[PATCH 0/2] start_kernel: omit stack canary

ndesaulniers
2023-04-12T18:32:19Z

A security research paper was recently published detailing Catch Handler
Oriented Programming (CHOP) attacks.
https://download.vusec.net/papers/chop_ndss23.pdf
The TL;DR being that C++ structured exception handling runtimes are
attractive gadgets for Jump Oriented Programming (JOP) attacks.

In response to this, a mitigation was developed under embargo in
clang-16 to check the stack canary before calling noreturn functions.
https://bugs.chromium.org/p/llvm/issues/detail?id=30

This started causing boot failures in Android kernel trees downstream of
stable linux-4.14.y that had proto-LTO support, as reported by Nathan
Chancellor.
https://github.com/ClangBuiltLinux/linux/issues/1815

Josh Poimboeuf recently sent a series to explicitly annotate more
functions as noreturn. Nathan noticed the series, and tested it finding
that it now caused boot failures with clang-16+ on mainline (raising the
visibility and urgency of the issue).
https://lore.kernel.org/cover.1680912057.git.jpoimboe@kernel.org/

Once the embargo was lifted, I asked questions such as "what does C++
structured exception handling have to do with C code" and "surely GCC
didn't ship the same mitigation for C code (narrator: 'They did not:
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=a25982ada523689c8745d7fb4b1b93c8f5dab2e7')?"

I now have a patch out for LLVM to undo this mess (or at least limit it
to C++ functions that may throw, similar to GCC's mitigation); it hasn't
landed yet but we're close to consensus and I expect it to land
imminently.
https://reviews.llvm.org/D147975

Remember this thread? (Pepperidge farms remembers...)
https://lore.kernel.org/all/20200314164451.346497-1-slyfox@gentoo.org/

That reminded me that years ago we discussed a function attribute for
no_stack_protector.
https://lore.kernel.org/all/20200316130414.GC12561@hirez.programming.kicks-ass.net/

GCC didn't have one at the time, it now does. In addition to the LLVM
fix, I'd like to introduce this in the kernel so that we might start
using it in additional places:
* https://lore.kernel.org/linux-pm/20200915172658.1432732-1-rkir@google.com/
* https://lore.kernel.org/lkml/20200918201436.2932360-30-samitolvanen@google.com/
And eventually remove the final macro expansion site of
prevent_tail_call_optimization.

With the LLVM fix, this series isn't required, but I'd like to start
paving the way to use these function attributes since I think they are a
sweet spot in terms of granularity (as opposed to trying to move
start_kernel to its own TU compiled with -fno-stack-protector).

(This is my first time using b4 to send a series, as per
https://people.kernel.org/monsieuricon/sending-a-kernel-patch-with-b4-part-1.
Here goes nothing!)

Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Nick Desaulniers (2):
start_kernel: add no_stack_protector fn attr
start_kernel: omit prevent_tail_call_optimization for newer toolchains

arch/powerpc/kernel/smp.c | 1 +
include/linux/compiler_attributes.h | 12 ++++++++++++
init/main.c | 8 +++++++-
3 files changed, 20 insertions(+), 1 deletion(-)
---
base-commit: 0bcc4025550403ae28d2984bddacafbca0a2f112
change-id: 20230412-no_stackp-a98168a2bb0a

Best regards,
--
Nick Desaulniers <ndesaulniers@google.com>