From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel Vetter Date: Fri, 25 Sep 2020 13:25:51 +0000 Subject: Re: [PATCH 0/3] Prevent out-of-bounds access for built-in font data buffers Message-Id: <20200925132551.GF438822@phenom.ffwll.local> List-Id: References: <0000000000006b9e8d059952095e@google.com> <3f754d60-1d35-899c-4418-147d922e29af@kernel.org> <20200925101300.GA890211@PWN> In-Reply-To: <20200925101300.GA890211@PWN> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Peilin Ye Cc: linux-fbdev@vger.kernel.org, Bartlomiej Zolnierkiewicz , Daniel Vetter , syzkaller-bugs@googlegroups.com, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, Greg Kroah-Hartman , Jiri Slaby , linux-kernel-mentees@lists.linuxfoundation.org On Fri, Sep 25, 2020 at 06:13:00AM -0400, Peilin Ye wrote: > Hi all! > > On Fri, Sep 25, 2020 at 08:46:04AM +0200, Jiri Slaby wrote: > > > In order to perform a reliable range check, fbcon_get_font() needs to know > > > `FONTDATAMAX` for each built-in font under lib/fonts/. Unfortunately, we > > > do not keep that information in our font descriptor, > > > `struct console_font`: > > > > > > (include/uapi/linux/kd.h) > > > struct console_font { > > > unsigned int width, height; /* font size */ > > > unsigned int charcount; > > > unsigned char *data; /* font data with height fixed to 32 */ > > > }; > > > > > > To make things worse, `struct console_font` is part of the UAPI, so we > > > cannot add a new field to keep track of `FONTDATAMAX`. > > > > Hi, > > > > but you still can define struct kernel_console_font containing struct > > console_font and the 4 more members you need in the kernel. See below. > > > > > Fortunately, the framebuffer layer itself gives us a hint of how to > > > resolve this issue without changing UAPI. When allocating a buffer for a > > > user-provided font, fbcon_set_font() reserves four "extra words" at the > > > beginning of the buffer: > > > > > > (drivers/video/fbdev/core/fbcon.c) > > > new_data = kmalloc(FONT_EXTRA_WORDS * sizeof(int) + size, GFP_USER); > > > > I might be missing something (like coffee in the morning), but why don't > > you just: > > 1) declare struct font_data as > > { > > unsigned sum, char_count, size, refcnt; > > const unsigned char data[]; > > } > > > > Or maybe "struct console_font font" instead of "const unsigned char > > data[]", if need be. > > > > 2) allocate by: > > kmalloc(struct_size(struct font_data, data, size)); > > > > 3) use container_of wherever needed > > > > That is you name the data on negative indexes using struct as you > > already have to define one. > > > > Then you don't need the ugly macros with negative indexes. And you can > > pass this structure down e.g. to fbcon_do_set_font, avoiding potential > > mistakes in accessing data[-1] and similar. > > Sorry that I didn't mention it in the cover letter, but yes, I've tried > this - a new `kernel_console_font` would be much cleaner than negative > array indexing. > > The reason I ended up giving it up was, frankly speaking, these macros > are being used at about 30 places, and I am not familiar enough with the > framebuffer and newport_con code, so I wasn't confident how to clean > them up and plug in `kernel_console_font` properly... > > Another reason was that, functions like fbcon_get_font() handle both user > fonts and built-in fonts, so I wanted a single solution for both of > them. I think we can't really introduce `kernel_console_font` while > keeping these macros, that would make the error handling logics etc. > very messy. > > I'm not very sure what to do now. Should I give it another try cleaning > up all the macros? > > And thank you for reviewing this! I think the only way to make this work is that we have one place which takes in the userspace uapi struct, and then converts it once into a kernel_console_font. With all the error checking. Then all internal code deals in terms of kernel_console_font, with properly typed and named struct members and helper functions and everything. And we might need a gradual conversion for this, so that first we can convert over invidual console drivers, then subsystems, until at the end we've pushed the conversion from uapi array to kernel_console_font all the way to the ioctl entry points. But that's indeed a huge pile of work, and fair warning: fbcon is semi-orphaned, so by doing this you'll pretty much volunteer for maintainership :-) But I'd be very happy to help get this done and throw some maintainership credentials at you in the proces ... Cheers, Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch