[PATCH] IB/mthca: Fix how mthca_map_user_db() calls gup

Davidlohr Bueso
2018-01-23T21:02:41Z

mthca_map_user_db() has two problems regarding the call to
get_user_pages():

(i) It is not done under mmap_sem.

(ii) It is done under the db_table mutex, which protects all
database related operations. Should any of these be called
under mmap_sem, we get an ABBA deadlock. In addition, gup can
be performance intensive, which could contend other mapping/
unmapping ops.

To fix this, we can drop the mutex while doing a gup_fast(),
once done, recheck to see the page was mapped while we didn't
hold the mutex, and exit out with the corresponding housekeeping.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---

- Compile tested only.

- Should I be wrong about no callers already holding mmap_sem,
I still think calling gup without the mutex makes sense for
improved paralellism. Now, if callers can hold the mmap_sem,
it's wrong to do copy_from_user right before calling mthca_map_user_db.

drivers/infiniband/hw/mthca/mthca_memfree.c | 20 +++++++++++++++++++-
1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/infiniband/hw/mthca/mthca_memfree.c b/drivers/infiniband/hw/mthca/mthca_memfree.c
index c6fe89d79248..046871878a02 100644
--- a/drivers/infiniband/hw/mthca/mthca_memfree.c
+++ b/drivers/infiniband/hw/mthca/mthca_memfree.c
@@ -472,9 +472,27 @@ int mthca_map_user_db(struct mthca_dev *dev, struct mthca_uar *uar,
goto out;
}

- ret = get_user_pages(uaddr & PAGE_MASK, 1, FOLL_WRITE, pages, NULL);
+ mutex_unlock(&db_tab->mutex);
+
+ ret = get_user_pages_fast(uaddr & PAGE_MASK, 1, FOLL_WRITE, pages);
if (ret < 0)
+ return ret;
+
+ mutex_lock(&db_tab->mutex);
+
+ if (db_tab->page[i].refcount >= MTHCA_DB_REC_PER_PAGE ||
+ (db_tab->page[i].uvirt && db_tab->page[i].uvirt != uaddr)) {
+ put_page(pages[0]);
+ ret = -EINVAL;
goto out;
+ }
+
+ /* page was already mapped by another task while we were doing gup */
+ if (db_tab->page[i].refcount) {
+ put_page(pages[0]);
+ ++db_tab->page[i].refcount;
+ goto out;
+ }

sg_set_page(&db_tab->page[i].mem, pages[0], MTHCA_ICM_PAGE_SIZE,
uaddr & ~PAGE_MASK);
--
2.13.6