linux-kernel - Re: [PATCH] binder: Address corner cases in deferred copy and fixup

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Date:   Tue, 19 Apr 2022 11:42:15 -0700
From:   Todd Kjos <tkjos@...gle.com>
To:     Alessandro Astone <ales.astone@...il.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>, tkjos@...roid.com,
        brauner@...nel.org, arve@...roid.com, linux-kernel@...r.kernel.org,
        maco@...roid.com, Joel Fernandes <joel@...lfernandes.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH] binder: Address corner cases in deferred copy and fixup

On Wed, Apr 13, 2022 at 4:20 AM Alessandro Astone <ales.astone@...il.com> wrote:
>
> On 13/04/2022 12:00, Greg KH wrote:
>
> > On Wed, Apr 13, 2022 at 10:54:27AM +0200, Alessandro Astone wrote:
> >> When handling BINDER_TYPE_FDA object we are pushing a parent fixup
> >> with a certain skip_size but no scatter-gather copy object, since
> >> the copy is handled standalone.
> >> If BINDER_TYPE_FDA is the last children the scatter-gather copy
> >> loop will never stop to skip it, thus we are left with an item in
> >> the parent fixup list. This will trigger the BUG_ON().
> >>
> >> Furthermore, it is possible to receive BINDER_TYPE_FDA object
> >> with num_fds=0 which will confuse the scatter-gather code.
> >>
> >> In the android userspace I could only find these usecases in the
> >> libstagefright OMX implementation, so it might be that they're
> >> doing something very weird, but nonetheless the kernel should not
> >> panic about it.
> >>
> >> Fixes: 09184ae9b575 ("binder: defer copies of pre-patched txn data")
> >> Signed-off-by: Alessandro Astone <ales.astone@...il.com>
> >> ---
> >>   drivers/android/binder.c | 11 +++++++++--
> >>   1 file changed, 9 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> >> index 8351c5638880..18ad6825ba30 100644
> >> --- a/drivers/android/binder.c
> >> +++ b/drivers/android/binder.c
> >> @@ -2295,7 +2295,7 @@ static int binder_do_deferred_txn_copies(struct binder_alloc *alloc,
> >>   {
> >>      int ret = 0;
> >>      struct binder_sg_copy *sgc, *tmpsgc;
> >> -    struct binder_ptr_fixup *pf =
> >> +    struct binder_ptr_fixup *tmppf, *pf =
> > Just make this a new line:
> >       struct binder_ptr_fixup *tmppf;
> > above the existing line.
> >
> Ack.
>
> >>              list_first_entry_or_null(pf_head, struct binder_ptr_fixup,
> >>                                       node);
> >>
> >> @@ -2349,7 +2349,11 @@ static int binder_do_deferred_txn_copies(struct binder_alloc *alloc,
> >>              list_del(&sgc->node);
> >>              kfree(sgc);
> >>      }
> >> -    BUG_ON(!list_empty(pf_head));
> > So you are hitting this BUG_ON() today?
> >
> Correct, both on 5.17, stable 5.17.2 and current master
>
> >> +    list_for_each_entry_safe(pf, tmppf, pf_head, node) {
> >> +            BUG_ON(pf->skip_size == 0);
> >> +            list_del(&pf->node);
> >> +            kfree(pf);
> >> +    }
> >>      BUG_ON(!list_empty(sgc_head));
> >>
> >>      return ret > 0 ? -EINVAL : ret;
> >> @@ -2486,6 +2490,9 @@ static int binder_translate_fd_array(struct list_head *pf_head,
> >>      struct binder_proc *proc = thread->proc;
> >>      int ret;
> >>
> >> +    if (fda->num_fds == 0)
> >> +            return 0;
> > Why return 0?
> >
> > This feels like a separate issue from above, should this be 2 different
> > commits?
> >
> return 0 because I want it to be handled as it was handled before
>         09184ae9b575 ("binder: defer copies of pre-patched txn data")
>
> Function `binder_do_deferred_txn_copies` distinguishes between a copy-fixup
> and a skip with `if (pf->skip_size)` so if the skip_size is 0, which happens
> if fda->num_fds is 0, it would accidentally enter the wrong branch.
> By returning 0 early i make sure a skip of size 0 is not added. It's not an
> error because it was never an error before commit 09184ae9b575 and some
> userspace in android is hitting this path.
>
> I would agree it's a separate issue.
> I originally merged it in this same patch because
> 1) Both are fixups to 09184ae9b575
> 2) Both are triggered by the same real-world android transaction that looks
>     something like this:
>       obj[0] BINDER_TYPE_PTR, parent
>       obj[1] BINDER_TYPE_PTR, child
>       obj[2] BINDER_TYPE_PTR, child
>       obj[3] BINDER_TYPE_FDA with num_fds=0
> 3) In the other hunk of this patch i replace the BUG_ON with:
>       BUG_ON(pf->skip_size == 0)
>     to only BUG if any item remaining in the pf_head list is not a skip,
>     but as observed we may receive skips of size 0.
> 4) With this hunk only, you would no longer reproduce the BUG_ON because the
>     only transaction we receive in android with BINDER_TYPE_FDA as last child
>     coincidentally always has num_fds=0. Certainly some weird behaviour...
>
> So if I split them, patch A would depend on patch B (see point 3), but the
> BUG of patch B would only be reproducible without patch A (see point 4).
>
> But let me know if you still prefer them split.

Thanks for the fix! Please go ahead and split it into two patches.

>
> > thanks,
> >
> > greg k-h