On Tue, Sep 8, 2020 at 8:40 PM Arvind Sankar <nived...@alum.mit.edu> wrote: > > On Tue, Sep 08, 2020 at 10:21:32AM -0700, Kees Cook wrote: > > On Tue, Sep 08, 2020 at 11:39:11AM +0200, Marco Elver wrote: > > > On Sun, 6 Sep 2020 at 00:23, Arvind Sankar <nived...@alum.mit.edu> wrote: > > > > > > > > String functions can be useful in early boot, but using instrumented > > > > versions can be problematic: eg on x86, some of the early boot code is > > > > executing out of an identity mapping rather than the kernel virtual > > > > addresses. Accessing any global variables at this point will lead to a > > > > crash. > > > > > > > > > > Ouch. > > > > > > We have found manifestations of bugs in lib/string.c functions, e.g.: > > > > > > https://groups.google.com/forum/#!msg/syzkaller-bugs/atbKWcFqE9s/x7AtoVoBAgAJ > > > > > > https://groups.google.com/forum/#!msg/syzkaller-bugs/iGBUm-FDhkM/chl05uEgBAAJ > > > > > > Is there any way this can be avoided? > > > > Agreed: I would like to keep this instrumentation; it's a common place > > to find bugs, security issues, etc. > > > > -- > > Kees Cook > > Ok, understood. I'll revise to open-code the strscpy instead. > > Is instrumentation supported on x86-32? load_ucode_bsp() on 32-bit is > called before paging is enabled, and load_ucode_bsp() itself, along with > eg lib/earlycpio and lib/string that it uses, don't have anything to > disable instrumentation. kcov, kasan, kcsan are unsupported already on > 32-bit, but the others like gcov and PROFILE_ALL_BRANCHES look like they > would just cause a crash if microcode loading is enabled.
I agree we should not disable instrumentation of such common functions. Instead of open-coding these functions maybe we could produce both instrumented and non-instrumented versions from the same source implementation. Namely, place implementation in a header function with always_inline attribute and include it from 2 source files, one with instrumentation enabled and another with instrumentation disabled. This way we could produce strscpy (instrumented) and __strscpy (non-instrumented) from the same source.
