linux-kernel - Re: [PATCH 1/3] binder: Fix race between mmap() and binder_alloc_print

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Message-Id: <BXTJFV9XU16K.1ARG78JWMXYE9@wittgenstein>
Date:   Sat, 19 Oct 2019 15:34:14 +0200
From:   "Christian Brauner" <christian.brauner@...ntu.com>
To:     "Jann Horn" <jannh@...gle.com>,
        "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" <christian@...uner.io>, <jannh@...gle.com>
Cc:     <devel@...verdev.osuosl.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] binder: Fix race between mmap() and
 binder_alloc_print_pages()

On Fri Oct 18, 2019 at 10:56 PM Jann Horn wrote:
> binder_alloc_print_pages() iterates over
> alloc->pages[0..alloc->buffer_size-1] under alloc->mutex.
> binder_alloc_mmap_handler() writes alloc->pages and alloc->buffer_size
> without holding that lock, and even writes them before the last bailout
> point.
> 
> Unfortunately we can't take the alloc->mutex in the ->mmap() handler
> because mmap_sem can be taken while alloc->mutex is held.
> So instead, we have to locklessly check whether the binder_alloc has been
> fully initialized with binder_alloc_get_vma(), like in
> binder_alloc_new_buf_locked().
> 
> Fixes: 8ef4665aa129 ("android: binder: Add page usage in binder stats")
> Cc: stable@...r.kernel.org
> Signed-off-by: Jann Horn <jannh@...gle.com>

Ok, I see a smp_wmb() in binder_alloc_set_vma() which is called in
binder_alloc_mmap_handler() paired with a smp_rmb() in
binder_alloc_get_vma(). That makes sense to me.

Acked-by: Christian Brauner <christian.brauner@...ntu.com>