From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrea Righi Date: Wed, 30 Oct 2019 20:12:01 +0000 Subject: Re: [PATCH] fbdev: potential information leak in do_fb_ioctl() Message-Id: <20191030201201.GA3209@xps-13> List-Id: References: <20191029182320.GA17569@mwanda> <87zhhjjryk.fsf@x220.int.ebiederm.org> <20191030074321.GD2656@xps-13> <87r22ujaqq.fsf@x220.int.ebiederm.org> In-Reply-To: <87r22ujaqq.fsf@x220.int.ebiederm.org> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: "Eric W. Biederman" Cc: Dan Carpenter , Bartlomiej Zolnierkiewicz , Daniel Vetter , Sam Ravnborg , Maarten Lankhorst , Peter Rosin , Gerd Hoffmann , dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org, security@kernel.org, Kees Cook , Julia Lawall On Wed, Oct 30, 2019 at 02:26:21PM -0500, Eric W. Biederman wrote: > Andrea Righi writes: > > > On Tue, Oct 29, 2019 at 02:02:11PM -0500, Eric W. Biederman wrote: > >> Dan Carpenter writes: > >> > >> > The "fix" struct has a 2 byte hole after ->ywrapstep and the > >> > "fix = info->fix;" assignment doesn't necessarily clear it. It depends > >> > on the compiler. > >> > > >> > Fixes: 1f5e31d7e55a ("fbmem: don't call copy_from/to_user() with mutex held") > >> > Signed-off-by: Dan Carpenter > >> > --- > >> > I have 13 more similar places to patch... I'm not totally sure I > >> > understand all the issues involved. > >> > >> What I have done in a similar situation with struct siginfo, is that > >> where the structure first appears I have initialized it with memset, > >> and then field by field. > >> > >> Then when the structure is copied I copy the structure with memcpy. > >> > >> That ensures all of the bytes in the original structure are initialized > >> and that all of the bytes are copied. > >> > >> The goal is to avoid memory that has values of the previous users of > >> that memory region from leaking to userspace. Which depending on who > >> the previous user of that memory region is could tell userspace > >> information about what the kernel is doing that it should not be allowed > >> to find out. > >> > >> I tried to trace through where "info" and thus presumably "info->fix" is > >> coming from and only made it as far as register_framebuffer. Given > >> that I suspect a local memset, and then a field by field copy right > >> before copy_to_user might be a sound solution. But ick. That is a lot > >> of fields to copy. > > > > I know it might sound quite inefficient, but what about making struct > > fb_fix_screeninfo __packed? > > > > This doesn't solve other potential similar issues, but for this > > particular case it could be a reasonable and simple fix. > > It is part of the user space ABI. As such you can't move the fields. > > Eric Oh, that's right! Then memset() + memcpy() is probably the best option, since copying all those fields one by one looks quite ugly to me... -Andrea