Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933738AbcKVWBe (ORCPT ); Tue, 22 Nov 2016 17:01:34 -0500 Received: from mail-yw0-f195.google.com ([209.85.161.195]:34436 "EHLO mail-yw0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933450AbcKVWBY (ORCPT ); Tue, 22 Nov 2016 17:01:24 -0500 MIME-Version: 1.0 In-Reply-To: <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> <20161122184742.GU1005@e106497-lin.cambridge.arm.com> From: Rob Clark Date: Tue, 22 Nov 2016 16:46:49 -0500 Message-ID: Subject: Re: [PATCH] drm: check for NULL parameter in exported drm_get_format_name() function. To: Liviu Dudau Cc: Sean Paul , =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= , Jani Nikula , Daniel Vetter , Eric Engestrom , LKML , DRI devel Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by mail.home.local id uAMM1lK4000947 Content-Length: 4416 Lines: 112 On Tue, Nov 22, 2016 at 1:47 PM, Liviu Dudau wrote: > 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. if you are a kernel module, in tree or not, all bets are off.. an out of tree module could just as easily call sprintf(NULL, "foo") directly.. BR, -R >> >> >> >> >> >> 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)_/¯