The rmap_walk can access vm_page_prot (and potentially vm_flags in the
pte/pmd manipulations). So it's not safe to wait the caller to update
the vm_page_prot/vm_flags after vma_merge returned potentially
removing the "next" vma and extending the "current" vma over the
next->vm_start,vm_end range, but still with the "current" vma
vm_page_prot, after releasing the rmap locks.
The vm_page_prot/vm_flags must be transferred from the "next" vma to
the current vma while vma_merge still holds the rmap locks.
The side effect of this race condition is pte corruption during
migrate as remove_migration_ptes when run on a address of the "next"
vma that got removed, used the vm_page_prot of the current vma.
migrate mprotect
------------ -------------
migrating in "next" vma
vma_merge() # removes "next" vma and
# extends "current" vma
# current vma is not with
# vm_page_prot updated
remove_migration_ptes
read vm_page_prot of current "vma"
establish pte with wrong permissions
vm_set_page_prot(vma) # too late!
change_protection in the old vma range
only, next range is not updated
This caused segmentation faults and potentially memory corruption in
heavy mprotect loads with some light page migration caused by
compaction in the background.
Hugh Dickins pointed out the comment about the Odd case 8 in vma_merge
which confirms the case 8 is only buggy one where the race can
trigger, in all other vma_merge cases the above cannot happen.
This fix removes the oddness factor from case 8 and it converts it
from:
AAAA
PPPPNNNNXXXX -> PPPPNNNNNNNN
to:
AAAA
PPPPNNNNXXXX -> PPPPXXXXXXXX
XXXX has the right vma properties for the whole merged vma returned by
vma_adjust, so it solves the problem fully. It has the added benefits
that the callers could stop updating vma properties when vma_merge
succeeds however the callers are not updated by this patch (there are
bits like VM_SOFTDIRTY that still need special care for the whole
range, as the vma merging ignores them, but as long as they're not
processed by rmap walks and instead they're accessed with the mmap_sem
at least for reading, they are fine not to be updated within
vma_adjust before releasing the rmap_locks).
Reported-by: Aditya Mandaleeka <adityam@xxxxxxxxxxxxx>
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---
include/linux/mm.h | 10 +++-
mm/mmap.c | 157 ++++++++++++++++++++++++++++++++++++++++++++---------
mm/mprotect.c | 1 +
3 files changed, 139 insertions(+), 29 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ef815b9..2334052 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1977,8 +1977,14 @@ void anon_vma_interval_tree_verify(struct anon_vma_chain *node);
/* mmap.c */
extern int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin);
-extern int vma_adjust(struct vm_area_struct *vma, unsigned long start,
- unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert);
+extern int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
+ unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert,
+ struct vm_area_struct *expand);
+static inline int vma_adjust(struct vm_area_struct *vma, unsigned long start,
+ unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert)
+{
+ return __vma_adjust(vma, start, end, pgoff, insert, NULL);
+}
extern struct vm_area_struct *vma_merge(struct mm_struct *,
struct vm_area_struct *prev, unsigned long addr, unsigned long end,
unsigned long vm_flags, struct anon_vma *, struct file *, pgoff_t,
diff --git a/mm/mmap.c b/mm/mmap.c
index f86fd39..eda3f07 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -597,14 +597,24 @@ static void __insert_vm_struct(struct mm_struct *mm, struct vm_area_struct *vma)
mm->map_count++;
}
-static inline void
-__vma_unlink(struct mm_struct *mm, struct vm_area_struct *vma,
- struct vm_area_struct *prev)
+static __always_inline void __vma_unlink_common(struct mm_struct *mm,
+ struct vm_area_struct *vma,
+ struct vm_area_struct *prev,
+ bool has_prev)
{
struct vm_area_struct *next;
vma_rb_erase(vma, &mm->mm_rb);
- prev->vm_next = next = vma->vm_next;
+ next = vma->vm_next;
+ if (has_prev)
+ prev->vm_next = next;
+ else {
+ prev = vma->vm_prev;
+ if (prev)
+ prev->vm_next = next;
+ else
+ mm->mmap = next;
+ }
if (next)
next->vm_prev = prev;
@@ -612,6 +622,19 @@ __vma_unlink(struct mm_struct *mm, struct vm_area_struct *vma,
vmacache_invalidate(mm);
}
+static inline void __vma_unlink_prev(struct mm_struct *mm,
+ struct vm_area_struct *vma,
+ struct vm_area_struct *prev)
+{
+ __vma_unlink_common(mm, vma, prev, true);
+}
+
+static inline void __vma_unlink(struct mm_struct *mm,
+ struct vm_area_struct *vma)
+{
+ __vma_unlink_common(mm, vma, NULL, false);
+}
+
/*
* We cannot adjust vm_start, vm_end, vm_pgoff fields of a vma that
* is already present in an i_mmap tree without adjusting the tree.
@@ -619,11 +642,12 @@ __vma_unlink(struct mm_struct *mm, struct vm_area_struct *vma,
* are necessary. The "insert" vma (if any) is to be inserted
* before we drop the necessary locks.
*/
-int vma_adjust(struct vm_area_struct *vma, unsigned long start,
- unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert)
+int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
+ unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert,
+ struct vm_area_struct *expand)
{
struct mm_struct *mm = vma->vm_mm;
- struct vm_area_struct *next = vma->vm_next;
+ struct vm_area_struct *next = vma->vm_next, *orig_vma = vma;
struct address_space *mapping = NULL;
struct rb_root *root = NULL;
struct anon_vma *anon_vma = NULL;
@@ -639,9 +663,38 @@ int vma_adjust(struct vm_area_struct *vma, unsigned long start,
/*
* vma expands, overlapping all the next, and
* perhaps the one after too (mprotect case 6).
+ * The only two other cases that gets here are
+ * case 1, case 7 and case 8.
*/
- remove_next = 1 + (end > next->vm_end);
- end = next->vm_end;
+ if (next == expand) {
+ /*
+ * The only case where we don't expand "vma"
+ * and we expand "next" instead is case 8.
+ */
+ VM_WARN_ON(end != next->vm_end);
+ /*
+ * remove_next == 3 means we're
+ * removing "vma" and that to do so we
+ * swapped "vma" and "next".
+ */
+ remove_next = 3;
+ VM_WARN_ON(file != next->vm_file);
+ swap(vma, next);
+ } else {
+ VM_WARN_ON(expand != vma);
+ /*
+ * case 1, 6, 7, remove_next == 2 is case 6,
+ * remove_next == 1 is case 1 or 7.
+ */
+ remove_next = 1 + (end > next->vm_end);
+ VM_WARN_ON(remove_next == 2 &&
+ end != next->vm_next->vm_end);
+ VM_WARN_ON(remove_next == 1 &&
+ end != next->vm_end);
+ /* trim end to next, for case 6 first pass */
+ end = next->vm_end;
+ }
+
exporter = next;
importer = vma;
@@ -660,6 +713,7 @@ int vma_adjust(struct vm_area_struct *vma, unsigned long start,
adjust_next = (end - next->vm_start) >> PAGE_SHIFT;
exporter = next;
importer = vma;
+ VM_WARN_ON(expand != importer);
} else if (end < vma->vm_end) {
/*
* vma shrinks, and !insert tells it's not
@@ -669,6 +723,7 @@ int vma_adjust(struct vm_area_struct *vma, unsigned long start,
adjust_next = -((vma->vm_end - end) >> PAGE_SHIFT);
exporter = vma;
importer = next;
+ VM_WARN_ON(expand != importer);
}
/*
@@ -686,7 +741,7 @@ int vma_adjust(struct vm_area_struct *vma, unsigned long start,
}
}
again:
- vma_adjust_trans_huge(vma, start, end, adjust_next);
+ vma_adjust_trans_huge(orig_vma, start, end, adjust_next);
if (file) {
mapping = file->f_mapping;
@@ -712,8 +767,8 @@ again:
if (!anon_vma && adjust_next)
anon_vma = next->anon_vma;
if (anon_vma) {
- VM_BUG_ON_VMA(adjust_next && next->anon_vma &&
- anon_vma != next->anon_vma, next);
+ VM_WARN_ON(adjust_next && next->anon_vma &&
+ anon_vma != next->anon_vma);
anon_vma_lock_write(anon_vma);
anon_vma_interval_tree_pre_update_vma(vma);
if (adjust_next)
@@ -753,7 +808,11 @@ again:
* vma_merge has merged next into vma, and needs
* us to remove next before dropping the locks.
*/
- __vma_unlink(mm, next, vma);
+ if (remove_next != 3)
+ __vma_unlink_prev(mm, next, vma);
+ else
+ /* vma is not before next if they've been swapped */
+ __vma_unlink(mm, next);
if (file)
__remove_shared_vm_struct(next, file, mapping);
} else if (insert) {
@@ -805,7 +864,27 @@ again:
* we must remove another next too. It would clutter
* up the code too much to do both in one go.
*/
- next = vma->vm_next;
+ if (remove_next != 3) {
+ /*
+ * If "next" was removed and vma->vm_end was
+ * expanded (up) over it, in turn
+ * "next->vm_prev->vm_end" changed and the
+ * "vma->vm_next" gap must be updated.
+ */
+ next = vma->vm_next;
+ } else {
+ /*
+ * For the scope of the comment "next" and
+ * "vma" considered pre-swap(): if "vma" was
+ * removed, next->vm_start was expanded (down)
+ * over it and the "next" gap must be updated.
+ * Because of the swap() the post-swap() "vma"
+ * actually points to pre-swap() "next"
+ * (post-swap() "next" as opposed is now a
+ * dangling pointer).
+ */
+ next = vma;
+ }
if (remove_next == 2) {
remove_next = 1;
end = next->vm_end;
@@ -934,13 +1013,24 @@ can_vma_merge_after(struct vm_area_struct *vma, unsigned long vm_flags,
* cannot merge might become might become might become
* PPNNNNNNNNNN PPPPPPPPPPNN PPPPPPPPPPPP 6 or
* mmap, brk or case 4 below case 5 below PPPPPPPPXXXX 7 or
- * mremap move: PPPPNNNNNNNN 8
+ * mremap move: PPPPXXXXXXXX 8
* AAAA
* PPPP NNNN PPPPPPPPPPPP PPPPPPPPNNNN PPPPNNNNNNNN
* might become case 1 below case 2 below case 3 below
*
- * Odd one out? Case 8, because it extends NNNN but needs flags of XXXX:
- * mprotect_fixup updates vm_flags & vm_page_prot on successful return.
+ * It is important for case 8 that the the vma NNNN overlapping the
+ * region AAAA is never going to extended over XXXX. Instead XXXX must
+ * be extended in region AAAA and NNNN must be removed. This way in
+ * all cases where vma_merge succeeds, the moment vma_adjust drops the
+ * rmap_locks, the properties of the merged vma will be already
+ * correct for the whole merged range. Some of those properties like
+ * vm_page_prot/vm_flags may be accessed by rmap_walks and they must
+ * be correct for the whole merged range immediately after the
+ * rmap_locks are released. Otherwise if XXXX would be removed and
+ * NNNN would be extended over the XXXX range, remove_migration_ptes
+ * or other rmap walkers (if working on addresses beyond the "end"
+ * parameter) may establish ptes with the wrong permissions of NNNN
+ * instead of the right permissions of XXXX.
*/
struct vm_area_struct *vma_merge(struct mm_struct *mm,
struct vm_area_struct *prev, unsigned long addr,
@@ -965,9 +1055,14 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
else
next = mm->mmap;
area = next;
- if (next && next->vm_end == end) /* cases 6, 7, 8 */
+ if (area && area->vm_end == end) /* cases 6, 7, 8 */
next = next->vm_next;
+ /* verify some invariant that must be enforced by the caller */
+ VM_WARN_ON(prev && addr <= prev->vm_start);
+ VM_WARN_ON(area && end > area->vm_end);
+ VM_WARN_ON(addr >= end);
+
/*
* Can it merge with the predecessor?
*/
@@ -988,11 +1083,12 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
is_mergeable_anon_vma(prev->anon_vma,
next->anon_vma, NULL)) {
/* cases 1, 6 */
- err = vma_adjust(prev, prev->vm_start,
- next->vm_end, prev->vm_pgoff, NULL);
+ err = __vma_adjust(prev, prev->vm_start,
+ next->vm_end, prev->vm_pgoff, NULL,
+ prev);
} else /* cases 2, 5, 7 */
- err = vma_adjust(prev, prev->vm_start,
- end, prev->vm_pgoff, NULL);
+ err = __vma_adjust(prev, prev->vm_start,
+ end, prev->vm_pgoff, NULL, prev);
if (err)
return NULL;
khugepaged_enter_vma_merge(prev, vm_flags);
@@ -1008,11 +1104,18 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
anon_vma, file, pgoff+pglen,
vm_userfaultfd_ctx)) {
if (prev && addr < prev->vm_end) /* case 4 */
- err = vma_adjust(prev, prev->vm_start,
- addr, prev->vm_pgoff, NULL);
- else /* cases 3, 8 */
- err = vma_adjust(area, addr, next->vm_end,
- next->vm_pgoff - pglen, NULL);
+ err = __vma_adjust(prev, prev->vm_start,
+ addr, prev->vm_pgoff, NULL, next);
+ else { /* cases 3, 8 */
+ err = __vma_adjust(area, addr, next->vm_end,
+ next->vm_pgoff - pglen, NULL, next);
+ /*
+ * In case 3 area is already equal to next and
+ * this is a noop, but in case 8 "area" has
+ * been removed and next was expanded over it.
+ */
+ area = next;
+ }
if (err)
return NULL;
khugepaged_enter_vma_merge(area, vm_flags);
diff --git a/mm/mprotect.c b/mm/mprotect.c
index a4830f0..e55e2c9 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -304,6 +304,7 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
vma->vm_userfaultfd_ctx);
if (*pprev) {
vma = *pprev;
+ VM_WARN_ON((vma->vm_flags ^ newflags) & ~VM_SOFTDIRTY);
goto success;
}
--
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