fortify: Add run-time WARN for cross-field memcpy()

fortify: Add run-time WARN for cross-field memcpy()

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>