| Message ID | 20220920192202.190793-3-keescook@chromium.org (mailing list archive) |
|---|---|
| State | Mainlined |
| Commit | fa35198f39571bbdae53c5b321020021eaad6bd2 |
| Headers | show |
| Series | fortify: Use __builtin_dynamic_object_size() when available | expand |
On 2022-09-20 15:22, Kees Cook wrote: > In preparation for replacing __builtin_object_size() with > __builtin_dynamic_object_size(), all the compile-time size checks need > to check that the bounds variables are, in fact, known at compile-time. > Enforce what was guaranteed with __bos(). In other words, since all uses > of __bos() were constant expressions, it was not required to test for > this. When these change to __bdos(), they _may_ be constant expressions, > and the checks are only valid when the prior condition holds. This > results in no binary differences. > > Signed-off-by: Kees Cook <keescook@chromium.org> > --- > include/linux/fortify-string.h | 50 +++++++++++++++++++++------------- > 1 file changed, 31 insertions(+), 19 deletions(-) > > diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h > index ff879efe94ed..71c0a432c638 100644 > --- a/include/linux/fortify-string.h > +++ b/include/linux/fortify-string.h > @@ -80,6 +80,12 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size) > #define POS __pass_object_size(1) > #define POS0 __pass_object_size(0) > > +#define __compiletime_lessthan(bounds, length) ( \ > + __builtin_constant_p(length) && \ > + __builtin_constant_p(bounds) && \ > + bounds < length \ > +) So with the gcc ranger, the compiler has lately been quite successful at computing a constant `bounds < length` even though bounds and length are not constant. So perhaps this: #define __compiletime_lessthan (bounds, length) ( \ __builtin_constant (bounds < length) && \ bounds < length \ ) might succeed in a few more cases. Thanks, Sid > + > /** > * strncpy - Copy a string to memory with non-guaranteed NUL padding > * > @@ -117,7 +123,7 @@ char *strncpy(char * const POS p, const char *q, __kernel_size_t size) > { > size_t p_size = __builtin_object_size(p, 1); > > - if (__builtin_constant_p(size) && p_size < size) > + if (__compiletime_lessthan(p_size, size)) > __write_overflow(); > if (p_size < size) > fortify_panic(__func__); > @@ -224,7 +230,7 @@ __FORTIFY_INLINE ssize_t strscpy(char * const POS p, const char * const POS q, s > * If size can be known at compile time and is greater than > * p_size, generate a compile time write overflow error. > */ > - if (__builtin_constant_p(size) && size > p_size) > + if (__compiletime_lessthan(p_size, size)) > __write_overflow(); > > /* > @@ -281,15 +287,16 @@ __FORTIFY_INLINE void fortify_memset_chk(__kernel_size_t size, > /* > * Length argument is a constant expression, so we > * can perform compile-time bounds checking where > - * buffer sizes are known. > + * buffer sizes are also known at compile time. > */ > > /* Error when size is larger than enclosing struct. */ > - if (p_size > p_size_field && p_size < size) > + if (__compiletime_lessthan(p_size_field, p_size) && > + __compiletime_lessthan(p_size, size)) > __write_overflow(); > > /* Warn when write size is larger than dest field. */ > - if (p_size_field < size) > + if (__compiletime_lessthan(p_size_field, size)) > __write_overflow_field(p_size_field, size); > } > /* > @@ -365,25 +372,28 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, > /* > * Length argument is a constant expression, so we > * can perform compile-time bounds checking where > - * buffer sizes are known. > + * buffer sizes are also known at compile time. > */ > > /* Error when size is larger than enclosing struct. */ > - if (p_size > p_size_field && p_size < size) > + if (__compiletime_lessthan(p_size_field, p_size) && > + __compiletime_lessthan(p_size, size)) > __write_overflow(); > - if (q_size > q_size_field && q_size < size) > + if (__compiletime_lessthan(q_size_field, q_size) && > + __compiletime_lessthan(q_size, size)) > __read_overflow2(); > > /* Warn when write size argument larger than dest field. */ > - if (p_size_field < size) > + if (__compiletime_lessthan(p_size_field, size)) > __write_overflow_field(p_size_field, size); > /* > * Warn for source field over-read when building with W=1 > * or when an over-write happened, so both can be fixed at > * the same time. > */ > - if ((IS_ENABLED(KBUILD_EXTRA_WARN1) || p_size_field < size) && > - q_size_field < size) > + if ((IS_ENABLED(KBUILD_EXTRA_WARN1) || > + __compiletime_lessthan(p_size_field, size)) && > + __compiletime_lessthan(q_size_field, size)) > __read_overflow2_field(q_size_field, size); > } > /* > @@ -494,7 +504,7 @@ __FORTIFY_INLINE void *memscan(void * const POS0 p, int c, __kernel_size_t size) > { > size_t p_size = __builtin_object_size(p, 0); > > - if (__builtin_constant_p(size) && p_size < size) > + if (__compiletime_lessthan(p_size, size)) > __read_overflow(); > if (p_size < size) > fortify_panic(__func__); > @@ -508,9 +518,9 @@ int memcmp(const void * const POS0 p, const void * const POS0 q, __kernel_size_t > size_t q_size = __builtin_object_size(q, 0); > > if (__builtin_constant_p(size)) { > - if (p_size < size) > + if (__compiletime_lessthan(p_size, size)) > __read_overflow(); > - if (q_size < size) > + if (__compiletime_lessthan(q_size, size)) > __read_overflow2(); > } > if (p_size < size || q_size < size) > @@ -523,7 +533,7 @@ void *memchr(const void * const POS0 p, int c, __kernel_size_t size) > { > size_t p_size = __builtin_object_size(p, 0); > > - if (__builtin_constant_p(size) && p_size < size) > + if (__compiletime_lessthan(p_size, size)) > __read_overflow(); > if (p_size < size) > fortify_panic(__func__); > @@ -535,7 +545,7 @@ __FORTIFY_INLINE void *memchr_inv(const void * const POS0 p, int c, size_t size) > { > size_t p_size = __builtin_object_size(p, 0); > > - if (__builtin_constant_p(size) && p_size < size) > + if (__compiletime_lessthan(p_size, size)) > __read_overflow(); > if (p_size < size) > fortify_panic(__func__); > @@ -547,7 +557,7 @@ __FORTIFY_INLINE void *kmemdup(const void * const POS0 p, size_t size, gfp_t gfp > { > size_t p_size = __builtin_object_size(p, 0); > > - if (__builtin_constant_p(size) && p_size < size) > + if (__compiletime_lessthan(p_size, size)) > __read_overflow(); > if (p_size < size) > fortify_panic(__func__); > @@ -563,11 +573,13 @@ char *strcpy(char * const POS p, const char * const POS q) > size_t size; > > /* If neither buffer size is known, immediately give up. */ > - if (p_size == SIZE_MAX && q_size == SIZE_MAX) > + if (__builtin_constant_p(p_size) && > + __builtin_constant_p(q_size) && > + p_size == SIZE_MAX && q_size == SIZE_MAX) > return __underlying_strcpy(p, q); > size = strlen(q) + 1; > /* Compile-time check for const size overflow. */ > - if (__builtin_constant_p(size) && p_size < size) > + if (__compiletime_lessthan(p_size, size)) > __write_overflow(); > /* Run-time check for dynamic size overflow. */ > if (p_size < size)
On Wed, Sep 21, 2022 at 07:48:44AM -0400, Siddhesh Poyarekar wrote: > On 2022-09-20 15:22, Kees Cook wrote: > > In preparation for replacing __builtin_object_size() with > > __builtin_dynamic_object_size(), all the compile-time size checks need > > to check that the bounds variables are, in fact, known at compile-time. > > Enforce what was guaranteed with __bos(). In other words, since all uses > > of __bos() were constant expressions, it was not required to test for > > this. When these change to __bdos(), they _may_ be constant expressions, > > and the checks are only valid when the prior condition holds. This > > results in no binary differences. > > > > Signed-off-by: Kees Cook <keescook@chromium.org> > > --- > > include/linux/fortify-string.h | 50 +++++++++++++++++++++------------- > > 1 file changed, 31 insertions(+), 19 deletions(-) > > > > diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h > > index ff879efe94ed..71c0a432c638 100644 > > --- a/include/linux/fortify-string.h > > +++ b/include/linux/fortify-string.h > > @@ -80,6 +80,12 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size) > > #define POS __pass_object_size(1) > > #define POS0 __pass_object_size(0) > > +#define __compiletime_lessthan(bounds, length) ( \ > > + __builtin_constant_p(length) && \ > > + __builtin_constant_p(bounds) && \ > > + bounds < length \ > > +) > > So with the gcc ranger, the compiler has lately been quite successful at > computing a constant `bounds < length` even though bounds and length are not > constant. So perhaps this: > > #define __compiletime_lessthan (bounds, length) ( \ > __builtin_constant (bounds < length) && \ > bounds < length \ > ) > > might succeed in a few more cases. Oh, interesting! That's very cool -- I never considered tossing a full expression into __bcp. Yeah, that seems to work just fine: https://godbolt.org/z/xrchErEx1
diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h index ff879efe94ed..71c0a432c638 100644 --- a/include/linux/fortify-string.h +++ b/include/linux/fortify-string.h @@ -80,6 +80,12 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size) #define POS __pass_object_size(1) #define POS0 __pass_object_size(0) +#define __compiletime_lessthan(bounds, length) ( \ + __builtin_constant_p(length) && \ + __builtin_constant_p(bounds) && \ + bounds < length \ +) + /** * strncpy - Copy a string to memory with non-guaranteed NUL padding * @@ -117,7 +123,7 @@ char *strncpy(char * const POS p, const char *q, __kernel_size_t size) { size_t p_size = __builtin_object_size(p, 1); - if (__builtin_constant_p(size) && p_size < size) + if (__compiletime_lessthan(p_size, size)) __write_overflow(); if (p_size < size) fortify_panic(__func__); @@ -224,7 +230,7 @@ __FORTIFY_INLINE ssize_t strscpy(char * const POS p, const char * const POS q, s * If size can be known at compile time and is greater than * p_size, generate a compile time write overflow error. */ - if (__builtin_constant_p(size) && size > p_size) + if (__compiletime_lessthan(p_size, size)) __write_overflow(); /* @@ -281,15 +287,16 @@ __FORTIFY_INLINE void fortify_memset_chk(__kernel_size_t size, /* * Length argument is a constant expression, so we * can perform compile-time bounds checking where - * buffer sizes are known. + * buffer sizes are also known at compile time. */ /* Error when size is larger than enclosing struct. */ - if (p_size > p_size_field && p_size < size) + if (__compiletime_lessthan(p_size_field, p_size) && + __compiletime_lessthan(p_size, size)) __write_overflow(); /* Warn when write size is larger than dest field. */ - if (p_size_field < size) + if (__compiletime_lessthan(p_size_field, size)) __write_overflow_field(p_size_field, size); } /* @@ -365,25 +372,28 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, /* * Length argument is a constant expression, so we * can perform compile-time bounds checking where - * buffer sizes are known. + * buffer sizes are also known at compile time. */ /* Error when size is larger than enclosing struct. */ - if (p_size > p_size_field && p_size < size) + if (__compiletime_lessthan(p_size_field, p_size) && + __compiletime_lessthan(p_size, size)) __write_overflow(); - if (q_size > q_size_field && q_size < size) + if (__compiletime_lessthan(q_size_field, q_size) && + __compiletime_lessthan(q_size, size)) __read_overflow2(); /* Warn when write size argument larger than dest field. */ - if (p_size_field < size) + if (__compiletime_lessthan(p_size_field, size)) __write_overflow_field(p_size_field, size); /* * Warn for source field over-read when building with W=1 * or when an over-write happened, so both can be fixed at * the same time. */ - if ((IS_ENABLED(KBUILD_EXTRA_WARN1) || p_size_field < size) && - q_size_field < size) + if ((IS_ENABLED(KBUILD_EXTRA_WARN1) || + __compiletime_lessthan(p_size_field, size)) && + __compiletime_lessthan(q_size_field, size)) __read_overflow2_field(q_size_field, size); } /* @@ -494,7 +504,7 @@ __FORTIFY_INLINE void *memscan(void * const POS0 p, int c, __kernel_size_t size) { size_t p_size = __builtin_object_size(p, 0); - if (__builtin_constant_p(size) && p_size < size) + if (__compiletime_lessthan(p_size, size)) __read_overflow(); if (p_size < size) fortify_panic(__func__); @@ -508,9 +518,9 @@ int memcmp(const void * const POS0 p, const void * const POS0 q, __kernel_size_t size_t q_size = __builtin_object_size(q, 0); if (__builtin_constant_p(size)) { - if (p_size < size) + if (__compiletime_lessthan(p_size, size)) __read_overflow(); - if (q_size < size) + if (__compiletime_lessthan(q_size, size)) __read_overflow2(); } if (p_size < size || q_size < size) @@ -523,7 +533,7 @@ void *memchr(const void * const POS0 p, int c, __kernel_size_t size) { size_t p_size = __builtin_object_size(p, 0); - if (__builtin_constant_p(size) && p_size < size) + if (__compiletime_lessthan(p_size, size)) __read_overflow(); if (p_size < size) fortify_panic(__func__); @@ -535,7 +545,7 @@ __FORTIFY_INLINE void *memchr_inv(const void * const POS0 p, int c, size_t size) { size_t p_size = __builtin_object_size(p, 0); - if (__builtin_constant_p(size) && p_size < size) + if (__compiletime_lessthan(p_size, size)) __read_overflow(); if (p_size < size) fortify_panic(__func__); @@ -547,7 +557,7 @@ __FORTIFY_INLINE void *kmemdup(const void * const POS0 p, size_t size, gfp_t gfp { size_t p_size = __builtin_object_size(p, 0); - if (__builtin_constant_p(size) && p_size < size) + if (__compiletime_lessthan(p_size, size)) __read_overflow(); if (p_size < size) fortify_panic(__func__); @@ -563,11 +573,13 @@ char *strcpy(char * const POS p, const char * const POS q) size_t size; /* If neither buffer size is known, immediately give up. */ - if (p_size == SIZE_MAX && q_size == SIZE_MAX) + if (__builtin_constant_p(p_size) && + __builtin_constant_p(q_size) && + p_size == SIZE_MAX && q_size == SIZE_MAX) return __underlying_strcpy(p, q); size = strlen(q) + 1; /* Compile-time check for const size overflow. */ - if (__builtin_constant_p(size) && p_size < size) + if (__compiletime_lessthan(p_size, size)) __write_overflow(); /* Run-time check for dynamic size overflow. */ if (p_size < size)
In preparation for replacing __builtin_object_size() with __builtin_dynamic_object_size(), all the compile-time size checks need to check that the bounds variables are, in fact, known at compile-time. Enforce what was guaranteed with __bos(). In other words, since all uses of __bos() were constant expressions, it was not required to test for this. When these change to __bdos(), they _may_ be constant expressions, and the checks are only valid when the prior condition holds. This results in no binary differences. Signed-off-by: Kees Cook <keescook@chromium.org> --- include/linux/fortify-string.h | 50 +++++++++++++++++++++------------- 1 file changed, 31 insertions(+), 19 deletions(-)