Enable run-time checking of dynamic memcpy() and memmove() lengths, issuing a WARN when a write would exceed the size of the target struct member, when built with CONFIG_FORTIFY_SOURCE=y. This would have caught all of the memcpy()-based buffer overflows in the last 3 years, specifically covering all the cases where the destination buffer size is known at compile time.

This change ONLY adds a run-time warning. As false positives are currently still expected, this will not block the overflow. The new warnings will look like this:

memcpy: detected field-spanning write (size N) of single field "var->dest" (size M) WARNING: CPU: n PID: pppp at source/file/path.c:nr function+0xXX/0xXX [module]

There may be false positives in the kernel where intentional field-spanning writes are happening. These need to be addressed similarly to how the compile-time cases were addressed: add a struct_group(), split the memcpy(), or some other refactoring.

In order to make counting/investigating instances of added runtime checks easier, each instance includes the destination variable name as a WARN argument, prefixed with 'field "'. Therefore, on an x86_64 defconfig build, it is trivial to inspect the build artifacts to find instances. For example on an x86_64 defconfig build, there are 78 new run-time memcpy() bounds checks added:

$ for i in vmlinux $(find . -name '*.ko'); do \ strings "$i" | grep '^field "'; done | wc -l 78

Simple cases where a destination buffer is known to be a dynamic size do not generate a WARN. For example:

struct normal_flex_array { void *a; int b; u32 c; size_t array_size; u8 flex_array[]; };

struct normal_flex_array *instance; ... /* These will be ignored for run-time bounds checking. */ memcpy(instance, src, len); memcpy(instance->flex_array, src, len);

However, one of the dynamic-sized destination cases is irritatingly unable to be detected by the compiler: when using memcpy() to target a composite struct member which contains a trailing flexible array struct. For example:

struct wrapper { int foo; char bar; struct normal_flex_array embedded; };

struct wrapper *instance; ... /* This will incorrectly WARN when len > sizeof(instance->embedded) */ memcpy(&instance->embedded, src, len);

These cases end up appearing to the compiler to be sized as if the flexible array had 0 elements. :( For more details see: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101832 https://godbolt.org/z/vW6x8vh4P

These "composite flexible array structure destination" cases will be need to be flushed out and addressed on a case-by-case basis.

Regardless, for the general case of using memcpy() on flexible array destinations, future APIs will be created to handle common cases. Those can be used to migrate away from open-coded memcpy() so that proper error handling (instead of trapping) can be used.

As mentioned, none of these bounds checks block any overflows currently. For users that have tested their workloads, do not encounter any warnings, and wish to make these checks stop any overflows, they can use a big hammer and set the sysctl panic_on_warn=1.

Signed-off-by: Kees Cook keescook@chromium.org --- include/linux/fortify-string.h | 70 ++++++++++++++++++++++++++++++++-- 1 file changed, 67 insertions(+), 3 deletions(-)

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h index 3b401fa0f374..ca3626c41785 100644 --- a/include/linux/fortify-string.h +++ b/include/linux/fortify-string.h @@ -3,6 +3,7 @@ #define _LINUX_FORTIFY_STRING_H_

#include <linux/const.h> +#include <linux/bug.h>

#define __FORTIFY_INLINE extern __always_inline __gnu_inline __overloadable #define __RENAME(x) __asm__(#x) @@ -319,7 +320,7 @@ __FORTIFY_INLINE void fortify_memset_chk(__kernel_size_t size, * V = vulnerable to run-time overflow (will need refactoring to solve) * */ -__FORTIFY_INLINE void fortify_memcpy_chk(__kernel_size_t size, +__FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, const size_t p_size, const size_t q_size, const size_t p_size_field, @@ -368,16 +369,79 @@ __FORTIFY_INLINE void fortify_memcpy_chk(__kernel_size_t size, if ((p_size != (size_t)(-1) && p_size < size) || (q_size != (size_t)(-1) && q_size < size)) fortify_panic(func); + + /* + * Warn when writing beyond destination field size. + * + * We must ignore p_size_field == 0 for existing 0-element + * fake flexible arrays, until they are all converted to + * proper flexible arrays. + * + * The implementation of __builtin_object_size() behaves + * like sizeof() when not directly referencing a flexible + * array member, which means there will be many bounds checks + * that will appear at run-time, without a way for them to be + * detected at compile-time (as can be done when the destination + * is specifically the flexible array member). + * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101832 + */ + if (p_size_field != 0 && p_size_field != (size_t)(-1) && + p_size != p_size_field && p_size_field < size) + return true; + + return false; }

#define __fortify_memcpy_chk(p, q, size, p_size, q_size, \ p_size_field, q_size_field, op) ({ \ size_t __fortify_size = (size_t)(size); \ - fortify_memcpy_chk(__fortify_size, p_size, q_size, \ - p_size_field, q_size_field, #op); \ + WARN_ONCE(fortify_memcpy_chk(__fortify_size, p_size, q_size, \ + p_size_field, q_size_field, #op), \ + #op ": detected field-spanning write (size %zu) of single %s (size %zu)\n", \ + __fortify_size, \ + "field "" #p "" at " __FILE__ ":" __stringify(__LINE__), \ + p_size_field); \ __underlying_##op(p, q, __fortify_size); \ })

+/* + * Notes about compile-time buffer size detection: + * + * With these types... + * + * struct middle { + * u16 a; + * u8 middle_buf[16]; + * int b; + * }; + * struct end { + * u16 a; + * u8 end_buf[16]; + * }; + * struct flex { + * int a; + * u8 flex_buf[]; + * }; + * + * void func(TYPE *ptr) { ... } + * + * Cases where destination size cannot be currently detected: + * - the size of ptr's object (seemingly by design, gcc & clang fail): + * __builtin_object_size(ptr, 1) == -1 + * - the size of flexible arrays in ptr's obj (by design, dynamic size): + * __builtin_object_size(ptr->flex_buf, 1) == -1 + * - the size of ANY array at the end of ptr's obj (gcc and clang bug): + * __builtin_object_size(ptr->end_buf, 1) == -1 + * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 + * + * Cases where destination size is currently detected: + * - the size of non-array members within ptr's object: + * __builtin_object_size(ptr->a, 1) == 2 + * - the size of non-flexible-array in the middle of ptr's obj: + * __builtin_object_size(ptr->middle_buf, 1) == 16 + * + */ + /* * __builtin_object_size() must be captured here to avoid evaluating argument * side-effects further into the macro layers.