linux-kernel - Re: [BUG] completely bonkers use of set_need_resched + VM_FAULT

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Message-ID: <CAKMK7uFkav0b+8AYkm_3eT=d6T6wvstHuOXkTwFt_Do4YmijQA@mail.gmail.com>
Date:	Fri, 13 Sep 2013 10:32:48 +0200
From:	Daniel Vetter <daniel.vetter@...ll.ch>
To:	Thomas Hellstrom <thellstrom@...are.com>
Cc:	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dave Airlie <airlied@...ux.ie>,
	intel-gfx <intel-gfx@...ts.freedesktop.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [BUG] completely bonkers use of set_need_resched + VM_FAULT_NOPAGE

On Fri, Sep 13, 2013 at 10:23 AM, Thomas Hellstrom
<thellstrom@...are.com> wrote:
> As previously mentioned, copy_from_user should return -EFAULT, since the
> VMAs are marked with VM_IO. It should not recurse into fault(), so evil
> user-space looses.

I haven't put a printk in the code to prove this, but gem mmap also
sets VM_IO in drm_gem_mmap_obj. And we can very much hit our own fault
handler and deadlock ....

On a _very_ quick reading (and definitely not enough coffee yet for
reading mm/* stuff) it looks like it's get_user_pages that will return
an -EFAULT when hitting upon a VM_IO mapping (which makes sense since
there's really no page backing it). Actually using get_user_pages was
the original slowpath we've had in a few places until we've noticed
that for pwrite that breaks legit userspace (the glBufferData(glMap))
use-case), so we've switched to lock dropping and proper slowpaths
using copy_*_user everywhere instead of trying to pin the userspace
storage with get_user_pages.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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