Re: [RFC][PATCH v3 4/5] Add new file in LKDTM to test fortified strscpy.

[Francis Laniel <laniel_francis@privacyrequired.com>]

Le samedi 24 octobre 2020, 07:23:01 CEST Kees Cook a écrit :
> On Wed, Oct 21, 2020 at 05:06:07PM +0200, laniel_francis@privacyrequired.com 
wrote:
> > From: Francis Laniel <laniel_francis@privacyrequired.com>
> > 
> > This new test generates a crash at runtime because there is a write
> > overflow in destination string.
> > 
> > Signed-off-by: Francis Laniel <laniel_francis@privacyrequired.com>
> > ---
> > 
> >  drivers/misc/lkdtm/Makefile             |  1 +
> >  drivers/misc/lkdtm/core.c               |  1 +
> >  drivers/misc/lkdtm/fortify.c            | 47 +++++++++++++++++++++++++
> >  drivers/misc/lkdtm/lkdtm.h              |  3 ++
> >  tools/testing/selftests/lkdtm/tests.txt |  1 +
> >  5 files changed, 53 insertions(+)
> >  create mode 100644 drivers/misc/lkdtm/fortify.c
> > 
> > diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
> > index c70b3822013f..d898f7b22045 100644
> > --- a/drivers/misc/lkdtm/Makefile
> > +++ b/drivers/misc/lkdtm/Makefile
> > @@ -10,6 +10,7 @@ lkdtm-$(CONFIG_LKDTM)		+= rodata_objcopy.o
> > 
> >  lkdtm-$(CONFIG_LKDTM)		+= usercopy.o
> >  lkdtm-$(CONFIG_LKDTM)		+= stackleak.o
> >  lkdtm-$(CONFIG_LKDTM)		+= cfi.o
> > 
> > +lkdtm-$(CONFIG_LKDTM)		+= fortify.o
> > 
> >  KASAN_SANITIZE_stackleak.o	:= n
> >  KCOV_INSTRUMENT_rodata.o	:= n
> > 
> > diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> > index a002f39a5964..4326e2d09870 100644
> > --- a/drivers/misc/lkdtm/core.c
> > +++ b/drivers/misc/lkdtm/core.c
> > @@ -180,6 +180,7 @@ static const struct crashtype crashtypes[] = {
> > 
> >  #ifdef CONFIG_X86_32
> >  
> >  	CRASHTYPE(DOUBLE_FAULT),
> >  
> >  #endif
> > 
> > +	CRASHTYPE(FORTIFIED_STRSCPY),
> > 
> >  };
> > 
> > diff --git a/drivers/misc/lkdtm/fortify.c b/drivers/misc/lkdtm/fortify.c
> > new file mode 100644
> > index 000000000000..cecdfbb8ba55
> > --- /dev/null
> > +++ b/drivers/misc/lkdtm/fortify.c
> > @@ -0,0 +1,47 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * Copyright (c) 2020 Francis Laniel <laniel_francis@privacyrequired.com>
> > + *
> > + * Add tests related to fortified functions in this file.
> > + */
> > +#include <linux/string.h>
> > +#include <linux/slab.h>
> > +#include "lkdtm.h"
> > +
> > +
> > +/*
> > + * Calls fortified strscpy to test that it returns the same result as
> > vanilla + * strscpy and generate a panic because there is a write
> > overflow (i.e. src + * length is greater than dst length).
> > + */
> > +void lkdtm_FORTIFIED_STRSCPY(void)
> > +{
> > +#if !defined(__NO_FORTIFY) && defined(__OPTIMIZE__) &&
> > defined(CONFIG_FORTIFY_SOURCE)
> I would drop the #if: just let it run and freak out on non-fortified
> kernels.

I will drop it for v4.

> > +	char *src;
> > +	char dst[3];
> > +
> > +	src = kstrdup("foobar", GFP_KERNEL);
> > +
> > +	if (src == NULL)
> > +		return;
> > +
> > +	/* Vanilla strscpy returns -E2BIG if size is 0. */
> > +	WARN_ON(strscpy(dst, src, 0) != -E2BIG);
> 
> For LKDTM, we have different reporting that "normal", in the sense that
> usually the WARN/BUG outcomes are _desirable_ (i.e. "freak out on stack
> overflow"). So, I would write this as:
> 
> 	if (strscpy(dst, src, 0) != -E2BIG)
> 		pr_warn("FAIL: strscpy() of 0 length did not return -E2BIG\n");

I was not sure if using WARN_* lile macro was a good idea in LKDTM since if I 
understood correctly the goal of a LKDTM test is to fail.
I will rewrite it the way you suggested it for the v4.

> > +
> > +	/* Vanilla strscpy returns -E2BIG if src is truncated. */
> > +	WARN_ON(strscpy(dst, src, sizeof(dst)) != -E2BIG);
> 
> Same.
> 
> > +
> > +	/* After above call, dst must contain "fo" because src was truncated. */
> > +	WARN_ON(strncmp(dst, "fo", sizeof(dst)) != 0);
> 
> Same.
> 
> > +
> > +	/*
> > +	 * Use strlen here so size cannot be known at compile time and there is
> > +	 * a runtime overflow.
> > +	 */
> > +	strscpy(dst, src, strlen(src));
> 
> I think we'll need a couple more corner cases, and any that need to Oops
> separately can be separate functions. Here's a corner case to test to
> strnlen():
> 
> struct {
> 	union {
> 	        char big[10];
> 	        char src[5];
> 	};
> } weird = { .big = "hello!" };
> char dst[sizeof(weird.src) + 1];
> 
> strscpy(dst, weird.src, sizeof(dst));
> 
> if (strcmp(dst, "hello") != 0)
> 	pr_warn("FAIL ...
> 
> But each of the cases being tested in the fortified strscpy() should be
> exercised.

I will try to add these cases and other tests, if I think to more, for the 
next version so the tests cover as most case as possible.

> > +
> > +	pr_info("Fail: No overflow in above strscpy call!\n");
> 
> pr_warn("FAIL: ...
> 

Will be modified for next release!

> > +
> > +	kfree(src);
> > +#endif
> > +}
> > diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
> > index 70c8b7c9460f..29c12dcdeab1 100644
> > --- a/drivers/misc/lkdtm/lkdtm.h
> > +++ b/drivers/misc/lkdtm/lkdtm.h
> > @@ -106,4 +106,7 @@ void lkdtm_STACKLEAK_ERASING(void);
> > 
> >  /* cfi.c */
> >  void lkdtm_CFI_FORWARD_PROTO(void);
> > 
> > +/* fortify.c */
> > +void lkdtm_FORTIFIED_STRSCPY(void);
> > +
> > 
> >  #endif
> > 
> > diff --git a/tools/testing/selftests/lkdtm/tests.txt
> > b/tools/testing/selftests/lkdtm/tests.txt index
> > 9d266e79c6a2..4234109579eb 100644
> > --- a/tools/testing/selftests/lkdtm/tests.txt
> > +++ b/tools/testing/selftests/lkdtm/tests.txt
> > @@ -70,3 +70,4 @@ USERCOPY_KERNEL
> > 
> >  USERCOPY_KERNEL_DS
> >  STACKLEAK_ERASING OK: the rest of the thread stack is properly erased
> >  CFI_FORWARD_PROTO
> > 
> > +FORTIFIED_STRSCPY
> > \ No newline at end of file