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Date:   Thu, 1 Sep 2022 14:19:54 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v4 00/21] KCFI support

Hi Sami,

On Tue, Aug 30, 2022 at 04:31:08PM -0700, Sami Tolvanen wrote:
> KCFI is a forward-edge control-flow integrity scheme in the upcoming
> Clang 16 release, which is more suitable for kernel use than the
> existing CFI scheme used by CONFIG_CFI_CLANG. KCFI doesn't require
> LTO, doesn't alter function references to point to a jump table, and
> won't break function address equality.
> 
> This series replaces the current arm64 CFI implementation with KCFI
> and adds support for x86_64.
> 
> KCFI requires assembly functions that are indirectly called from C
> code to be annotated with type identifiers. As type information is
> only available in C, the compiler emits expected type identifiers
> into the symbol table, so they can be referenced from assembly
> without having to hardcode type hashes. Patch 6 adds helper macros
> for annotating functions, and patches 9 and 19 add annotations.
> 
> In case of a type mismatch, KCFI always traps. To support error
> handling, the compiler generates a .kcfi_traps section for x86_64,
> which contains the locations of each trap, and for arm64, encodes
> the necessary register information to the ESR. Patches 10 and 21 add
> arch-specific error handlers.
> 
> To test this series, you'll a ToT Clang toolchain. The series is
> also available in GitHub:
> 
>   https://github.com/samitolvanen/linux/commits/kcfi-v4

I took this series for a spin on arm64 and x86_64.

I did not see any runtime issues on my arm64 or AMD test machines but I
do see a set of failures on my two Intel test machines when accessing
the files in /sys/devices/pci0000:00/0000:00:02.0/drm/card0/gt/gt0:

  $ ls -1 /sys/devices/pci0000:00/0000:00:02.0/drm/card0/gt/gt0
  id
  punit_req_freq_mhz
  rc6_enable
  rc6_residency_ms
  rps_act_freq_mhz
  rps_boost_freq_mhz
  rps_cur_freq_mhz
  rps_max_freq_mhz
  rps_min_freq_mhz
  rps_rp0_freq_mhz
  rps_rp1_freq_mhz
  rps_rpn_freq_mhz
  throttle_reason_pl1
  throttle_reason_pl2
  throttle_reason_pl4
  throttle_reason_prochot
  throttle_reason_ratl
  throttle_reason_status
  throttle_reason_thermal
  throttle_reason_vr_tdc
  throttle_reason_vr_thermalert

  $ cat /sys/devices/pci0000:00/0000:00:02.0/drm/card0/gt/gt0/*
  ...

  $ sudo dmesg | rg "CFI failure at"
  [  481.234522] CFI failure at kobj_attr_show+0x19/0x30 (target: max_freq_mhz_show+0x0/0xe0 [i915]; expected type: 0xc527b809)
  [  481.234699] CFI failure at kobj_attr_show+0x19/0x30 (target: act_freq_mhz_show+0x0/0xe0 [i915]; expected type: 0xc527b809)
  [  481.235067] CFI failure at kobj_attr_show+0x19/0x30 (target: boost_freq_mhz_show+0x0/0xe0 [i915]; expected type: 0xc527b809)
  [  481.235194] CFI failure at kobj_attr_show+0x19/0x30 (target: min_freq_mhz_show+0x0/0xe0 [i915]; expected type: 0xc527b809)
  [  481.235320] CFI failure at kobj_attr_show+0x19/0x30 (target: punit_req_freq_mhz_show+0x0/0x40 [i915]; expected type: 0xc527b809)
  [  481.235447] CFI failure at kobj_attr_show+0x19/0x30 (target: throttle_reason_bool_show+0x0/0x50 [i915]; expected type: 0xc527b809)
  [  481.235570] CFI failure at kobj_attr_show+0x19/0x30 (target: id_show+0x0/0x70 [i915]; expected type: 0xc527b809)
  [  481.235694] CFI failure at kobj_attr_show+0x19/0x30 (target: throttle_reason_bool_show+0x0/0x50 [i915]; expected type: 0xc527b809)
  [  481.235821] CFI failure at kobj_attr_show+0x19/0x30 (target: throttle_reason_bool_show+0x0/0x50 [i915]; expected type: 0xc527b809)
  [  481.235945] CFI failure at kobj_attr_show+0x19/0x30 (target: cur_freq_mhz_show+0x0/0xe0 [i915]; expected type: 0xc527b809)
  [  481.236075] CFI failure at kobj_attr_show+0x19/0x30 (target: throttle_reason_bool_show+0x0/0x50 [i915]; expected type: 0xc527b809)
  [  481.236201] CFI failure at kobj_attr_show+0x19/0x30 (target: rc6_enable_show+0x0/0x40 [i915]; expected type: 0xc527b809)
  [  481.236327] CFI failure at kobj_attr_show+0x19/0x30 (target: throttle_reason_bool_show+0x0/0x50 [i915]; expected type: 0xc527b809)
  [  481.236453] CFI failure at kobj_attr_show+0x19/0x30 (target: RP0_freq_mhz_show+0x0/0xe0 [i915]; expected type: 0xc527b809)
  [  481.236582] CFI failure at kobj_attr_show+0x19/0x30 (target: throttle_reason_bool_show+0x0/0x50 [i915]; expected type: 0xc527b809)
  [  481.236707] CFI failure at kobj_attr_show+0x19/0x30 (target: throttle_reason_bool_show+0x0/0x50 [i915]; expected type: 0xc527b809)
  [  481.236836] CFI failure at kobj_attr_show+0x19/0x30 (target: throttle_reason_bool_show+0x0/0x50 [i915]; expected type: 0xc527b809)
  [  481.236958] CFI failure at kobj_attr_show+0x19/0x30 (target: RPn_freq_mhz_show+0x0/0xe0 [i915]; expected type: 0xc527b809)
  [  481.237079] CFI failure at kobj_attr_show+0x19/0x30 (target: RP1_freq_mhz_show+0x0/0xe0 [i915]; expected type: 0xc527b809)
  [  481.237224] CFI failure at kobj_attr_show+0x19/0x30 (target: throttle_reason_bool_show+0x0/0x50 [i915]; expected type: 0xc527b809)
  [  481.237377] CFI failure at kobj_attr_show+0x19/0x30 (target: rc6_residency_ms_show+0x0/0x270 [i915]; expected type: 0xc527b809)

The source of those is drivers/gpu/drm/i915/gt/intel_gt_sysfs_pm.c. I
have not looked too closely yet but the fix should be something along
the lines of commit 58606220a2f1 ("drm/i915: Fix CFI violation with
show_dynamic_id()"). I will note that the 'dev' variable does appear to
be used so the fix might not be as trivial as that one. Not sure I will
have a chance to look into it before Plumbers but I am happy to test a
patch if you happen to see an obvious fix.

Interestingly, I do not see the KVM failure [1] that I reported anymore.
I do not see an obvious fix for it in this series or -next though, could
it have been an issue with an earlier revision of kCFI on the compiler
side?

I do see a few new objtool warnings as well:

vmlinux.o: warning: objtool: apply_relocate_add+0x34: relocation to !ENDBR: memcpy+0x0
vmlinux.o: warning: objtool: ___ksymtab+__memcpy+0x0: data relocation to !ENDBR: memcpy+0x0
vmlinux.o: warning: objtool: ___ksymtab+memcpy+0x0: data relocation to !ENDBR: memcpy+0x0

As a result of the above:

Tested-by: Nathan Chancellor <nathan@...nel.org>

[1]: https://github.com/ClangBuiltLinux/linux/issues/1644

Cheers,
Nathan

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