Re: [RFC PATCH 1/2] lib/string: Disable instrumentation

[Kees Cook]

On Tue, Sep 08, 2020 at 11:39:11AM +0200, Marco Elver wrote:
> On Sun, 6 Sep 2020 at 00:23, Arvind Sankar <nivedita@xxxxxxxxxxxx> 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.
> >
> > Tracing and KCOV are already disabled, and CONFIG_AMD_MEM_ENCRYPT will
> > additionally disable KASAN and stack protector.
> >
> > Additionally disable GCOV, UBSAN, KCSAN, STACKLEAK_PLUGIN and branch
> > profiling, and make it unconditional to allow safe use of string
> > functions.
> >
> > Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
> > ---
> >  lib/Makefile | 11 +++++++----
> >  1 file changed, 7 insertions(+), 4 deletions(-)
> >
> > diff --git a/lib/Makefile b/lib/Makefile
> > index a4a4c6864f51..5e421769bbc6 100644
> > --- a/lib/Makefile
> > +++ b/lib/Makefile
> > @@ -8,7 +8,6 @@ ccflags-remove-$(CONFIG_FUNCTION_TRACER) += $(CC_FLAGS_FTRACE)
> >  # These files are disabled because they produce lots of non-interesting and/or
> >  # flaky coverage that is not a function of syscall inputs. For example,
> >  # rbtree can be global and individual rotations don't correlate with inputs.
> > -KCOV_INSTRUMENT_string.o := n
> >  KCOV_INSTRUMENT_rbtree.o := n
> >  KCOV_INSTRUMENT_list_debug.o := n
> >  KCOV_INSTRUMENT_debugobjects.o := n
> > @@ -20,12 +19,16 @@ KCOV_INSTRUMENT_fault-inject.o := n
> >  # them into calls to themselves.
> >  CFLAGS_string.o := -ffreestanding
> >
> > -# Early boot use of cmdline, don't instrument it
> > -ifdef CONFIG_AMD_MEM_ENCRYPT
> > +# Early boot use of string functions, disable instrumentation
> > +GCOV_PROFILE_string.o := n
> > +KCOV_INSTRUMENT_string.o := n
> >  KASAN_SANITIZE_string.o := n
> > +UBSAN_SANITIZE_string.o := n
> > +KCSAN_SANITIZE_string.o := n
> 
> 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