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Message-ID: <202010161555.A73EEE9@keescook> Date: Fri, 16 Oct 2020 16:16:36 -0700 From: Kees Cook <keescook@...omium.org> To: laniel_francis@...vacyrequired.com Cc: linux-hardening@...r.kernel.org, Daniel Axtens <dja@...ens.net> Subject: Re: [RFC][PATCH v2] Fortify string function strscpy. 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. 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. > 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. > + > + 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. > 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. > 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. > +{ > + 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. > + /* > + * 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 > + /* 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. > + /* > + * 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) > + __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__); > + /* > + * 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. > +} > + > /* defined after fortified strlen and strnlen to reuse them */ > __FORTIFY_INLINE char *strncat(char *p, const char *q, __kernel_size_t count) > { > -- > 2.20.1 > -- Kees Cook
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