linux-kernel - Re: [PATCH resend] binder: Prevent context manager from incrementing ref 0

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Date:   Fri, 10 Jul 2020 12:27:08 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <christian@...uner.io>,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        Mattias Nissler <mnissler@...gle.com>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH resend] binder: Prevent context manager from incrementing
 ref 0

On Fri, Jul 10, 2020 at 8:54 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> On Fri, Jul 10, 2020 at 12:39:48AM +0200, Jann Horn wrote:
> > Binder is designed such that a binder_proc never has references to
> > itself. If this rule is violated, memory corruption can occur when a
> > process sends a transaction to itself; see e.g.
> > <https://syzkaller.appspot.com/bug?extid=09e05aba06723a94d43d>.
> >
> > There is a remaining edgecase through which such a transaction-to-self
> > can still occur from the context of a task with BINDER_SET_CONTEXT_MGR
> > access:
> >
> >  - task A opens /dev/binder twice, creating binder_proc instances P1
> >    and P2
> >  - P1 becomes context manager
> >  - P2 calls ACQUIRE on the magic handle 0, allocating index 0 in its
> >    handle table
> >  - P1 dies (by closing the /dev/binder fd and waiting a bit)
> >  - P2 becomes context manager
> >  - P2 calls ACQUIRE on the magic handle 0, allocating index 1 in its
> >    handle table
> >    [this triggers a warning: "binder: 1974:1974 tried to acquire
> >    reference to desc 0, got 1 instead"]
> >  - task B opens /dev/binder once, creating binder_proc instance P3
> >  - P3 calls P2 (via magic handle 0) with (void*)1 as argument (two-way
> >    transaction)
> >  - P2 receives the handle and uses it to call P3 (two-way transaction)
> >  - P3 calls P2 (via magic handle 0) (two-way transaction)
> >  - P2 calls P2 (via handle 1) (two-way transaction)
> >
> > And then, if P2 does *NOT* accept the incoming transaction work, but
> > instead closes the binder fd, we get a crash.
> >
> > Solve it by preventing the context manager from using ACQUIRE on ref 0.
> > There shouldn't be any legitimate reason for the context manager to do
> > that.
> >
> > Additionally, print a warning if someone manages to find another way to
> > trigger a transaction-to-self bug in the future.
> >
> > Cc: stable@...r.kernel.org
> > Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
> > Signed-off-by: Jann Horn <jannh@...gle.com>
> > Acked-by: Todd Kjos <tkjos@...gle.com>
> > ---
> > sending again because I forgot to CC LKML the first time... sorry about
> > the spam.
>
> This spits out a bunch of warnings when built, how did it work on your
> end?

... by creating the patch file before fixing the warnings. Sigh. I'll
send the proper patch as v2...