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From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	live-patching@vger.kernel.org, Michal Marek <mmarek@suse.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andi Kleen <andi@firstfloor.org>, Pedro Alves <palves@redhat.com>,
	Namhyung Kim <namhyung@gmail.com>,
	Bernd Petrovitsch <bernd@petrovitsch.priv.at>,
	Chris J Arges <chris.j.arges@canonical.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jiri Slaby <jslaby@suse.cz>,
	Arnaldo Carvalho de Melo <acme@kernel.org>
Subject: Re: [PATCH v15 13/25] x86/reboot: Add ljmp instructions to stacktool whitelist
Date: Fri, 15 Jan 2016 00:06:52 -0600	[thread overview]
Message-ID: <20160115060652.GA16760@treble.redhat.com> (raw)
In-Reply-To: <20160113105503.GB11575@gmail.com>

On Wed, Jan 13, 2016 at 11:55:03AM +0100, Ingo Molnar wrote:
> 
> * Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> 
> > > Well, I can't say that I'm crazy about all those new tools adding markers to 
> > > unrelated kernel code.
> > > 
> > > Can't you teach stacktool to ignore the whole machine_real_restart() function 
> > > simply?
> > 
> > Well, these STACKTOOL_IGNORE whitelist markers are only needed in a handful of 
> > places, and only for code that does very weird things.  Yes, they're a bit ugly, 
> > but IMO they also communicate valuable information: "be careful, this code does 
> > something very weird."
> 
> How common are these markers? Like with lockdep, it all depends on magnitude:
> 
>  - If it's less than 10 I'd say it's OK.
> 
>  - If it's dozens then it's ho-hum.
> 
>  - If certain types of annotations can go over 100, then they are unacceptable.
> 
> all such in-code overhead has to be balanced against the utility of the tooling.

Sounds reasonable.  After looking at all the warnings on an allyesconfig
kernel,  I'm pretty sure it'll be less than 10.  Most of the cases are
mentioned below.

> > As for whether to put the whitelist info in the code vs hard-coding it in 
> > stacktool, I think it's clearer and less "magical" to put them directly in the 
> > code.
> 
> That's true - but I think Boris tried to ask something slightly different: can 
> stacktool be taught to detect weird signatures automatically, and to ignore them 
> automatically?
> 
> Stuff like 16-bit code sure wounds 'weird' and the tool could detect that?

Some of the 'weird' cases:

- Some functions use 'ljmp' or 'lret'.  After looking at this some more,
  I think it would be safe for stacktool to translate the use of these
  instructions to mean "I know what I'm doing" and just ignore the
  function.  100% of the functions are safe to ignore anyway.  So we can
  get rid of those markers and just make stacktool smarter.

- xen_cpuid() uses some custom xen instructions which start with
  XEN_EMULATE_PREFIX.  It corresponds to the following x86 instructions:

    ffffffff8107e572:       0f 0b                   ud2
    ffffffff8107e574:       78 65                   js ffffffff8107e5db <xen_get_debugreg+0xa>
    ffffffff8107e576:       6e                      outsb %ds:(%rsi),(%dx)

  Apparently(?) xen treats the ud2 special when it's followed by "78 65
  6e".  This is confusing for stacktool because ud2 is normally a dead
  end, and it thinks the instructions after it will never run.
  
  (In theory stacktool could be taught to understand this hack, but
  that's a bad idea IMO)

- The error path in arch/x86/net/bpf_jit.S uses 'leaveq' to do a double
  return so that it returns from its caller's context.  stacktool
  doesn't know how to distinguish this from a frame pointer programming
  bug.  I think the only way to avoid a whitelist marker here would be
  to rewrite the bpf code to conform with more traditional rbp usage
  (but I don't know if that would really be a good idea because it would
  probably result in slower/more code).

- __bpf_prog_run() uses a jump table:

    goto *jumptable[insn->code];

  stacktool doesn't have an x86 emulator, so it doesn't know how to
  deterministically follow all possible branches for a dynamic jump.

- schedule() mucks with the frame pointer which is normally not allowed.

-- 
Josh

  reply	other threads:[~2016-01-15  6:06 UTC|newest]

Thread overview: 67+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-12-18 12:39 [PATCH v15 00/25] Compile-time stack metadata validation Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 01/25] tools: Fix formatting of the "make -C tools" help message Josh Poimboeuf
2016-01-13  9:40   ` [tip:perf/urgent] " tip-bot for Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 02/25] tools: Make list.h self-sufficient Josh Poimboeuf
2016-01-12 12:35   ` Borislav Petkov
2016-01-12 14:54     ` Arnaldo Carvalho de Melo
2016-01-12 15:59       ` Borislav Petkov
2016-01-12 17:16         ` Arnaldo Carvalho de Melo
2016-01-13  9:40   ` [tip:perf/urgent] " tip-bot for Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 03/25] tools subcmd: Add missing NORETURN define for parse-options.h Josh Poimboeuf
2016-01-13  9:41   ` [tip:perf/urgent] " tip-bot for Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 04/25] x86/asm: Frame pointer macro cleanup Josh Poimboeuf
2016-01-19 13:39   ` [tip:x86/asm] x86/asm: Clean up frame pointer macros tip-bot for Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 05/25] x86/asm: Add C versions of " Josh Poimboeuf
2016-01-19 13:40   ` [tip:x86/asm] " tip-bot for Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 06/25] x86/stacktool: Compile-time stack metadata validation Josh Poimboeuf
2016-01-12 14:48   ` Borislav Petkov
2016-01-12 15:06     ` Josh Poimboeuf
2016-01-12 16:10       ` Borislav Petkov
2016-01-19 12:02   ` Ingo Molnar
2015-12-18 12:39 ` [PATCH v15 07/25] x86/stacktool: Add file and directory ignores Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 08/25] x86/stacktool: Add ignore macros Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 09/25] x86/xen: Add stack frame dependency to hypercall inline asm calls Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 10/25] x86/paravirt: Add stack frame dependency to PVOP " Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 11/25] x86/paravirt: Create a stack frame in PV_CALLEE_SAVE_REGS_THUNK Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 12/25] x86/amd: Set ELF function type for vide() Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 13/25] x86/reboot: Add ljmp instructions to stacktool whitelist Josh Poimboeuf
2016-01-12 16:47   ` Borislav Petkov
2016-01-12 17:43     ` Josh Poimboeuf
2016-01-12 17:55       ` Borislav Petkov
2016-01-12 18:56         ` Josh Poimboeuf
2016-01-12 19:37           ` Borislav Petkov
2016-01-13 10:55       ` Ingo Molnar
2016-01-15  6:06         ` Josh Poimboeuf [this message]
2016-01-15 10:41           ` Borislav Petkov
2016-01-15 11:00             ` Ingo Molnar
2016-01-15 11:11               ` Borislav Petkov
2016-01-15 11:13                 ` Ingo Molnar
2016-01-20  5:42               ` Josh Poimboeuf
2016-01-20  5:50                 ` H. Peter Anvin
2016-01-20  6:09                   ` Josh Poimboeuf
2016-01-20 10:44                 ` Borislav Petkov
2016-01-15 10:56           ` Ingo Molnar
2015-12-18 12:39 ` [PATCH v15 14/25] x86/xen: Add xen_cpuid() and xen_setup_gdt() to stacktool whitelists Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 15/25] x86/asm/crypto: Create stack frames in aesni-intel_asm.S Josh Poimboeuf
2016-01-12 16:53   ` Borislav Petkov
2016-01-12 16:54     ` Borislav Petkov
2015-12-18 12:39 ` [PATCH v15 16/25] x86/asm/crypto: Move .Lbswap_mask data to .rodata section Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 17/25] x86/asm/crypto: Move jump_table " Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 18/25] x86/asm/crypto: Create stack frames in clmul_ghash_mul/update() Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 19/25] x86/asm/entry: Create stack frames in thunk functions Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 20/25] x86/asm/acpi: Create a stack frame in do_suspend_lowlevel() Josh Poimboeuf
2015-12-20 16:13   ` Rafael J. Wysocki
2015-12-18 12:39 ` [PATCH v15 21/25] x86/asm: Create stack frames in rwsem functions Josh Poimboeuf
2016-01-12 12:41   ` Borislav Petkov
2016-01-12 14:36     ` Josh Poimboeuf
2016-01-12 14:40       ` Borislav Petkov
2015-12-18 12:39 ` [PATCH v15 22/25] x86/asm/efi: Create a stack frame in efi_call() Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 23/25] x86/asm/power: Create stack frames in hibernate_asm_64.S Josh Poimboeuf
2015-12-20 16:14   ` Rafael J. Wysocki
2015-12-18 12:39 ` [PATCH v15 24/25] x86/uaccess: Add stack frame output operand in get_user inline asm Josh Poimboeuf
2015-12-18 12:39 ` [PATCH v15 25/25] x86/stacktool: Ignore head_$(BITS) files Josh Poimboeuf
2016-01-12 14:58 ` [PATCH v15 00/25] Compile-time stack metadata validation Arnaldo Carvalho de Melo
2016-01-12 17:17 ` Borislav Petkov
2016-01-12 17:50   ` Josh Poimboeuf
2016-01-12 18:04     ` Borislav Petkov
2016-01-13 10:18   ` Ingo Molnar

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