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Message-ID: <2224107.5bobqytM52@machine> Date: Sat, 17 Oct 2020 11:22:04 +0200 From: Francis Laniel <laniel_francis@...vacyrequired.com> To: Kees Cook <keescook@...omium.org> Cc: linux-hardening@...r.kernel.org, Daniel Axtens <dja@...ens.net> Subject: Re: [RFC][PATCH v2] Fortify string function strscpy. Le samedi 17 octobre 2020, 01:16:36 CEST Kees Cook a écrit : > On Fri, Oct 16, 2020 at 02:38:09PM +0200, laniel_francis@...vacyrequired.com wrote: > > From: Francis Laniel <laniel_francis@...vacyrequired.com> > > > > Thanks to kees advices (see: > > https://github.com/KSPP/linux/issues/96#issuecomment-709620337) I wrote a > > LKDTM test for the fortified version of strscpy I added in the v1 of this > > patch. The test panics due to write overflow. > > Ah nice, thanks! I am reminded about this series as well: > https://lore.kernel.org/lkml/20200120045424.16147-1-dja@axtens.net > I think we can likely do this all at the same time, merge the > complementary pieces, etc. You are welcome! Just to be sure I understand correctly: you want me to add work of Daniel Axtens to my local version, then add my modifications on top of his work and republish the whole patch set? > > Notes below... > > > 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 | 37 +++++++++++++++++++++++++++++ > > drivers/misc/lkdtm/lkdtm.h | 17 ++++++++------ > > Yay tests! These should, however, be a separate patch. Ok, I will separate it. If I understand correctly: one semantic modification = one commit. > > > include/linux/string.h | 45 ++++++++++++++++++++++++++++++++++++ > > 5 files changed, 94 insertions(+), 7 deletions(-) > > 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 a5e344df9166..979f9e3feefd 100644 > > --- a/drivers/misc/lkdtm/core.c > > +++ b/drivers/misc/lkdtm/core.c > > @@ -178,6 +178,7 @@ 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..0397d2def66d > > --- /dev/null > > +++ b/drivers/misc/lkdtm/fortify.c > > @@ -0,0 +1,37 @@ > > +// 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 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) + char *src; > > + char dst[3]; > > + > > + src = kmalloc(7, GFP_KERNEL); > > + src[0] = 'f'; > > + src[1] = 'o'; > > + src[2] = 'o'; > > + src[3] = 'b'; > > + src[4] = 'a'; > > + src[5] = 'r'; > > + src[6] = '\0'; > > Hah, yes, I guess we need to bypass the common utilities. ;) I wonder if > using __underlying_strcpy() might be easier. I am sorry but I did not understand. If we use here __underlying_strcpy() the function this will not profit from the protection added in fortified version of strscpy()? > > > + > > + strscpy(dst, src, 1000); > > + > > + kfree(dst); > > + > > + pr_info("Fail: No overflow in above strscpy call!\n"); > > +#endif > > +} > > One thing I'd love to see is a _compile-time_ test too: but it needs to > be a negative failure case, which Makefiles are not well suited to > dealing with. e.g. something like: > > good.o: nop.c bad.c > if $(CC) .... -o bad.o bad.c $< ; then exit 1; else $(CC) ... -o good.c > nop.c ; fi > > I'm not sure how to do it. > This is a good idea, I though to it but I did not see an easy way to deal with it. I will investigate one it, but I cannot guarantee the next version will come with this feature. > > diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h > > index 8878538b2c13..8e5e90eb0e00 100644 > > --- a/drivers/misc/lkdtm/lkdtm.h > > +++ b/drivers/misc/lkdtm/lkdtm.h > > @@ -6,7 +6,7 @@ > > > > #include <linux/kernel.h> > > > > -/* lkdtm_bugs.c */ > > +/* bugs.c */ > > oops, yes. Can you split change from the others, since it's an unrelated > clean-up. Understand, it will be done for next version! > > > void __init lkdtm_bugs_init(int *recur_param); > > void lkdtm_PANIC(void); > > void lkdtm_BUG(void); > > > > @@ -34,7 +34,7 @@ void lkdtm_UNSET_SMEP(void); > > > > void lkdtm_DOUBLE_FAULT(void); > > void lkdtm_CORRUPT_PAC(void); > > > > -/* lkdtm_heap.c */ > > +/* heap.c */ > > > > void __init lkdtm_heap_init(void); > > void __exit lkdtm_heap_exit(void); > > void lkdtm_OVERWRITE_ALLOCATION(void); > > > > @@ -46,7 +46,7 @@ void lkdtm_SLAB_FREE_DOUBLE(void); > > > > void lkdtm_SLAB_FREE_CROSS(void); > > void lkdtm_SLAB_FREE_PAGE(void); > > > > -/* lkdtm_perms.c */ > > +/* perms.c */ > > > > void __init lkdtm_perms_init(void); > > void lkdtm_WRITE_RO(void); > > void lkdtm_WRITE_RO_AFTER_INIT(void); > > > > @@ -61,7 +61,7 @@ void lkdtm_EXEC_NULL(void); > > > > void lkdtm_ACCESS_USERSPACE(void); > > void lkdtm_ACCESS_NULL(void); > > > > -/* lkdtm_refcount.c */ > > +/* refcount.c */ > > > > void lkdtm_REFCOUNT_INC_OVERFLOW(void); > > void lkdtm_REFCOUNT_ADD_OVERFLOW(void); > > void lkdtm_REFCOUNT_INC_NOT_ZERO_OVERFLOW(void); > > > > @@ -82,10 +82,10 @@ void lkdtm_REFCOUNT_SUB_AND_TEST_SATURATED(void); > > > > void lkdtm_REFCOUNT_TIMING(void); > > void lkdtm_ATOMIC_TIMING(void); > > > > -/* lkdtm_rodata.c */ > > +/* rodata.c */ > > > > void lkdtm_rodata_do_nothing(void); > > > > -/* lkdtm_usercopy.c */ > > +/* usercopy.c */ > > > > void __init lkdtm_usercopy_init(void); > > void __exit lkdtm_usercopy_exit(void); > > void lkdtm_USERCOPY_HEAP_SIZE_TO(void); > > > > @@ -98,10 +98,13 @@ void lkdtm_USERCOPY_STACK_BEYOND(void); > > > > void lkdtm_USERCOPY_KERNEL(void); > > void lkdtm_USERCOPY_KERNEL_DS(void); > > > > -/* lkdtm_stackleak.c */ > > +/* stackleak.c */ > > > > void lkdtm_STACKLEAK_ERASING(void); > > > > /* cfi.c */ > > void lkdtm_CFI_FORWARD_PROTO(void); > > > > +/* fortify.c */ > > +void lkdtm_FORTIFIED_STRSCPY(void); > > + > > > > #endif > > > > diff --git a/include/linux/string.h b/include/linux/string.h > > index b1f3894a0a3e..b661863619e0 100644 > > --- a/include/linux/string.h > > +++ b/include/linux/string.h > > @@ -6,6 +6,8 @@ > > > > #include <linux/compiler.h> /* for inline */ > > #include <linux/types.h> /* for size_t */ > > #include <linux/stddef.h> /* for NULL */ > > > > +#include <linux/bug.h> /* for WARN_ON_ONCE */ > > +#include <linux/errno.h> /* for E2BIG */ > > > > #include <stdarg.h> > > #include <uapi/linux/string.h> > > > > @@ -357,6 +359,49 @@ __FORTIFY_INLINE size_t strlcpy(char *p, const char > > *q, size_t size)> > > return ret; > > > > } > > > > +/* defined after fortified strlen to reuse it */ > > +extern ssize_t __real_strscpy(char *, const char *, size_t) > > __RENAME(strscpy); +__FORTIFY_INLINE ssize_t strscpy(char *p, const char > > *q, size_t count) > I would name "count" as "size" to match the other helpers. > I decided to keep count because it is the argument name in unfortified version of strscpy I will change the name for next version to stick with all the fortified functions arguments. > > +{ > > + size_t len; > > + size_t p_size = __builtin_object_size(p, 0); > > + size_t q_size = __builtin_object_size(q, 0); > > These can be using ", 1" instead of ", 0". And I'll grab the related > changes from the mentioned series above. > I looked Daniel Axtens patch and understood why it is better to use 1 instead of 0 so I will add it for the next version. > > + /* > > + * If p_size and q_size cannot be known at compile time we just had to > > + * trust this function caller. > > + */ > > + if (p_size == (size_t)-1 && q_size == (size_t)-1) > > + return __real_strscpy(p, q, count); > > + len = strlen(q); > > + if (count) { > > This test isn't needed; it'll work itself out correctly. :P > Indeed, if this condition is met, __real_strscpy will be called later. > > + /* If count is bigger than INT_MAX, strscpy returns -E2BIG. */ > > + if (WARN_ON_ONCE(count > INT_MAX)) > > + return -E2BIG; > > This is already handled in strscpy, I'd drop this here. I though of it at first, but since the patch modify count/size before giving it to __real_strscpy(), real one will never return -E2BIG due to that. So removing this modification will lead to difference between returned value of fortified strscpy() and __real_strscpy(). > > > + /* > > + * strscpy handles read overflows by stop reading q when '\0' is > > + * met. > > + * We stick to this behavior here. > > + */ > > + len = (len >= count) ? count : len; > > + /* > > + * If len can be known at compile time and is greater than > > + * p_size, generate a compile time write overflow error. > > + */ > > + if (__builtin_constant_p(len) && len > p_size) > > This won't work (len wasn't an argument and got assigned); you need: > > if (__builtin_constant_p(size) && p_size < size) > You are right, len is unknown at compile time... So, I will correct it for next version! > > + __write_overflow(); > > + /* Otherwise generate a runtime write overflow error. */ > > + if (len > p_size) > > + fortify_panic(__func__); > > I think this just needs to be: > > if (p_size < size) > fortify_panic(__func__); > I am not really sure. If p_size is 4, size is 1000 and q is "foo\0", then what you suggested will panic but there is not need to panic since __real_strscpy will truncate size and copy just 4 bytes into p (because of '\0' in q). Am I correct? > > + /* > > + * Still use count as third argument to correctly compute max > > + * inside strscpy. > > + */ > > + return __real_strscpy(p, q, count); > > + } > > + /* If count is 0, strscpy return -E2BIG. */ > > + return -E2BIG; > > I'd let __real_strscpy() handle this. > See my three times above comment. __real_strscpy is called only if count > 0, so it will never return -E2BIG due to this. So it will lead to difference in returned value between fortified strscpy() and __real_strscpy(). > > +} > > + > > > > /* defined after fortified strlen and strnlen to reuse them */ > > __FORTIFY_INLINE char *strncat(char *p, const char *q, __kernel_size_t > > count) {
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