorman
Date: Wed Feb 13 2013 - 07:14:57 EST

Commit-ID: 0ee364eb316348ddf3e0dfcd986f5f13f528f821
Gitweb: http://git.kernel.org/tip/0ee364eb316348ddf3e0dfcd986f5f13f528f821
Author: Mel Gorman <mgorman@xxxxxxx>
AuthorDate: Mon, 11 Feb 2013 14:52:36 +0000
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 13 Feb 2013 10:02:55 +0100

x86/mm: Check if PUD is large when validating a kernel address

A user reported the following oops when a backup process reads

BUG: unable to handle kernel paging request at ffffbb00ff33b000
IP: [<ffffffff8103157e>] kern_addr_valid+0xbe/0x110

Call Trace:
[<ffffffff811b8aaa>] read_kcore+0x17a/0x370
[<ffffffff811ad847>] proc_reg_read+0x77/0xc0
[<ffffffff81151687>] vfs_read+0xc7/0x130
[<ffffffff811517f3>] sys_read+0x53/0xa0
[<ffffffff81449692>] system_call_fastpath+0x16/0x1b

Investigation determined that the bug triggered when reading
system RAM at the 4G mark. On this system, that was the first
address using 1G pages for the virt->phys direct mapping so the
PUD is pointing to a physical address, not a PMD page.

The problem is that the page table walker in kern_addr_valid() is
not checking pud_large() and treats the physical address as if
it was a PMD. If it happens to look like pmd_none then it'll
silently fail, probably returning zeros instead of real data. If
the data happens to look like a present PMD though, it will be
walked resulting in the oops above.

This patch adds the necessary pud_large() check.

Unfortunately the problem was not readily reproducible and now
they are running the backup program without accessing
/proc/kcore so the patch has not been validated but I think it
makes sense.

Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>
Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Link: http://lkml.kernel.org/r/20130211145236.GX21389@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
arch/x86/include/asm/pgtable.h | 5 +++++
arch/x86/mm/init_64.c | 3 +++
2 files changed, 8 insertions(+)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 5199db2..1c1a955 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -142,6 +142,11 @@ static inline unsigned long pmd_pfn(pmd_t pmd)
return (pmd_val(pmd) & PTE_PFN_MASK) >> PAGE_SHIFT;

+static inline unsigned long pud_pfn(pud_t pud)
+ return (pud_val(pud) & PTE_PFN_MASK) >> PAGE_SHIFT;
#define pte_page(pte) pfn_to_page(pte_pfn(pte))

static inline int pmd_large(pmd_t pte)
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 2ead3c8..75c9a6a 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -831,6 +831,9 @@ int kern_addr_valid(unsigned long addr)
if (pud_none(*pud))
return 0;

+ if (pud_large(*pud))
+ return pfn_valid(pud_pfn(*pud));
pmd = pmd_offset(pud, addr);
if (pmd_none(*pmd))
return 0;
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