Re: [PATCH v2 00/14] x86: Add support for Clang CFI

[Tom Stellard]

On 8/23/21 10:20 AM, Sami Tolvanen wrote:
> On Mon, Aug 23, 2021 at 10:16 AM Tom Stellard <tstellar@xxxxxxxxxx> wrote:
> > On 8/23/21 10:13 AM, 'Sami Tolvanen' via Clang Built Linux 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
> > >
> > > Version 2 depends on Clang >=14, where we fixed the issue with
> > > referencing static functions from inline assembly. Based on the
> > > feedback for v1, this version also changes the declaration of
> > > functions that are not callable from C to use an opaque type,
> > > which stops the compiler from replacing references to them. This
> > > avoids the need to sprinkle function_nocfi() macros in the kernel
> > > code.
> >
> > How invasive are the changes in clang 14 necessary to make CFI work?
> > Would it be possible to backport them to LLVM 13?
>
> I'm not sure what the LLVM backport policy is, but this specific fix
> was quite simple:
>
> https://reviews.llvm.org/rG7ce1c4da7726

That looks like something we could backport, I filed a bug to track
the backport: https://bugs.llvm.org/show_bug.cgi?id=51588.

Do you have any concerns about backporting it or do you think it's pretty
safe?

-Tom


> Sami