Re: [PATCH v2 1/3] ubsan: Add trap instrumentation option

[Kees Cook]

On Mon, Dec 16, 2019 at 10:26:56AM +0000, Will Deacon wrote:
> Hi Kees,
> 
> On Thu, Nov 21, 2019 at 10:15:17AM -0800, Kees Cook wrote:
> > The Undefined Behavior Sanitizer can operate in two modes: warning
> > reporting mode via lib/ubsan.c handler calls, or trap mode, which uses
> > __builtin_trap() as the handler. Using lib/ubsan.c means the kernel
> > image is about 5% larger (due to all the debugging text and reporting
> > structures to capture details about the warning conditions). Using the
> > trap mode, the image size changes are much smaller, though at the loss
> > of the "warning only" mode.
> > 
> > In order to give greater flexibility to system builders that want
> > minimal changes to image size and are prepared to deal with kernel code
> > being aborted and potentially destabilizing the system, this introduces
> > CONFIG_UBSAN_TRAP. The resulting image sizes comparison:
> > 
> >    text    data     bss       dec       hex     filename
> > 19533663   6183037  18554956  44271656  2a38828 vmlinux.stock
> > 19991849   7618513  18874448  46484810  2c54d4a vmlinux.ubsan
> > 19712181   6284181  18366540  44362902  2a4ec96 vmlinux.ubsan-trap
> > 
> > CONFIG_UBSAN=y:      image +4.8% (text +2.3%, data +18.9%)
> > CONFIG_UBSAN_TRAP=y: image +0.2% (text +0.9%, data +1.6%)
> > 
> > Additionally adjusts the CONFIG_UBSAN Kconfig help for clarity and
> > removes the mention of non-existing boot param "ubsan_handle".
> > 
> > Suggested-by: Elena Petrova <lenaptr@xxxxxxxxxx>
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> >  lib/Kconfig.ubsan      | 22 ++++++++++++++++++----
> >  lib/Makefile           |  2 ++
> >  scripts/Makefile.ubsan |  9 +++++++--
> >  3 files changed, 27 insertions(+), 6 deletions(-)
> > 
> > diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
> > index 0e04fcb3ab3d..9deb655838b0 100644
> > --- a/lib/Kconfig.ubsan
> > +++ b/lib/Kconfig.ubsan
> > @@ -5,11 +5,25 @@ config ARCH_HAS_UBSAN_SANITIZE_ALL
> >  config UBSAN
> >  	bool "Undefined behaviour sanity checker"
> >  	help
> > -	  This option enables undefined behaviour sanity checker
> > +	  This option enables the Undefined Behaviour sanity checker.
> >  	  Compile-time instrumentation is used to detect various undefined
> > -	  behaviours in runtime. Various types of checks may be enabled
> > -	  via boot parameter ubsan_handle
> > -	  (see: Documentation/dev-tools/ubsan.rst).
> > +	  behaviours at runtime. For more details, see:
> > +	  Documentation/dev-tools/ubsan.rst
> > +
> > +config UBSAN_TRAP
> > +	bool "On Sanitizer warnings, abort the running kernel code"
> > +	depends on UBSAN
> > +	depends on $(cc-option, -fsanitize-undefined-trap-on-error)
> > +	help
> > +	  Building kernels with Sanitizer features enabled tends to grow
> > +	  the kernel size by around 5%, due to adding all the debugging
> > +	  text on failure paths. To avoid this, Sanitizer instrumentation
> > +	  can just issue a trap. This reduces the kernel size overhead but
> > +	  turns all warnings (including potentially harmless conditions)
> > +	  into full exceptions that abort the running kernel code
> > +	  (regardless of context, locks held, etc), which may destabilize
> > +	  the system. For some system builders this is an acceptable
> > +	  trade-off.
> 
> Slight nit, but I wonder if it would make sense to move all this under a
> 'menuconfig UBSAN' entry, so the dependencies can be dropped? Then you could
> have all of the suboptions default to on and basically choose which
> individual compiler options to disable based on your own preferences.

Sure; I can do that. I'll respin the series.

-- 
Kees Cook