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From: Peter Zijlstra <peterz@infradead.org>
To: Sami Tolvanen <samitolvanen@google.com>
Cc: x86@kernel.org, Kees Cook <keescook@chromium.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org,
	clang-built-linux@googlegroups.com,
	Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH v2 00/14] x86: Add support for Clang CFI
Date: Tue, 24 Aug 2021 21:46:52 +0200	[thread overview]
Message-ID: <20210824194652.GB17784@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20210823171318.2801096-1-samitolvanen@google.com>

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.

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.

  parent reply	other threads:[~2021-08-24 19:47 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-23 17:13 Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 01/14] objtool: Add CONFIG_CFI_CLANG support Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 02/14] objtool: Add ASM_STACK_FRAME_NON_STANDARD Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 03/14] linkage: Add DECLARE_ASM_FUNC_SYMBOL Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 04/14] ftrace: Use an opaque type for functions not callable from C Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 05/14] lkdtm: Disable UNSET_SMEP with CFI Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 06/14] lkdtm: Use an opaque type for lkdtm_rodata_do_nothing Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 07/14] x86: Use an opaque type for functions not callable from C Sami Tolvanen
2021-08-26 16:54   ` Andy Lutomirski
2021-08-26 22:11     ` Sami Tolvanen
2021-08-26 23:23       ` Andy Lutomirski
2021-08-26 23:45         ` Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 08/14] x86/extable: Do not mark exception callback as CFI Sami Tolvanen
2021-08-26 16:56   ` Andy Lutomirski
2021-08-30 19:57     ` Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 09/14] x86/purgatory: Disable CFI Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 10/14] x86, relocs: Ignore __typeid__ relocations Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 11/14] x86, module: " Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 12/14] x86, cpu: Use LTO for cpu.c with CFI Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 13/14] x86, kprobes: Fix optprobe_template_func type mismatch Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 14/14] x86, build: Allow CONFIG_CFI_CLANG to be selected Sami Tolvanen
2021-08-23 17:16 ` [PATCH v2 00/14] x86: Add support for Clang CFI Tom Stellard
2021-08-23 17:20   ` Sami Tolvanen
2021-08-24 17:26     ` Tom Stellard
2021-08-24 17:30       ` Sami Tolvanen
2021-08-24 19:46 ` Peter Zijlstra [this message]
2021-08-25 15:49   ` Sami Tolvanen
2021-08-26 11:43     ` Peter Zijlstra
2021-08-26 21:52       ` Sami Tolvanen

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