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Message-ID: <87y2k2v3gh.fsf@dja-thinkpad.axtens.net> Date: Mon, 19 Oct 2020 22:51:26 +1100 From: Daniel Axtens <dja@...ens.net> To: Francis Laniel <laniel_francis@...vacyrequired.com>, Kees Cook <keescook@...omium.org> Cc: linux-hardening@...r.kernel.org Subject: Re: [RFC][PATCH v2] Fortify string function strscpy. Francis Laniel <laniel_francis@...vacyrequired.com> writes: > 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? That would make sense to me: apply my patches from the list and include them in your next patch series. If you need to modify my patches, just add your Signed-off-by: after mine. I don't know if my patches still apply to a current tree: let me know if you need any help getting them up to date. Kind regards, Daniel > >> >> 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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