Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756642AbcKVTMj (ORCPT ); Tue, 22 Nov 2016 14:12:39 -0500 Received: from foss.arm.com ([217.140.101.70]:36788 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756625AbcKVTMi (ORCPT ); Tue, 22 Nov 2016 14:12:38 -0500 Date: Tue, 22 Nov 2016 18:47:42 +0000 From: Liviu Dudau To: Sean Paul Cc: Ville =?utf-8?B?U3lyasOkbMOk?= , Rob Clark , Jani Nikula , Daniel Vetter , Eric Engestrom , LKML , DRI devel Subject: Re: [PATCH] drm: check for NULL parameter in exported drm_get_format_name() function. Message-ID: <20161122184742.GU1005@e106497-lin.cambridge.arm.com> References: <20161122164106.31852-1-Liviu.Dudau@arm.com> <20161122165017.GC31595@intel.com> <20161122173157.GD31595@intel.com> <20161122180619.GE31595@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4073 Lines: 104 On Tue, Nov 22, 2016 at 01:15:08PM -0500, Sean Paul wrote: > On Tue, Nov 22, 2016 at 1:06 PM, Ville Syrjälä > wrote: > > On Tue, Nov 22, 2016 at 12:35:53PM -0500, Rob Clark wrote: > >> On Tue, Nov 22, 2016 at 12:31 PM, Ville Syrjälä > >> wrote: > >> > On Tue, Nov 22, 2016 at 12:23:59PM -0500, Rob Clark wrote: > >> >> On Tue, Nov 22, 2016 at 11:50 AM, Ville Syrjälä > >> >> wrote: > >> >> > On Tue, Nov 22, 2016 at 04:41:06PM +0000, Liviu Dudau wrote: > >> >> >> drm_get_format_name() de-references the buf parameter without checking > >> >> >> if the pointer was not NULL. Given that the function is EXPORT-ed, lets > >> >> >> sanitise the parameters before proceeding. > >> >> >> > >> >> >> Fixes: b3c11ac267d461d3d5 ("drm: move allocation out of drm_get_format_name()) > >> >> >> Cc: Eric Engestrom > >> >> >> Cc: Rob Clark > >> >> >> Cc: Jani Nikula > >> >> >> Cc: Daniel Vetter > >> >> >> > >> >> >> Signed-off-by: Liviu Dudau > >> >> >> --- > >> >> >> drivers/gpu/drm/drm_fourcc.c | 3 +++ > >> >> >> 1 file changed, 3 insertions(+) > >> >> >> > >> >> >> diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c > >> >> >> index 90d2cc8..0a3ff0b 100644 > >> >> >> --- a/drivers/gpu/drm/drm_fourcc.c > >> >> >> +++ b/drivers/gpu/drm/drm_fourcc.c > >> >> >> @@ -85,6 +85,9 @@ EXPORT_SYMBOL(drm_mode_legacy_fb_format); > >> >> >> */ > >> >> >> const char *drm_get_format_name(uint32_t format, struct drm_format_name_buf *buf) > >> >> >> { > >> >> >> + if (!buf) > >> >> >> + return NULL; > >> >> >> + > >> >> > > >> >> > Seems rather pointless to me. Why would you ever pass NULL to this guy? I don't know, maybe because I'm a malicious driver writer that wants to crash the kernel? The function is exported, so it can be called by any module, even out-of-tree. Looks like an easy DoS to me. > >> >> > >> >> perhaps BUG_ON(!buf)... > >> > > >> > And how does that differ from just buf->foo? > >> > >> it gets you a file and line # in the error splat.. not that > >> drm_get_format_name() is such a big function that it would be > >> difficult to decipher the null deref crash, but if we added anything > >> it should be BUG_ON() to make it clear that passing null isn't a > >> caller error. > > > > Yeah, BUG_ON() at least documents the intent, so it's better than > > the null check. But for something like this even BUG_ON() is > > just wasted bytes IMO. > > +1 > > The patch has a Fixes line, but AFAICT, the referenced patch didn't > introduce any NULL opportunities that weren't previously there. Really? Before the referenced patch the function only had one parameter, an uint32_t. True that kmalloc could return a NULL pointer and we would've crashed there as well, but at least we were running out of memory. Now I can crash the kernel any time I want with a badly written driver/module. Best regards, Liviu > > Sean > > > > > > BUG_ON() can be useful for those weird bugs where somewhere deep > > down you hit a null pointer and you can't figure out where the > > bad pointer came from. So you might sprinkle a few BUG_ONs() > > further up to catch it sooner. Esp. if you can't reproduce the > > bug yourself and have to rely on user(s) to find it for you. > > > > Even WARN_ON() w/ or w/o an early bailout might be a decent idea > > sometimes since it might have a slightly higher chance of keeping > > the kernel in working condition, but IMO just blindly throwing > > it around everywhere is not a good approach. > > > > -- > > Ville Syrjälä > > Intel OTC > > _______________________________________________ > > dri-devel mailing list > > dri-devel@lists.freedesktop.org > > https://lists.freedesktop.org/mailman/listinfo/dri-devel -- ==================== | I would like to | | fix the world, | | but they're not | | giving me the | \ source code! / --------------- ¯\_(ツ)_/¯