[Mesa-dev] [PATCH] nouveau: Use dup fd as key in drm-winsys hash table to fix ZaphodHeads.

[Ilia Mirkin]

How about:

/* Use dupfd in hash table, to avoid errors if the original fd gets
* closed by its owner. The hash key needs to live at least as long as
* the screen.
*/


On Sun, Jun 28, 2015 at 12:57 AM, Mario Kleiner
<mario.kleiner.de at gmail.com> wrote:
> On 06/28/2015 06:25 AM, Ilia Mirkin wrote:
>>
>> But that's the thing... in this case, the fd lifetime is owned by the
>> X server. In fact, nouveau doesn't touch that fd in mesa beyond
>> dup'ing it, and then exclusively using the dup'd fd.
>>
>
> True, it's not owned by nouveau, but instead by mesa core code? I think the
> x-server itself is only involved in authenticating the fd under DRI2, after
> mesa has opened it? Although under DRI3 the ready made fd comes from a
> xcb_dri3_open() trip to the server.
>
> See the open() call on the device file:
> dri2CreateScreen() in mesa/src/glx/dri2_glx.c
>
> xcb_dri3_open():
> dri3_create_screen() in mesa/src/glx/dri3_glx.c
>
> And the close() calls for the fd's are also inside those files.
>
> Maybe those winsys functions we fixed here are also involved in things like
> EGL for wayland etc.? Haven't checked this.
>
> Other than that your new commit message is fine. Also i am possibly just
> confused about this, and your commit message is an improvement in clarity
> anyway :)
>
> So i think it is fine to leave it as is.
>
> thanks,
> -mario
>
>
>
>> On Sun, Jun 28, 2015 at 12:23 AM, Mario Kleiner
>> <mario.kleiner.de at gmail.com> wrote:
>>>
>>> Ok, maybe one thing for the commit message, as i just made the same mixup
>>> in
>>> my reply: The fd's are not owned by the x-server, but by the mesa screens
>>> (pipe screens? dri screens?) they represent, so if such a screen goes
>>> away,
>>> the fd goes away. Using "X server" would be confusing, especially under
>>> dri3, although each such "mesa screen" corresponds to a x-screen.
>>>
>>> -mario
>>>
>>>
>>> On 06/28/2015 06:03 AM, Mario Kleiner wrote:
>>>>
>>>>
>>>> On 06/28/2015 05:41 AM, Ilia Mirkin wrote:
>>>>>
>>>>>
>>>>> Oh duh. Thanks for the super-detailed explanation. To rephrase what
>>>>> you said in a slightly shorter manner:
>>>>>
>>>>> See bug https://bugs.freedesktop.org/show_bug.cgi?id=79823 for why we
>>>>> need to dupfd (which we are already, although radeon might not be).
>>>>>
>>>>
>>>> Radeon doesn't so far. My 2nd patch for radeon from earlier today adds
>>>> that.
>>>>
>>>>> Except instead of sticking the dup'd fd into the hash table, whose
>>>>> lifetime matches that of the device, we stick the other one in, which
>>>>> is effectively owned by the X server. As a result, those fstat's might
>>>>> fail down the line, and all sorts of hell will break loose.
>>>>>
>>>>
>>>> Yes. Essentially we should make sure the fd's we keep around have the
>>>> same lifetime as the data structure in which we need to use it. That was
>>>> the point, the server owned fd went away while we still needed it.
>>>>
>>>>> Would you be opposed to me rewriting your commit message to basically
>>>>> reflect the above? Perhaps your original one did as well, but it
>>>>> wasn't clear to me. I'd also like to throw some assert's in to make
>>>>> sure the fstat's don't fail -- does that sound reasonable?
>>>>>
>>>>
>>>> Oh please, yes! My commit messages often have this disease that they are
>>>> either too terse, when i think the problem is obvious, or too long and
>>>> somewhat convoluted when i'm going overboard in the other direction. Any
>>>> editing/shortening for clarity more than happily accepted :)
>>>>
>>>> thanks,
>>>> -mario
>>>>
>>>>> Another solution, btw, is to stick <stat.st_dev, stat.st_ino,
>>>>> stat.st_rdev> as the real key in the hash, although that'd involve
>>>>> making pointers. Probably not worth it.
>>>>>
>>>>> Cheers,
>>>>>
>>>>>     -ilia
>>>>>
>>>>> On Sat, Jun 27, 2015 at 11:13 PM, Mario Kleiner
>>>>> <mario.kleiner.de at gmail.com> wrote:
>>>>>>
>>>>>>
>>>>>> On 06/28/2015 03:48 AM, Ilia Mirkin wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Fri, Jun 5, 2015 at 9:36 AM, Mario Kleiner
>>>>>>> <mario.kleiner.de at gmail.com> wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> The dup'ed fd owned by the nouveau_screen for a device node
>>>>>>>> must also be used as key for the winsys hash table, instead
>>>>>>>> of using the original fd passed in for a screen, to make
>>>>>>>> multi-x-screen ZaphodHeads configurations work on nouveau.
>>>>>>>>
>>>>>>>> This prevents the following crash scenario that was observed
>>>>>>>> when a dynamically loaded rendering plugin used OpenGL on a
>>>>>>>> ZaphodHeads setup, e.g., on a dual x-screen setup. At first
>>>>>>>> load the plugin worked, but after unloading and reloading it,
>>>>>>>> the next rendering operation crashed:
>>>>>>>>
>>>>>>>> 1. Client, e.g., a plugin, calls glXQueryVersion.
>>>>>>>>
>>>>>>>> 2. DRI screens for the x-screens 0 and 1 are created, one shared
>>>>>>>>       nouveau_screen is created for the shared device node of both
>>>>>>>>       screens, but the original fd of x-screen 0 is used as
>>>>>>>> identifying
>>>>>>>>       key in the hash table, instead of the dup()ed fd of x-screen 0
>>>>>>>>       which is owned by the nouveau_screen. nouveau_screen's
>>>>>>>> refcount
>>>>>>>>       is now 2.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> See below, but it shouldn't matter which fd gets used.
>>>>>>>
>>>>>>>>
>>>>>>>> 3. Regular rendering happens by the client plugin, then the plugin
>>>>>>>>       gets unloaded.
>>>>>>>>
>>>>>>>> 4. XCloseDisplay(). x-screen 0 gets its DRI screen destroyed,
>>>>>>>>       nouveau_drm_screen_unref() drops the refcount to 1, calling
>>>>>>>> mesa
>>>>>>>>       code then closes the fd of x-screen 0, so now the fd which is
>>>>>>>>       used as key in the hash table is invalid. x-screen 1 gets
>>>>>>>>       destroyed, nouveau_drm_screen_unref() drops the refcount to 0,
>>>>>>>>       the nouveau_screen gets destroyed, but removal of its entry
>>>>>>>>       in the hash table fails, because the invalid fd in the hash
>>>>>>>>       table no longer matches anything (fstat() on the fd is used
>>>>>>>>       for hashing and key comparison, but fstat() on an already
>>>>>>>> closed
>>>>>>>>       fd fails and returns bogus results). x-screen 1 closes its fd.
>>>>>>>>
>>>>>>>>       Now all fd's are closed, the nouveau_screen destroyed, but
>>>>>>>>       there is a dangling reference to the nouveau_screen in the
>>>>>>>>       hash table.
>>>>>>>>
>>>>>>>> 5. Some OpenGL client plugin gets loaded again and calls
>>>>>>>>       glXQueryVersion. Step 2 above repeats, but because a
>>>>>>>>       dangling reference with a matching fd is found in the winsys
>>>>>>>>       hash table, no new nouveau_screen is created this time.
>>>>>>>> Instead
>>>>>>>>       the invalid pointer to the old nouveau_screen is recycled,
>>>>>>>>       which points to nirvana -> Crash.
>>>>>>>>
>>>>>>>> This problem is avoided by use of the dup()ed fd which is
>>>>>>>> owned by the nouveau_screen and has the same lifetime as
>>>>>>>> the nouveau_screen itself.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> I need to think about this some more, but... this shouldn't happen :)
>>>>>>>
>>>>>>> In fact, the whole dupfd thing was added there for ZaphodHeads
>>>>>>> screens
>>>>>>> in the first place. See
>>>>>>> https://bugs.freedesktop.org/show_bug.cgi?id=79823 and commit
>>>>>>> a59f2bb17bcc which fixed it.
>>>>>>>
>>>>>>> Note that the hash has the following hash/eq functions:
>>>>>>>
>>>>>>> static unsigned hash_fd(void *key)
>>>>>>> {
>>>>>>>        int fd = pointer_to_intptr(key);
>>>>>>>        struct stat stat;
>>>>>>>        fstat(fd, &stat);
>>>>>>>
>>>>>>>        return stat.st_dev ^ stat.st_ino ^ stat.st_rdev;
>>>>>>> }
>>>>>>>
>>>>>>> static int compare_fd(void *key1, void *key2)
>>>>>>> {
>>>>>>>        int fd1 = pointer_to_intptr(key1);
>>>>>>>        int fd2 = pointer_to_intptr(key2);
>>>>>>>        struct stat stat1, stat2;
>>>>>>>        fstat(fd1, &stat1);
>>>>>>>        fstat(fd2, &stat2);
>>>>>>>
>>>>>>>        return stat1.st_dev != stat2.st_dev ||
>>>>>>>               stat1.st_ino != stat2.st_ino ||
>>>>>>>               stat1.st_rdev != stat2.st_rdev;
>>>>>>> }
>>>>>>>
>>>>>>> so fd and dupfd should get hashed to the same thing. I suspect
>>>>>>> there's
>>>>>>> something else going on in your application...
>>>>>>>
>>>>>>
>>>>>> My application is a set of dynamically loaded plugins running inside
>>>>>> another
>>>>>> host app (Psychtoolbox-3 inside GNU/Octave), so what happens often is
>>>>>> that
>>>>>> the OpenGL using plugin gets unloaded and reloaded at runtime, so a
>>>>>> after a
>>>>>> OpenGL session has ended with a XCloseDisplay() tearing the winsys
>>>>>> down, you
>>>>>> can easily have it restart at plugin reload with another XOpenDisplay
>>>>>> ->
>>>>>> glXQueryVersion -> .... sequence, where the new call to
>>>>>> glXQueryVersion will
>>>>>> trigger a recreation of the winsys, which will find the stale entry
>>>>>> in the
>>>>>> hash table pointing to nowhere instead of the previously released
>>>>>> nouveau_screen -> use-after-free -> boom!
>>>>>>
>>>>>> The reason this fails is because during destruction of the 2nd, 3rd
>>>>>> etc.
>>>>>> x-screen, the already closed fd associated with the 1st x-screen is
>>>>>> fed into
>>>>>> the compare_fd function, so fstat() errors out on the invalid fd. I
>>>>>> added
>>>>>> printf's etc. on both nouveau and now radeon to verify the fstat
>>>>>> gives me
>>>>>> EBADF errors. So the hash calculation goes wrong when trying to find
>>>>>> the
>>>>>> matching element in the hash table with a fd that has a matching hash
>>>>>> -> The
>>>>>> element which should be removed is ignored/not removed because it
>>>>>> contains
>>>>>> an already closed fd for which no proper hash can be calculated
>>>>>> anymore ->
>>>>>> hash comparison during search goes wrong.
>>>>>>
>>>>>> This is because multiple x-screens, e.g., 2 x-screens are destroyed
>>>>>> in order
>>>>>> 0, 1 by FreeScreenConfigs() as part of XCloseDisplay().
>>>>>> FreeScreenConfigs()
>>>>>> calls dri2DestroyScreen() in dri2.c or dri3_destroy_screen in dri3.c,
>>>>>> which
>>>>>> are essentially identical, e.g.,:
>>>>>>
>>>>>> static void
>>>>>> dri2DestroyScreen(struct glx_screen *base)
>>>>>> {
>>>>>>      struct dri2_screen *psc = (struct dri2_screen *) base;
>>>>>>
>>>>>>      /* Free the direct rendering per screen data */
>>>>>>      (*psc->core->destroyScreen) (psc->driScreen);
>>>>>>
>>>>>> --> This core->destroyScreen calls eventually into the winsys, which
>>>>>> then
>>>>>> drops the refcount of the nouveau_screen or radeon winsys structure
>>>>>> by one,
>>>>>> and tries to release the struct and remove the hash table entry once
>>>>>> the
>>>>>> refcount drops to zero.
>>>>>>
>>>>>>      driDestroyConfigs(psc->driver_configs);
>>>>>>      close(psc->fd);
>>>>>>
>>>>>> -> This close() closes the fd of a screen immediately after closing
>>>>>> down
>>>>>> that screen. At the time screen 1 is closed down, the fd associated
>>>>>> with
>>>>>> screen 0 is already closed.
>>>>>>
>>>>>>      free(psc);
>>>>>> }
>>>>>>
>>>>>> So you have this sequence during creation:
>>>>>>
>>>>>> glXQueryVersion() -> AllocAndFetchScreenConfigs():
>>>>>>
>>>>>> create screen 0 and its fd, e.g., fd==5, dup(fd), e.g., fd==6 for the
>>>>>> nouveau_screen, but store x-screen 0's fd==5 *itself* in the hash
>>>>>> table as
>>>>>> key.
>>>>>>
>>>>>> create screen 1. This now creates an fd 7
>>>>>>
>>>>>> => refcount is now 2.
>>>>>>
>>>>>>
>>>>>> And this sequence during destruction at XCloseDisplay();
>>>>>>
>>>>>> FreeScreenConfigs():
>>>>>>
>>>>>> free screen 0: core->destroyScreen() -> ... ->
>>>>>> nouveau_drm_screen_unref() =>
>>>>>> refcount-- is now 1
>>>>>> close(fd==5) of screen 0 and thereby the fd==5 stored inside the hash
>>>>>> table
>>>>>> as key. From now on using fcntl(fd==5) for hash calculation on that
>>>>>> element
>>>>>> will malfunction.
>>>>>>
>>>>>> free screen 1: -> nouveau_drm_screen_unref -> refcount-- is now 0. ->
>>>>>> Try to
>>>>>> remove hash table entry => fails because fcntl(fd==5) on the already
>>>>>> closed
>>>>>> fd==5 of screen 0 fails with EBADF. => Hash table keeps its stale
>>>>>> element
>>>>>> referencing the struct nouveau_screen.
>>>>>>
>>>>>> Free nouveau_screen struct (close dup()ed fd==6 for nouveau_screen,
>>>>>> close
>>>>>> down stuff, free the struct).
>>>>>> -> close(fd==7) associated with screen 1.
>>>>>>
>>>>>> Now all fd's (5,6,7) are closed, the struct is gone, the hash table
>>>>>> contains
>>>>>> a stale element with a no longer existent fd==5 and a pointer to an
>>>>>> already
>>>>>> free()d struct nouveau_screen.
>>>>>>
>>>>>> Now we reload the plugin and do glXQueryVersion() again, so
>>>>>> AllocAndFetchScreenConfigs() is executed again to create the winsys
>>>>>> etc. for
>>>>>> screens 0 and 1.
>>>>>>
>>>>>> create screen 0 and its fd: Here the first unused fd in the
>>>>>> filedescriptor
>>>>>> table is recycled, which happens to be fd==5, just as in the first
>>>>>> session
>>>>>> with the plugin! Now suddenly fd=5 is again the valid fd for x-screen
>>>>>> 0, and
>>>>>> it points to the same /dev/drm/card0 device file as before. That
>>>>>> means when
>>>>>> nouveau_drm_screen_create(fd==5) is called, it finds the stale
>>>>>> element in
>>>>>> the hash table from the previous session, because the previously
>>>>>> closed fd 5
>>>>>> in that element is now valid and open again and points to the same
>>>>>> device
>>>>>> file as in the previous session, ergo has the same hash etc.
>>>>>> nouveau_drm_screen_create() concludes that a fully initialized
>>>>>> nouveau_screen already exists for this session, except it doesn't
>>>>>> exist -
>>>>>> the pointer in the hash table points to invalid previously freed
>>>>>> memory.
>>>>>> When it tries to access that memory we have a use-after-free and the
>>>>>> application segfaults.
>>>>>>
>>>>>> So it depends a bit on what happens in the applications memory
>>>>>> between the
>>>>>> two consecutive OpenGL sessions etc. how long it takes for an invalid
>>>>>> memory
>>>>>> access to happen, but eventually it hits invalid data. Maybe
>>>>>> occassionally
>>>>>> it gets lucky and the freed memory is still accessible and the struct
>>>>>> intact, so stuff works by chance.
>>>>>>
>>>>>> If the application happened to open other files inbetween the 1st and
>>>>>> 2nd
>>>>>> session, then the relevant fd in the 2nd session may not be exactly
>>>>>> the same
>>>>>> as in the first session, because recycling that fd didn't work, so we
>>>>>> are
>>>>>> only left with a dead but harmless entry in the hash table instead of
>>>>>> a
>>>>>> crasher. I assume something like that prevented my crashes on radeon
>>>>>> in the
>>>>>> past, because when i wasn't expecting/looking for trouble i didn't use
>>>>>> a
>>>>>> test sequence carefully made to trigger the bug for certain.
>>>>>>
>>>>>> Similar things happen on radeon for the same reason, ergo the 2nd
>>>>>> patch with
>>>>>> a similar fix.
>>>>>>
>>>>>> I don't have the gdb backtrace for nouveau anymore, but the traces for
>>>>>> radeon are attached to this mail to show you the flow of execution,
>>>>>> similar
>>>>>> to nouveau.
>>>>>>
>>>>>> -mario