Date: Fri, 24 Feb 2017 00:15:10 +0100 From: Rasmus Villemoes <linux@...musvillemoes.dk> To: Kees Cook <keescook@...omium.org> Cc: Laura Abbott <labbott@...hat.com>, "kernel-hardening\@lists.openwall.com" <kernel-hardening@...ts.openwall.com> Subject: Re: Format string gcc-plugin? On Tue, Feb 14 2017, Kees Cook <keescook@...omium.org> wrote: > On Tue, Feb 14, 2017 at 10:01 AM, Laura Abbott <labbott@...hat.com> wrote: >> Hi, >> >> The recent discussions about %pk led me to start playing around >> with writing a gcc plugin (notes about writing a gcc plugin are >> forthcoming). My idea was to expand checking of the kernel's additional >> formats (%pK, %pR etc.) or possibly even change the format >> strings. The big issue is that most of vsprintf.c would end up >> in the plugin which would be ugly to maintain. This concept could >> be used for some of the problems with kernel pointer leaks >> https://kernsec.org/wiki/index.php/Bug_Classes/Kernel_pointer_leak >> but I'm interested if anyone else has thoughts or ideas about >> how better to do this (or even not to do this). > > If we can leverage common code between the plugin and vsprintf.c, it > should be doable. I haven't looked at processing format strings at > all, though I assume the plugin would need to check the > __printf-generated attributes. > smatch already does type checking of the kernel's %p extensions, plus some additional sanity checks (e.g. passing a signed char to %02x, duplicate KERN_* or KERN_* prefixes not at the beginning, 0x%d, etc.). Maybe Dan Carpenter is the only one actually running it periodically. I never managed to get sparse to grok the printf attribute, so it uses a hardcoded list of printf-like functions. It would be nice to do these checks in a gcc plugin to get wider coverage and piggyback on the actual printf attributes, and it would probably be easier to keep up with the endless stream of new %pX stuff that way. Rasmus
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