path: root/arch/x86/xen/mmu.c
diff options
authorKonrad Rzeszutek Wilk <>2012-05-03 16:14:14 -0400
committerKonrad Rzeszutek Wilk <>2012-05-07 10:21:13 -0400
commitb7e5ffe5d83fa40d702976d77452004abbe35791 (patch)
tree65f50b54a7c974f5de0a27c9cd3c954e24a9c077 /arch/x86/xen/mmu.c
parent558daa289a402dbcce0c065c6ff3cc2e00ffeac8 (diff)
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: [<ffffffff8106aa51>] 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 was 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. 'ptdump_show' bitshifts it and gets and invalid value that it tries to dereference. Instead of doing all of that, we detect the ~0 case and just return !_PAGE_PRESENT. This bug has been in existence .. at least until 2.6.37 (yikes!) CC: Signed-off-by: Konrad Rzeszutek Wilk <>
Diffstat (limited to 'arch/x86/xen/mmu.c')
1 files changed, 6 insertions, 1 deletions
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index b8e279479a6b..69f5857660ac 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;