Messages in this thread | | | Date | Thu, 26 Aug 2021 13:43:59 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 00/14] x86: Add support for Clang CFI |
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On Wed, Aug 25, 2021 at 08:49:36AM -0700, Sami Tolvanen wrote: > On Tue, Aug 24, 2021 at 12:47 PM Peter Zijlstra <peterz@infradead.org> wrote: > > > > On Mon, Aug 23, 2021 at 10:13:04AM -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: > > > > If I understand this right; tp_stub_func() in kernel/tracepoint.c > > violates this (as would much of the HAVE_STATIC_CALL=n code, luckily > > that is not a valid x86_64 configuration). > > > > Specifically, we assign &tp_stub_func to tracepoint_func::func, but that > > function pointer is only ever indirectly called when cast to the > > tracepoint prototype: > > > > ((void(*)(void *, proto))(it_func))(__data, args); > > > > (see DEFINE_TRACE_FN() in linux/tracepoint.h) > > > > This means the indirect function type and the target function type > > mismatch, resulting in that runtime check you added to trigger. > > Thanks for pointing this out. Yes, that would clearly trip CFI. > > Any concerns about just writing a magic value to the slot instead of > pointing it to a stub function, and checking for it before the call?
Performance :-) that compare is going to be useless roughly 100% of the time.
> > Hitting tp_stub_func() at runtime is exceedingly rare, but possible. > > > > I realize this is strictly UB per C, but realistically any CDECL ABI > > requires that any function with arbitrary signature: > > > > void foo(...) > > { > > } > > > > translates to the exact same code. Specifically on x86-64, the super > > impressive: > > > > foo: > > RET > > > > And as such this works just fine. Except now you wrecked it. > > Sure. Another option is to disable CFI for the functions that perform > the call, but I would rather avoid that whenever possible.
Is there no means of teaching the compiler about these magical functions? There's only two possible stubs:
void foo(...) { }
and
unsigned long bar(...) { return 0; }
Both exist in the kernel. We can easily give them a special function attribute to call them out.
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