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Date:   Fri, 6 Jan 2023 19:59:18 +0100
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Hang Zhang <zh.nvgt@gmail.com>
Cc:     Daniel Vetter <daniel@ffwll.ch>, Helge Deller <deller@gmx.de>,
        Javier Martinez Canillas <javierm@redhat.com>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Sam Ravnborg <sam@ravnborg.org>,
        Alex Deucher <alexander.deucher@amd.com>,
        linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fbmem: prevent potential use-after-free issues with
 console_lock()
Message-ID: <Y7hvhtHeivfsnBtE@phenom.ffwll.local>
Mail-Followup-To: Hang Zhang <zh.nvgt@gmail.com>,
        Helge Deller <deller@gmx.de>,
        Javier Martinez Canillas <javierm@redhat.com>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Sam Ravnborg <sam@ravnborg.org>,
        Alex Deucher <alexander.deucher@amd.com>,
        linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org
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On Thu, Jan 05, 2023 at 01:38:54PM -0500, Hang Zhang wrote:
> On Thu, Jan 5, 2023 at 5:25 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> >
> > On Thu, 5 Jan 2023 at 11:21, Daniel Vetter <daniel@ffwll.ch> wrote:
> > >
> > > Hi Helge
> > >
> > > On Mon, 2 Jan 2023 at 16:28, Helge Deller <deller@gmx.de> wrote:
> > > >
> > > > On 12/30/22 07:35, Hang Zhang wrote:
> > > > > In do_fb_ioctl(), user specified "fb_info" can be freed in the callee
> > > > > fbcon_get_con2fb_map_ioctl() -> set_con2fb_map() ->
> > > > > con2fb_release_oldinfo(), this free operation is protected by
> > > > > console_lock() in fbcon_set_con2fb_map_ioctl(), it also results in
> > > > > the change of certain states such as "minfo->dead" in matroxfb_remove(),
> > > > > so that it can be checked to avoid use-after-free before the use sites
> > > > > (e.g., the check at the beginning of matroxfb_ioctl()). However,
> > > > > the problem is that the use site is not protected by the same locks
> > > > > as for the free operation, e.g., "default" case in do_fb_ioctl()
> > > > > can lead to "matroxfb_ioctl()" but it's not protected by console_lock(),
> > > > > which can invalidate the aforementioned state set and check in a
> > > > > concurrent setting.
> > > > >
> > > > > Prevent the potential use-after-free issues by protecting the "default"
> > > > > case in do_fb_ioctl() with console_lock(), similarly as for many other
> > > > > cases like "case FBIOBLANK" and "case FBIOPAN_DISPLAY".
> > > > >
> > > > > Signed-off-by: Hang Zhang <zh.nvgt@gmail.com>
> > > >
> > > > applied to fbdev git tree.
> > >
> > > The patch above makes no sense at all to me:
> > >
> > > - fb_info is protected by lock_fb_info and
> > > - the lifetime of fb_info is protected by the get/put functions
> > > - yes there's the interaction with con2fb, which is protected by
> > > console_lock, but the lifetime guarantees are ensured by the device
> > > removal
> > > - which means any stuff happening in matroxfb_remove is also not a
> > > concern here (unless matroxfb completely gets all the device lifetime
> > > stuff wrong, but it doesn't look like it's any worse than any of the
> > > other fbdev drivers that we haven't recently fixed up due to the
> > > takeover issues with firmware drivers
> >
> > I have also a really hard timing finding the con2fb map use in the
> > matroxfb ioctl code, but that just might be that I didn't look
> > carefully enough. Maybe that would shed some light on this.
> > -Daniel
> >
> >
> > >
> > > On the very clear downside this now means we take console_lock for the
> > > vblank ioctl (which is a device driver extension for reasons, despite
> > > that it's a standard fbdev ioctl), which is no good at all given how
> > > console_lock() is a really expensive lock.
> > >
> > > Unless I'm massively missing something, can you pls push the revert
> > > before this lands in Linus' tree?
> > >
> > > Thanks, Daniel
> 
> Hi, Daniel. Thank you for your feedback. We're not developers of the
> video subsystem and thus may be short in domain knowledge (e.g., the
> performance of console_lock() and the complex lifetime management).
> This patch initially intended to bring up the potential use-after-free
> issues here to the community - we have performed a best-effort code
> review and cannot exclude the possibility based on our understanding.
> 
> What we have observed is that the call chain leading to the free site
> (do_fb_ioctl()->fbcon_set_con2fb_map_ioctl()->set_con2fb_map()->
> con2fb_release_oldinfo()-> ... ->matroxfb_remove()) is only protected
> by console_lock() but not lock_fb_info(), while the potential use
> site (call chain starts from the default case in do_fb_ioctl()) is
> only protected by lock_fb_info() but not console_lock().
> We thus propose to add this extra console_lock() to the default case,
> which is inspired by the lock protection of many other existing
> switch-case terms in the same function.
> 
> Since we do not have deep domain knowledge of this subsystem, we will
> rely on the developers to make a decision regarding the patch. Thank
> you again for your review and help!

Can you please elaborate where you've found this use-after-free and how?
I'm still not understanding how you even got here - this is orthogonal to
whether the patch is the right fix or not.
-Daniel

> 
> Best,
> Hang
> 
> > >
> > > > Thanks,
> > > > Helge
> > > >
> > > > > ---
> > > > >   drivers/video/fbdev/core/fbmem.c | 2 ++
> > > > >   1 file changed, 2 insertions(+)
> > > > >
> > > > > diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> > > > > index 1e70d8c67653..8b1a1527d18a 100644
> > > > > --- a/drivers/video/fbdev/core/fbmem.c
> > > > > +++ b/drivers/video/fbdev/core/fbmem.c
> > > > > @@ -1182,6 +1182,7 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
> > > > >               console_unlock();
> > > > >               break;
> > > > >       default:
> > > > > +             console_lock();
> > > > >               lock_fb_info(info);
> > > > >               fb = info->fbops;
> > > > >               if (fb->fb_ioctl)
> > > > > @@ -1189,6 +1190,7 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
> > > > >               else
> > > > >                       ret = -ENOTTY;
> > > > >               unlock_fb_info(info);
> > > > > +             console_unlock();
> > > > >       }
> > > > >       return ret;
> > > > >   }
> > > >
> > >
> > >
> > > --
> > > Daniel Vetter
> > > Software Engineer, Intel Corporation
> > > http://blog.ffwll.ch
> >
> >
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch