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Message-ID: <3172137.tozOrHZ9KS@machine> Date: Sat, 24 Oct 2020 12:19:44 +0200 From: Francis Laniel <laniel_francis@...vacyrequired.com> To: Kees Cook <keescook@...omium.org> Cc: linux-hardening@...r.kernel.org, dja@...ens.net Subject: Re: [RFC][PATCH v3 4/5] Add new file in LKDTM to test fortified strscpy. 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@...vacyrequired.com wrote: > > From: Francis Laniel <laniel_francis@...vacyrequired.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@...vacyrequired.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@...vacyrequired.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
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