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Subject[PATCH 15/22] mm/hmm: Poison hmm_range during unregister
Date
From: Jason Gunthorpe <jgg@mellanox.com>

Trying to misuse a range outside its lifetime is a kernel bug. Use poison
bytes to help detect this condition. Double unregister will reliably crash.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Tested-by: Philip Yang <Philip.Yang@amd.com>
---
mm/hmm.c | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 2ef14b2b5505..c30aa9403dbe 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -925,19 +925,21 @@ void hmm_range_unregister(struct hmm_range *range)
{
struct hmm *hmm = range->hmm;

- /* Sanity check this really should not happen. */
- if (hmm == NULL || range->end <= range->start)
- return;
-
mutex_lock(&hmm->lock);
list_del_init(&range->list);
mutex_unlock(&hmm->lock);

/* Drop reference taken by hmm_range_register() */
- range->valid = false;
mmput(hmm->mm);
hmm_put(hmm);
- range->hmm = NULL;
+
+ /*
+ * The range is now invalid and the ref on the hmm is dropped, so
+ * poison the pointer. Leave other fields in place, for the caller's
+ * use.
+ */
+ range->valid = false;
+ memset(&range->hmm, POISON_INUSE, sizeof(range->hmm));
}
EXPORT_SYMBOL(hmm_range_unregister);

--
2.20.1
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