linux-kernel - Re: [PATCH 3/3] binder: Handle start==NULL in binder_update_page

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Message-Id: <BXTJW0B3Y8RD.1TSRR5G3JNB20@wittgenstein>
Date:   Sat, 19 Oct 2019 15:55:18 +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 3/3] binder: Handle start==NULL in
 binder_update_page_range()

On Fri Oct 18, 2019 at 10:56 PM Jann Horn wrote:
> The old loop wouldn't stop when reaching `start` if `start==NULL`, instead
> continuing backwards to index -1 and crashing.
> 
> Luckily you need to be highly privileged to map things at NULL, so it's not
> a big problem.
> 
> Fix it by adjusting the loop so that the loop variable is always in bounds.
> 
> This patch is deliberately minimal to simplify backporting, but IMO this
> function could use a refactor. The jump labels in the second loop body are
> horrible (the error gotos should be jumping to free_range instead), and
> both loops would look nicer if they just iterated upwards through indices.
> And the up_read()+mmput() shouldn't be duplicated like that.
> 
> Cc: stable@...r.kernel.org
> Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
> Signed-off-by: Jann Horn <jannh@...gle.com>

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