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Subject[PATCH 4.19 038/110] mm/memory.c: recheck page table entry with page table lock held
Date
4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>

commit ff09d7ec9786be4ad7589aa987d7dc66e2dd9160 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/memory.c | 34 ++++++++++++++++++++++++++++++----
1 file changed, 30 insertions(+), 4 deletions(-)

--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3745,10 +3745,36 @@ static vm_fault_t do_fault(struct vm_fau
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)
- ret = VM_FAULT_SIGBUS;
- else if (!(vmf->flags & FAULT_FLAG_WRITE))
+ /*
+ * 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 {
+ 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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