linux-hardening - Re: [PATCH v4] fortify: Work around Clang inlining bugs

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Date:   Mon, 31 Jan 2022 13:13:30 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v4] fortify: Work around Clang inlining bugs

On Mon, Jan 31, 2022 at 11:04:36AM -0800, Nick Desaulniers wrote:
> On Sun, Jan 30, 2022 at 10:22 AM Kees Cook <keescook@...omium.org> wrote:
> >
> > To enable FORTIFY_SOURCE support for Clang, the kernel must work around
> > a pair of bugs, related to Clang's inlining:
> >
> > 1) Change all the fortified string APIs into macros with different
> > inline names to bypass Clang's broken inline-of-a-builtin detection:
> > https://bugs.llvm.org/show_bug.cgi?id=50322
> >
> > 2) Lift all misbehaving __builtin_object_size() calls into the macros
> > to bypass Clang's broken __builtin_object_size() arguments-of-an-inline
> > visibility:
> > https://github.com/ClangBuiltLinux/linux/issues/1401
> 
> ^ mentions a difference in compilers for mode 1. I wonder if this
> patch could "hoist" the BOS calls into the macro ONLY for mode 1 and
> not mode 0 usage? i.e. the str* functions, not the mem* functions.

Everything (with a couple exceptions) is using mode 1 after the earlier
patches in the series. e.g.:

+#define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s, \
+		__builtin_object_size(p, 0), __builtin_object_size(q, 0), \
+		__builtin_object_size(p, 1), __builtin_object_size(q, 1), \
+		memcpy)

> It's too late to fix these in clang-13.  If we get a fix in clang-14
> or later, what does that look like for this header?  Is there a way we

If the bos mode 1 got fixed for Clang 14, this patch would likely be
dropped and the Clang + FORTIFY version check would be moved to Clang
14.

> can provide a different header than include/linux/fortify-string.h
> just for clang-13 (or whatever versions until the above are fixed)?

So much of it would be identical. This macro-ification is least
invasive, and it's pretty invasive.

> I don't see this series getting backported to stable, where older
> versions of clang may still be in use.

Right.

> I'm tempted to say "let's get get these 2 fixed in clang-14" but we'll
> probably have to trade something off the existing TODO list.

Agreed.

-- 
Kees Cook