Re: [PATCH v2] lib/string.c: implement stpcpy

[Kees Cook]

On Fri, Aug 14, 2020 at 07:09:44PM -0700, Nick Desaulniers wrote:
> LLVM implemented a recent "libcall optimization" that lowers calls to
> `sprintf(dest, "%s", str)` where the return value is used to
> `stpcpy(dest, str) - dest`. This generally avoids the machinery involved
> in parsing format strings.  Calling `sprintf` with overlapping arguments
> was clarified in ISO C99 and POSIX.1-2001 to be undefined behavior.
> 
> `stpcpy` is just like `strcpy` except it returns the pointer to the new
> tail of `dest`. This allows you to chain multiple calls to `stpcpy` in
> one statement.

O_O What?

No; this is a _terrible_ API: there is no bounds checking, there are no
buffer sizes. Anything using the example sprintf() pattern is _already_
wrong and must be removed from the kernel. (Yes, I realize that the
kernel is *filled* with this bad assumption that "I'll never write more
than PAGE_SIZE bytes to this buffer", but that's both theoretically
wrong ("640k is enough for anybody") and has been known to be wrong in
practice too (e.g. when suddenly your writing routine is reachable by
splice(2) and you may not have a PAGE_SIZE buffer).

But we cannot _add_ another dangerous string API. We're already in a
terrible mess trying to remove strcpy[1], strlcpy[2], and strncpy[3]. This
needs to be addressed up by removing the unbounded sprintf() uses. (And
to do so without introducing bugs related to using snprintf() when
scnprintf() is expected[4].)

-Kees

[1] https://github.com/KSPP/linux/issues/88
[2] https://github.com/KSPP/linux/issues/89
[3] https://github.com/KSPP/linux/issues/90
[4] https://lore.kernel.org/lkml/20200810092100.GA42813@2f5448a72a42/

-- 
Kees Cook