hugetlbfs: fix kernel BUG at fs/hugetlbfs/inode.c:444!

This bug has been experienced several times by the Oracle DB team.  The
BUG is in remove_inode_hugepages() as follows:

	/*
	 * If page is mapped, it was faulted in after being
	 * unmapped in caller.  Unmap (again) now after taking
	 * the fault mutex.  The mutex will prevent faults
	 * until we finish removing the page.
	 *
	 * This race can only happen in the hole punch case.
	 * Getting here in a truncate operation is a bug.
	 */
	if (unlikely(page_mapped(page))) {
		BUG_ON(truncate_op);

In this case, the elevated map count is not the result of a race.
Rather it was incorrectly incremented as the result of a bug in the huge
pmd sharing code.  Consider the following:

 - Process A maps a hugetlbfs file of sufficient size and alignment
   (PUD_SIZE) that a pmd page could be shared.

 - Process B maps the same hugetlbfs file with the same size and
   alignment such that a pmd page is shared.

 - Process B then calls mprotect() to change protections for the mapping
   with the shared pmd. As a result, the pmd is 'unshared'.

 - Process B then calls mprotect() again to chage protections for the
   mapping back to their original value. pmd remains unshared.

 - Process B then forks and process C is created. During the fork
   process, we do dup_mm -> dup_mmap -> copy_page_range to copy page
   tables. Copying page tables for hugetlb mappings is done in the
   routine copy_hugetlb_page_range.

In copy_hugetlb_page_range(), the destination pte is obtained by:

	dst_pte = huge_pte_alloc(dst, addr, sz);

If pmd sharing is possible, the returned pointer will be to a pte in an
existing page table.  In the situation above, process C could share with
either process A or process B.  Since process A is first in the list,
the returned pte is a pointer to a pte in process A's page table.

However, the check for pmd sharing in copy_hugetlb_page_range is:

	/* If the pagetables are shared don't copy or take references */
	if (dst_pte == src_pte)
		continue;

Since process C is sharing with process A instead of process B, the
above test fails.  The code in copy_hugetlb_page_range which follows
assumes dst_pte points to a huge_pte_none pte.  It copies the pte entry
from src_pte to dst_pte and increments this map count of the associated
page.  This is how we end up with an elevated map count.

To solve, check the dst_pte entry for huge_pte_none.  If !none, this
implies PMD sharing so do not copy.

Link: http://lkml.kernel.org/r/20181105212315.14125-1-mike.kravetz@oracle.com
Fixes: c5c99429fa57 ("fix hugepages leak due to pagetable page sharing")
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Prakash Sangappa <prakash.sangappa@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index c007fb5..7f2a28a 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3233,7 +3233,7 @@ static int is_hugetlb_entry_hwpoisoned(pte_t pte)
 int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
                            struct vm_area_struct *vma)
 {
-       pte_t *src_pte, *dst_pte, entry;
+       pte_t *src_pte, *dst_pte, entry, dst_entry;
        struct page *ptepage;
        unsigned long addr;
        int cow;
@@ -3261,15 +3261,30 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
                        break;
                }
 
-               /* If the pagetables are shared don't copy or take references */
-               if (dst_pte == src_pte)
+               /*
+                * If the pagetables are shared don't copy or take references.
+                * dst_pte == src_pte is the common case of src/dest sharing.
+                *
+                * However, src could have 'unshared' and dst shares with
+                * another vma.  If dst_pte !none, this implies sharing.
+                * Check here before taking page table lock, and once again
+                * after taking the lock below.
+                */
+               dst_entry = huge_ptep_get(dst_pte);
+               if ((dst_pte == src_pte) || !huge_pte_none(dst_entry))
                        continue;
 
                dst_ptl = huge_pte_lock(h, dst, dst_pte);
                src_ptl = huge_pte_lockptr(h, src, src_pte);
                spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
                entry = huge_ptep_get(src_pte);
-               if (huge_pte_none(entry)) { /* skip none entry */
+               dst_entry = huge_ptep_get(dst_pte);
+               if (huge_pte_none(entry) || !huge_pte_none(dst_entry)) {
+                       /*
+                        * Skip if src entry none.  Also, skip in the
+                        * unlikely case dst entry !none as this implies
+                        * sharing with another vma.
+                        */
                        ;
                } else if (unlikely(is_hugetlb_entry_migration(entry) ||
                                    is_hugetlb_entry_hwpoisoned(entry))) {