Commit ff09d7ec authored Oct 26, 2018 by Aneesh Kumar K.V Committed by Linus Torvalds Oct 26, 2018

mm/memory.c: recheck page table entry with page table lock held

We clear the pte temporarily during read/modify/write update of the pte.
If we take a page fault while the pte is cleared, the application can get
SIGBUS.  One such case is with remap_pfn_range without a backing
vm_ops->fault callback.  do_fault will return SIGBUS in that case.

cpu 0		 				cpu1
mprotect()
ptep_modify_prot_start()/pte cleared.
.
.						page fault.
.
.
prep_modify_prot_commit()

Fix this by taking page table lock and rechecking for pte_none.

[aneesh.kumar@linux.ibm.com: fix crash observed with syzkaller run]
  Link: http://lkml.kernel.org/r/87va6bwlfg.fsf@linux.ibm.com
Link: http://lkml.kernel.org/r/20180926031858.9692-1-aneesh.kumar@linux.ibm.com


Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Ido Schimmel <idosch@idosch.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent cc4b8c79

mm/memory.c

+30 −4

Original line number	Diff line number	Diff line
		@@ -3496,10 +3496,36 @@ static vm_fault_t do_fault(struct vm_fault *vmf)
		struct vm_area_struct *vma = vmf->vma;
		vm_fault_t ret;

		/* The VMA was not fully populated on mmap() or missing VM_DONTEXPAND */
		if (!vma->vm_ops->fault)
		/*
		* The VMA was not fully populated on mmap() or missing VM_DONTEXPAND
		*/
		if (!vma->vm_ops->fault) {
		/*
		* If we find a migration pmd entry or a none pmd entry, which
		* should never happen, return SIGBUS
		*/
		if (unlikely(!pmd_present(*vmf->pmd)))
		ret = VM_FAULT_SIGBUS;
		else if (!(vmf->flags & FAULT_FLAG_WRITE))
		else {
		vmf->pte = pte_offset_map_lock(vmf->vma->vm_mm,
		vmf->pmd,
		vmf->address,
		&vmf->ptl);
		/*
		* Make sure this is not a temporary clearing of pte
		* by holding ptl and checking again. A R/M/W update
		* of pte involves: take ptl, clearing the pte so that
		* we don't have concurrent modification by hardware
		* followed by an update.
		*/
		if (unlikely(pte_none(*vmf->pte)))
		ret = VM_FAULT_SIGBUS;
		else
		ret = VM_FAULT_NOPAGE;

		pte_unmap_unlock(vmf->pte, vmf->ptl);
		}
		} else if (!(vmf->flags & FAULT_FLAG_WRITE))
		ret = do_read_fault(vmf);
		else if (!(vma->vm_flags & VM_SHARED))
		ret = do_cow_fault(vmf);

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