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x86/mm: Check if PUD is large when validating a kernel address
A user reported the following oops when a backup process reads /proc/kcore: 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@suse.de> Reviewed-by: Rik van Riel <riel@redhat.coM> Reviewed-by: Michal Hocko <mhocko@suse.cz> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Cc: stable@vger.kernel.org Cc: linux-mm@kvack.org Link: http://lkml.kernel.org/r/20130211145236.GX21389@suse.de Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -142,6 +142,11 @@ static inline unsigned long pmd_pfn(pmd_t pmd)
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return (pmd_val(pmd) & PTE_PFN_MASK) >> PAGE_SHIFT;
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}
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static inline unsigned long pud_pfn(pud_t pud)
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{
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return (pud_val(pud) & PTE_PFN_MASK) >> PAGE_SHIFT;
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}
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#define pte_page(pte) pfn_to_page(pte_pfn(pte))
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static inline int pmd_large(pmd_t pte)
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@ -831,6 +831,9 @@ int kern_addr_valid(unsigned long addr)
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if (pud_none(*pud))
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return 0;
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if (pud_large(*pud))
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return pfn_valid(pud_pfn(*pud));
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pmd = pmd_offset(pud, addr);
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if (pmd_none(*pmd))
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return 0;
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