[LLVMdev] [RFC] Simple control-flow integrity

[Tom Roeder]

On Thu, Apr 3, 2014 at 7:44 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:

> 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.
>

I have an implementation of my proposal that generates the right
instructions and only those instructions. This is because of the other
annotations on the jumptable function: optnone, noreturn, naked, and
unwind. I've just posted my current version of the patch as
http://llvm-reviews.chandlerc.com/D3292


>
> 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.
>

I guess this boils down to the question of whether or not it's possible to
insist on naked functions with no optimization. My jumptable intrinsic is
guaranteed by the IR verifier constraints to be the only reachable IR
instruction in the function at code-generation time.

Because this instruction is an intrinsic and only undergoes lowering to a
jump to a symbol, and because the function is annotated with naked,
optnone, noreturn, and nounwind, I believe that I have complete control
over the instructions in that function, and that there will be no
preamble/postamble. Are there mechanisms that can insert other code in
these circumstances?

>From a perusal of CodeGen, it looks like the attribute optnone translates
to CodeGenOpt::None, which in turn seems to turn off lots of work that
would otherwise be performed on the function.

I can certainly add test cases that make sure that no extra code is added
now and that will break if anyone ever adds code that does this.



>
> 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.
>

I see the value in terms of the security guarantees, though isn't it the
case that other code could in principle be added to the function unless the
jump-table generation was literally the last transform to run?

My concern about it is more to do with code duplication and maintainability
for multiple backends. Consider the set of changes that the jump-table
transformation does:

1. find all address-taken functions
2. create a jump-table function for each one
3. replace all address-taking sites with the appropriate jump-table
function address
3. ensure that the jump-table functions are compiled into a form that
admits useful runtime checks

Most of this is not target-specific, and these are all very natural IR
operations. And I want them to work at least for X86 and ARM (I currently
support both with my inline asm version), and probably for other backends,
too. The only target-specific step is the lowering of the jump-table
intrinsic to its equivalent branch statement, and the only reason that this
is target-specific is that there's no way to represent an unconditional
jump to a function in IR. So, I'd really like to confine the
target-specific code to exactly that lowering.

Note that there are CFI transformations that don't depend on the tightness
of the jump table: e.g., if the check code just compares a given pointer
against the start and the end functions in the table, then function section
annotations would suffice, since that would guarantee that all and only
jumptable functions would be in a given section. I would view the tightness
of the jump table more as an optimization than as a requirement.

It seems to me to be unnecessarily painful to have to fiddle with the
lowering, e.g., of function symbols to their associated jump-table
functions in the target backend because we don't want to represent the
jump-table entries as functions at IR time.


>
> 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
>
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