Date: Fri, 3 Nov 2017 11:44:44 +0300 From: Tycho Andersen <tycho@...ker.com> To: "Tobin C. Harding" <me@...in.cc> Cc: kernel-hardening@...ts.openwall.com, Kees Cook <keescook@...omium.org> Subject: Re: [RFC] vla: add VLA macro and testing On Thu, Nov 02, 2017 at 10:50:40AM +1100, Tobin C. Harding 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. > > thanks, > Tobin. > > include/linux/vla.h | 24 ++++++++++ > lib/Kconfig.debug | 3 ++ > lib/Makefile | 1 + > lib/test_vla.c | 98 ++++++++++++++++++++++++++++++++++++++ > tools/testing/selftests/lib/config | 1 + > 5 files changed, 127 insertions(+) > create mode 100644 include/linux/vla.h > create mode 100644 lib/test_vla.c > > diff --git a/include/linux/vla.h b/include/linux/vla.h > new file mode 100644 > index 000000000000..e8f1572bbf42 > --- /dev/null > +++ b/include/linux/vla.h > @@ -0,0 +1,24 @@ > +/* > + * This software is licensed under the terms of the GNU General Public > + * License version 2, as published by the Free Software Foundation, and > + * may be copied, distributed, and modified under those terms. > + * > + * This program is distributed in the hope that it will be useful, > + * but WITHOUT ANY WARRANTY; without even the implied warranty of > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the > + * GNU General Public License for more details. > + * > + */ > +#ifndef __LINUX_VLA_H > +#define __LINUX_VLA_H > + > +/* > + * When declaring a local variable using VLA(), VLA() must be the last > + * declaration otherwise a compiler warning [-Wdeclaration-after-statement] will > + * be generated > + */ > +#define VLA(type, name, size, max) \ > + type name[(*size < max ? *size : max)]; \ > + *size = (*size < max ? *size : max) This requires you to have your VLAs at the end of the declarations, which is annoying and potentially limiting (limited to one VLA, etc.). I also don't think you need the pointer. Something like: #define VLA(type, name, size, max) \ type name[(size < max ? max : (size = max))] should work I think. It would also be nice to see some users of the VLA macro as part of this series. Cheers, Tycho
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