Messages in this thread | | | Date | Fri, 17 Dec 2021 13:34:50 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH 06/17] fortify: Detect struct member overflows in memcpy() at compile-time |
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On Thu, Dec 16, 2021 at 10:00:09AM -0800, Kees Cook wrote: > On Thu, Dec 16, 2021 at 11:08:26AM +0000, Mark Rutland wrote: > > On Mon, Dec 13, 2021 at 02:33:20PM -0800, Kees Cook wrote: > > > memcpy() is dead; long live memcpy() > > > > > > tl;dr: In order to eliminate a large class of common buffer overflow > > > flaws that continue to persist in the kernel, have memcpy() (under > > > CONFIG_FORTIFY_SOURCE) perform bounds checking of the destination struct > > > member when they have a known size. This would have caught all of the > > > memcpy()-related buffer write overflow flaws identified in at least the > > > last three years. > > > > > > > Hi Kees, > > > > Since there's a *lot* of context below, it's very easy to miss some key details > > (e.g. that the compile-time warnings are limited to W=1 builds). It would be > > really nice if the summary above could say something like: > > Hm, I do need to write a better summary! I think there's still some > misunderstanding, and I will attempt some clarity here... :)
Thanks! Sorry if that came across as a complaint; this looks really useful and I just couldn't figure out if I was holding things wrong or whether I was hitting an unexpected issue. :)
> > This patch makes it possible to detect when memcpy() of a struct member may > > go past the bounds of that member. When CONFIG_FORTIFY_SOURCE=y, runtime > > checks are always emitted where the compiler cannot guarantee a memcpy() is > > safely bounded, and compile-time warnings are enabled for W=1 builds. > > For GCC and Clang 14, compile-time _write_ overflow warnings are meant > to be emitted under FORTIFY_SOURCE. _read_ overflow warnings are meant > to be emitted under FORTIFY_SOURCE + W=1 (or when the same statement > also has a write overflow).
Cool.
I'll await a clang 14 release, or go build my own copy in the mean time.
> > This catches a large class of common buffer overflow flaws, and would have > > caught all of the memcpy()-related buffer write overflow flaws identified in > > the last three years. > > > > As an aside, since W=1 is chock-full of (IMO useless) warnings, is there any > > way to enable *just* the FORTIFY_SOURCE warnings? > > To see them all (i.e. not shove some into W=1), you can remove the "W=1 > or write overflow" part of the read overflow test in fortify-string.h. > e.g.: > > - if ((IS_ENABLED(KBUILD_EXTRA_WARN1) || p_size_field < size) && > - q_size_field < size) > + if (q_size_field < size) > > > I had a go at testing this on arm64, and could get build-time warnings from GCC > > 11.1.0, but not from Clang 13.0.0. > > This is correct and expected due to Clang 13's lack of support for > compiletime_warning().
Thanks for confirming!
Thanks, Mark.
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