Am 18.06.21 um 17:17 schrieb Daniel Vetter:
[SNIP]
Ignoring _all_ fences is officially ok for pinned dma-buf. This is
what v4l does. Aside from it's definitely not just i915 that does this
even on the drm side, we have a few more drivers nowadays.
No it seriously isn't. If drivers are doing this they are more than broken.
See the comment in dma-resv.h
* Based on bo.c which bears the following copyright notice,
* but is dual licensed:
....
The handling in ttm_bo.c is and always was that the exclusive fence is
used for buffer moves.
As I said multiple times now the *MAIN* purpose of the dma_resv object
is memory management and *NOT* synchronization.
Those restrictions come from the original design of TTM where the
dma_resv object originated from.
The resulting consequences are that:
a) If you access the buffer without waiting for the exclusive fence you
run into a potential information leak.
We kind of let that slip for V4L since they only access the buffers
for writes, so you can't do any harm there.
b) If you overwrite the exclusive fence with a new one without waiting
for the old one to signal you open up the possibility for userspace to
access freed up memory.
This is a complete show stopper since it means that taking over the
system is just a typing exercise.
What you have done by allowing this in is ripping open a major security
hole for any DMA-buf import in i915 from all TTM based driver.
This needs to be fixed ASAP, either by waiting in i915 and all other
drivers doing this for the exclusive fence while importing a DMA-buf or
by marking i915 and all other drivers as broken.
Sorry, but if you allowed that in you seriously have no idea what you
are talking about here and where all of this originated from.
Regards,
Christian.
