[PATCH v2] dma-buf: Move BUG_ON from _add_shared_fence to _add_shared_inplace

From: Michel DÃnzer
Date: Wed Jul 04 2018 - 11:15:26 EST


From: Michel DÃnzer <michel.daenzer@xxxxxxx>

Fixes the BUG_ON spuriously triggering under the following
circumstances:

* reservation_object_reserve_shared is called with shared_count ==
shared_max - 1, so obj->staged is freed in preparation of an in-place
update.

* reservation_object_add_shared_fence is called with the first fence,
after which shared_count == shared_max.

* reservation_object_add_shared_fence is called with a follow-up fence
from the same context.

In the second reservation_object_add_shared_fence call, the BUG_ON
triggers. However, nothing bad would happen in
reservation_object_add_shared_inplace, since both fences are from the
same context, so they only occupy a single slot.

Prevent this by moving the BUG_ON to where an overflow would actually
happen (e.g. if a buggy caller didn't call
reservation_object_reserve_shared before).

v2:
* Fix description of breaking scenario (Christian KÃnig)
* Add bugzilla reference

Cc: stable@xxxxxxxxxxxxxxx
Bugzilla: https://bugs.freedesktop.org/106418
Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> # v1
Reviewed-by: Christian KÃnig <christian.koenig@xxxxxxx> # v1
Signed-off-by: Michel DÃnzer <michel.daenzer@xxxxxxx>
---
drivers/dma-buf/reservation.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
index 314eb1071cce..532545b9488e 100644
--- a/drivers/dma-buf/reservation.c
+++ b/drivers/dma-buf/reservation.c
@@ -141,6 +141,7 @@ reservation_object_add_shared_inplace(struct reservation_object *obj,
if (signaled) {
RCU_INIT_POINTER(fobj->shared[signaled_idx], fence);
} else {
+ BUG_ON(fobj->shared_count >= fobj->shared_max);
RCU_INIT_POINTER(fobj->shared[fobj->shared_count], fence);
fobj->shared_count++;
}
@@ -230,10 +231,9 @@ void reservation_object_add_shared_fence(struct reservation_object *obj,
old = reservation_object_get_list(obj);
obj->staged = NULL;

- if (!fobj) {
- BUG_ON(old->shared_count >= old->shared_max);
+ if (!fobj)
reservation_object_add_shared_inplace(obj, old, fence);
- } else
+ else
reservation_object_add_shared_replace(obj, old, fobj, fence);
}
EXPORT_SYMBOL(reservation_object_add_shared_fence);
--
2.18.0