Commit b7e5ffe5 authored May 03, 2012 by Konrad Rzeszutek Wilk

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: stable@kernel.org
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

parent 558daa28

arch/x86/xen/mmu.c

+6 −1

Original line number	Original line	Diff line number	Diff line
	@@ -353,8 +353,13 @@ static pteval_t pte_mfn_to_pfn(pteval_t val)
	{		{
	if (val & _PAGE_PRESENT) {		if (val & _PAGE_PRESENT) {
	unsigned long mfn = (val & PTE_PFN_MASK) >> PAGE_SHIFT;		unsigned long mfn = (val & PTE_PFN_MASK) >> PAGE_SHIFT;
			unsigned long pfn = mfn_to_pfn(mfn);

	pteval_t flags = val & PTE_FLAGS_MASK;		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;		return val;

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