Date: Wed, 8 Nov 2017 17:23:33 -0600 From: Josh Poimboeuf <jpoimboe@...hat.com> To: Kees Cook <keescook@...omium.org> Cc: "Tobin C. Harding" <me@...in.cc>, kernel-hardening@...ts.openwall.com, Nick Desaulniers <ndesaulniers@...gle.com> Subject: Re: [RFC] vla: add VLA macro and testing On Wed, Nov 08, 2017 at 02:58:19PM -0800, Kees Cook wrote: > On Wed, Nov 1, 2017 at 4:50 PM, Tobin C. Harding <me@...in.cc> wrote: > > Variable Length Arrays (VLA) pose a risk to the stack if the variable > > passed into the array declaration is too large. If the variable used can > > be controlled by a malicious party then this poses a security risk to > > the kernel. > > > > Add a macro for declaring VLA's. Macro includes a requested size and a > > maximum size, if requested size is larger than maximum size then > > requested size is capped at maximum. Requested size is passed by > > reference and updated by macro so caller has access to size of array > > after declaration. > > > > Signed-off-by: Tobin C. Harding <me@...in.cc> > > > > --- > > > > I was unable to get the test module to integrate with the kernel build system > > correctly. The attempt was to mirror the way `lib/test_printf.c` functions. This > > effort was unsuccessful, it is included in the patch in the hope of getting > > better suggestions. To test, the test module was built out of tree and all tests > > pass. > > > > The macro needs some work. It functions as intended but > > > > Checkpatch emits ERROR: Macros with multiple statements should be enclosed in a > > do - while loop. > > > > Also for each use of VLA() checkpatch emits WARNING: Missing a blank line after > > declarations. > > > > Also I was unsure where to put the macro definition, appreciate any suggestions. > > While I'd certainly like to see SOME kind of sanity checking for VLAs, > my preference would be to entirely eliminate them at get -Werror=vla > added to the Makefile. Even Linus has expressed irritation over the > idea of VLAs in structs in the past: > > https://lkml.org/lkml/2013/9/23/500 > > I imagine VLAs make Josh's life harder too. Would objtool have an > easier time if there were no VLAs? As it turns out, it doesn't make objtool's life that much harder. GCC sets up a frame pointer before doing the stack allocation anyway. But, I strongly agree that VLAs should just go away. -- Josh
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