linux-kernel - Re: [PATCH 1/7] binder: fix alloc->vma_vm

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Message-ID: <Yw5nwaNI5ewExYtC@google.com>
Date:   Tue, 30 Aug 2022 19:40:49 +0000
From:   Carlos Llamas <cmllamas@...gle.com>
To:     Liam Howlett <liam.howlett@...cle.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <brauner@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "kernel-team@...roid.com" <kernel-team@...roid.com>,
        "syzbot+f7dc54e5be28950ac459@...kaller.appspotmail.com" 
        <syzbot+f7dc54e5be28950ac459@...kaller.appspotmail.com>,
        "syzbot+a75ebe0452711c9e56d9@...kaller.appspotmail.com" 
        <syzbot+a75ebe0452711c9e56d9@...kaller.appspotmail.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/7] binder: fix alloc->vma_vm_mm null-ptr dereference

On Tue, Aug 30, 2022 at 07:06:37PM +0000, Liam Howlett wrote:
> > diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> > index 51f4e1c5cd01..9b1778c00610 100644
> > --- a/drivers/android/binder_alloc.c
> > +++ b/drivers/android/binder_alloc.c
> > @@ -322,7 +322,6 @@ static inline void binder_alloc_set_vma(struct binder_alloc *alloc,
> >  	 */
> >  	if (vma) {
> >  		vm_start = vma->vm_start;
> > -		alloc->vma_vm_mm = vma->vm_mm;
> 
> Is this really the null pointer dereference?  We check for vma above..?
> 

Not here. The sequence leading to the null-ptr-deref happens when we try
to take alloc->vma_vm_mm->mmap_lock in binder_alloc_new_buf_locked() and
in binder_alloc_print_pages() without initializing alloc->vma_vm_mm
first (e.g. mmap() was never called). These sequences are described in
the commit message but basically they translate to mmap_read_lock(NULL)
calls.