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Subject[PATCH 16/22] mm/hmm: Remove confusing comment and logic from hmm_release
Date
From: Jason Gunthorpe <jgg@mellanox.com>

hmm_release() is called exactly once per hmm. ops->release() cannot
accidentally trigger any action that would recurse back onto
hmm->mirrors_sem.

This fixes a use after-free race of the form:

CPU0 CPU1
hmm_release()
up_write(&hmm->mirrors_sem);
hmm_mirror_unregister(mirror)
down_write(&hmm->mirrors_sem);
up_write(&hmm->mirrors_sem);
kfree(mirror)
mirror->ops->release(mirror)

The only user we have today for ops->release is an empty function, so this
is unambiguously safe.

As a consequence of plugging this race drivers are not allowed to
register/unregister mirrors from within a release op.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Philip Yang <Philip.Yang@amd.com>
---
mm/hmm.c | 28 +++++++++-------------------
1 file changed, 9 insertions(+), 19 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index c30aa9403dbe..b224ea635a77 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -130,26 +130,16 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
*/
WARN_ON(!list_empty_careful(&hmm->ranges));

- down_write(&hmm->mirrors_sem);
- mirror = list_first_entry_or_null(&hmm->mirrors, struct hmm_mirror,
- list);
- while (mirror) {
- list_del_init(&mirror->list);
- if (mirror->ops->release) {
- /*
- * Drop mirrors_sem so the release callback can wait
- * on any pending work that might itself trigger a
- * mmu_notifier callback and thus would deadlock with
- * us.
- */
- up_write(&hmm->mirrors_sem);
+ down_read(&hmm->mirrors_sem);
+ list_for_each_entry(mirror, &hmm->mirrors, list) {
+ /*
+ * Note: The driver is not allowed to trigger
+ * hmm_mirror_unregister() from this thread.
+ */
+ if (mirror->ops->release)
mirror->ops->release(mirror);
- down_write(&hmm->mirrors_sem);
- }
- mirror = list_first_entry_or_null(&hmm->mirrors,
- struct hmm_mirror, list);
}
- up_write(&hmm->mirrors_sem);
+ up_read(&hmm->mirrors_sem);

hmm_put(hmm);
}
@@ -279,7 +269,7 @@ void hmm_mirror_unregister(struct hmm_mirror *mirror)
struct hmm *hmm = mirror->hmm;

down_write(&hmm->mirrors_sem);
- list_del_init(&mirror->list);
+ list_del(&mirror->list);
up_write(&hmm->mirrors_sem);
hmm_put(hmm);
}
--
2.20.1
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