Messages in this thread | | | Date | Wed, 27 Oct 2021 12:17:21 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v5 00/15] x86: Add support for Clang CFI |
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On Wed, Oct 27, 2021 at 10:02:56AM +0000, David Laight wrote: > From: Peter Zijlstra > > Sent: 26 October 2021 21:16 > > > > On Wed, Oct 13, 2021 at 11:16:43AM -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 > > > > So, if I understand this right, the compiler emits, for every function > > two things: 1) the actual funcion and 2) a jump-table entry. > > > > Then, every time the address of a function is taken, 2) is given instead > > of the expected 1), right? > > > > But how does this work with things like static_call(), which we give a > > function address (now a jump-table entry) and use that to write direct > > call instructions? > > > > Should not this jump-table thingy get converted to an actual function > > address somewhere around arch_static_call_transform() ? This also seems > > relevant for arm64 (which already has CLANG_CFI supported) given: > > > > https://lkml.kernel.org/r/20211025122102.46089-3-frederic@kernel.org > > > > Or am I still not understanding this CFI thing? > > From what I remember the compiler adds code prior to every jump indirect > to check that the function address is in the list of valid functions > (with a suitable prototype - or some similar check).
It definitely mucks about with the address too; see here:
https://lkml.kernel.org/r/YW6a67fGzM2AyHot@hirez.programming.kicks-ass.net
I'm thinking static_call() wants the real actual function address before it writes instructions.
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