[LLVMdev] [RFC] Simple control-flow integrity

[Peter Collingbourne]

On Thu, Apr 03, 2014 at 06:54:55PM -0700, Reid Kleckner wrote:
> I think it's a little scary to assume things about LLVM's x86 code
> generation.  I haven't really finished the codegen side of the change, but
> I'm pretty sure in it's current state it will emit extra loads and stores,
> even if they are unnecessary.

Right, I had similar concerns. Now that I've thought about it a little more,
I think that representing the jump table entries as IR functions in any
form presents a risk that a change in code generation could endanger the
correctness of the CFI mechanism.

The other alternative we were thinking about was to put the responsibility for
generating jump tables in the hands of the code generator using attributes
[1].  From a security perspective, I think that might be preferable since
the generation of the jump table would be tightly controlled.

Peter

[1] http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-March/071340.html

> 
> 
> On Thu, Apr 3, 2014 at 6:28 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
> 
> > On Wed, Apr 02, 2014 at 05:28:04PM -0700, Tom Roeder wrote:
> > > On Fri, Mar 21, 2014 at 1:46 PM, Peter Collingbourne <peter at pcc.me.uk
> > >wrote:
> > >
> > > > On Fri, Mar 21, 2014 at 12:54:07PM -0700, Tom Roeder wrote:
> > > > > On Fri, Mar 21, 2014 at 12:15 PM, Peter Collingbourne <
> > peter at pcc.me.uk>
> > > > wrote:
> > > > > >> The way I've implemented it (see the patch I sent to llvm-commits
> > > > > >> yesterday), it's not just metadata: the intrinsic lowers to the
> > > > > >> jumptable entry code given above. The CFI pass then generates a
> > > > > >> function for each jump table; the function consists solely of
> > these
> > > > > >> intrinsic calls.
> > > > > >
> > > > > > Well, the intrinsic you proposed has no effect on the caller and
> > has
> > > > > > non-local effects on other specified functions. I'm not aware of
> > any
> > > > other
> > > > > > intrinsic with similar behavior.
> > > > >
> > > > > I agree that it's not very similar to other intrinsics. But I don't
> > > > > exactly follow these statements. There are definitely intrinsics that
> > > > > have no effect on the caller, like llvm.var.annotation.
> > > >
> > > > Yes but the purpose of such intrinsics is to communicate information
> > about
> > > > a specific value that may have an effect on analysis, optimization or
> > code
> > > > generation for that caller. On the other hand, the intrinsic you are
> > > > proposing
> > > > has nothing to do with the caller.
> > > >
> > > > > And AFAIK,
> > > > > there is no non-local behavior: all the intrinsic does is lower to
> > the
> > > > > labeled jump instruction; the changes to address-taken functions are
> > > > > done separately by the CFI pass. Note that in the patch I sent, the
> > > > > intrinsic only takes one argument: the function to jump to. Are there
> > > > > other effects in this case?
> > > >
> > > > The non-local effect is that the intrinsic describes the definition of
> > a
> > > > function in the global scope. Normally such definitions come from
> > top-level
> > > > entities.
> > > >
> > > > > So, maybe it would be better to call it something like
> > > > > @llvm.unconditional.jump(i8*)? I could then make it only lower to the
> > > > > jump and add an intrinsic that lowered to a function label as well.
> > > >
> > > > I'd imagine that might present more problems. For example, if you used
> > > > either intrinsic in the middle of a regular function the behavior
> > would not
> > > > necessarily be well defined. It would be necessary to carefully
> > document
> > > > where and how these intrinsics may be used.
> > > >
> > >
> > > Taking these considerations into account, I propose this:
> > >
> > > Jump functions created for the jump table are actual functions, marked
> > with
> > > a new jumptable attribute, as well as naked and noreturn and optnone.
> > Each
> > > table is put in a special section using the section attribute, and
> > > alignment of these functions is done using the align attribute. The
> > > combination of these attributes means that the function has no preamble
> > or
> > > postamble, so it consists of exactly a global function label and its
> > > instructions.
> > >
> > > Then I think it would make sense to create an intrinsic like
> > > llvm.jumptable.instr(i8*) that would be a placeholder for an
> > unconditional
> > > jump. I can add code to the verifier that insists on two conditions:
> > >
> > > 1. functions marked as jumptable must have exactly two instructions: an
> > > llvm.jumptable.instr followed by a unreachable.
> > > 2. llvm.jumptable.instr can only occur in a jumptable function.
> > >
> > > I think this handles the problem of using llvm.jumptable.instr in normal
> > > functions: that isn't allowed. And I think it deals with the problems of
> > > non-local behavior by making functions out of the things that really are
> > > functions. Each entry in the jump table really is a function, albeit a
> > > rather strange one, so it should look like a function at the IR level.
> > >
> > > In other words, this change would add a new attribute that marks a very
> > > specialized kind of function and an intrinsic that can only occur in this
> > > kind of function.
> > >
> > > What do you think?
> >
> > I think you might be close.
> >
> > The llvm.jumptable.instr intrinsic you have proposed is similar to the
> > 'musttail' call marker that Reid (cc'd) is working on in [1]. That marker
> > will
> > also have verifier support. I think it might be reasonable to require the
> > code generator to emit the body of a function containing only a 'musttail'
> > function call as a single branch instruction. Then you could emit such
> > functions in the jump table section.
> >
> > Reid, what do you think?
> >
> > Thanks,
> > --
> > Peter
> >
> > [1] http://llvm-reviews.chandlerc.com/D3240
> >

-- 
Peter