From e12676ff205b12743d84199eec0d45fa84eaf4cc Mon Sep 17 00:00:00 2001 From: Konrad Rzeszutek Wilk Date: Thu, 3 May 2012 16:14:14 -0400 Subject: [PATCH] xen/pte: Fix crashes when trying to see non-existent PGD/PMD/PUD/PTEs If I try to do "cat /sys/kernel/debug/kernel_page_tables" I end up with: BUG: unable to handle kernel paging request at ffffc7fffffff000 IP: [] ptdump_show+0x221/0x480 PGD 0 Oops: 0000 [#1] SMP CPU 0 .. snip.. RAX: 0000000000000000 RBX: ffffc00000000fff RCX: 0000000000000000 RDX: 0000800000000000 RSI: 0000000000000000 RDI: ffffc7fffffff000 which is due to the fact we are trying to access a PFN that is not accessible to us. The reason (at least in this case) was that PGD[256] is set to __HYPERVISOR_VIRT_START which setup (by the hypervisor) to point to a read-only linear map of the MFN->PFN array. During our parsing we would get the MFN (a valid one), try to look it up in the MFN->PFN tree and find it invalid and return ~0 as PFN. Then pte_mfn_to_pfn would happilly feed that in, attach the flags and return it back to the caller. In this case the ptdump_show bitshifts it and we get this invalid value. Instead of doing all of that, we detect the ~0 case and just return !_PAGE_PRESENT. CC: stable@kernel.org Signed-off-by: Konrad Rzeszutek Wilk --- arch/x86/xen/mmu.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c index faf30556e983..86da5b8e7e62 100644 --- a/arch/x86/xen/mmu.c +++ b/arch/x86/xen/mmu.c @@ -353,8 +353,13 @@ static pteval_t pte_mfn_to_pfn(pteval_t val) { if (val & _PAGE_PRESENT) { unsigned long mfn = (val & PTE_PFN_MASK) >> PAGE_SHIFT; + unsigned long pfn = mfn_to_pfn(mfn); + pteval_t flags = val & PTE_FLAGS_MASK; - val = ((pteval_t)mfn_to_pfn(mfn) << PAGE_SHIFT) | flags; + if (unlikely(pfn == ~0)) + val = flags & ~_PAGE_PRESENT; + else + val = ((pteval_t)pfn << PAGE_SHIFT) | flags; } return val; -- 2.50.1