Re: [PATCH v4 00/15] x86: Add support for Clang CFI

[Josh Poimboeuf]

On Tue, Oct 05, 2021 at 02:52:46PM -0700, Sami Tolvanen wrote:
> On Tue, Oct 5, 2021 at 1:37 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> >
> > On Thu, Sep 30, 2021 at 11:05:16AM -0700, Sami Tolvanen wrote:
> > > This series adds support for Clang's Control-Flow Integrity (CFI)
> > > checking to x86_64. With CFI, the compiler injects a runtime
> > > check before each indirect function call to ensure the target is
> > > a valid function with the correct static type. This restricts
> > > possible call targets and makes it more difficult for an attacker
> > > to exploit bugs that allow the modification of stored function
> > > pointers. For more details, see:
> > >
> > >   https://clang.llvm.org/docs/ControlFlowIntegrity.html
> > >
> > > Note that v4 is based on tip/master. The first two patches contain
> > > objtool support for CFI, the remaining patches change function
> > > declarations to use opaque types, fix type mismatch issues that
> > > confuse the compiler, and disable CFI where it can't be used.
> > >
> > > You can also pull this series from
> > >
> > >   https://github.com/samitolvanen/linux.git x86-cfi-v4
> >
> > Does this work for indirect calls made from alternatives?
> 
> It works in the sense that indirect calls made from alternatives won't
> trip CFI. The compiler doesn't instrument inline assembly.
> 
> > I'm also wondering whether this works on CONFIG_RETPOLINE systems which
> > disable retpolines at runtime, combined with Peter's patch to use
> > objtool to replace retpoline thunk calls with indirect branches:
> >
> >   9bc0bb50727c ("objtool/x86: Rewrite retpoline thunk calls")
> >
> > Since presumably objtool runs after the CFI stuff is inserted.
> 
> The indirect call checking is before the retpoline thunk call, so
> replacing the call with an indirect call isn't a problem.

Ah right.  I managed to forget how this worked and was thinking this
intercepted the indirect call rather than the function pointer.

-- 
Josh