Re: [PATCH 00/33] Compile-time stack metadata validation

Josh Poimboeuf
2016-02-23T15:02:03Z

Hi Ingo,

On Tue, Feb 23, 2016 at 09:14:06AM +0100, Ingo Molnar wrote:
> So I tried out this latest stacktool series and it looks mostly good for an
> upstream merge.
>
> To help this effort move forward I've applied the preparatory/fix patches that are
> part of this series to tip:x86/debug - that's 26 out of 31 patches. (I've
> propagated all the acks that the latest submission got into the changelogs.)

Thanks very much for your review and for applying the fixes!

A few issues relating to the merge:

- The tip:x86/debug branch fails to build because it depends on
ec5186557abb ("x86/asm: Add C versions of frame pointer macros") which
is in tip:x86/asm.

- As Pavel mentioned, the tip-bot seems to be spitting out garbage
emails from:
=?UTF-8?B?dGlwLWJvdCBmb3IgSm9zaCBQb2ltYm9ldWYgPHRpcGJvdEB6eXRvci5jb20+?=@zytor.com.

> I have 5 (easy to address) observations that need to be addressed before we can
> move on with the rest of the merge:
>
> 1)
>
> Due to recent changes to x86 exception handling, I get a lot of bogus warnings
> about exception table sizes:
>
> stacktool: arch/x86/kernel/cpu/mcheck/mce.o: __ex_table size not a multiple of 24
> stacktool: arch/x86/kernel/cpu/mtrr/generic.o: __ex_table size not a multiple of 24
> stacktool: arch/x86/kernel/cpu/mtrr/cleanup.o: __ex_table size not a multiple of 24
>
> this is due to:
>
> 548acf19234d x86/mm: Expand the exception table logic to allow new handling options

Ok, I'll fix it up.

> 2)
>
> The fact that 'stacktool' already checks about assembly details like __ex_table[]
> shows that my review feedback early iterations of this series, that the
> 'stacktool' name is too specific, was correct.
>
> We really need to rename it before it gets upstream and the situation gets worse.
> __ex_table[] has obviously nothing to do with the 'stack layout' of binaries.
>
> Another suitable name would be 'asmtool' or 'objtool'. For example the following
> would naturally express what it does:
>
> objtool check kernel/built-in.o
>
> the name expresses that the tool checks object files, independently of the
> existing toolchain. Its primary purpose right now is the checking of stack layout
> details, but the tool is not limited to that at all.

Fair enough. I'll rename it to objtool if there are no objections.

> 3)
>
> There's quite a bit of overhead when running the tool on larger object files, most
> prominently in cmd_check():
>
> 62.06% stacktool stacktool [.] cmd_check
> 6.72% stacktool stacktool [.] find_rela_by_dest_range
>
> I added -g to the CFLAGS, which made it visible to annotated output of perf top:
>
> 0.00 : 40430d: lea 0x4(%rdx,%rax,1),%r13
> : find_instruction():
> : struct section *sec,
> : unsigned long offset)
> : {
> : struct instruction *insn;
> :
> : list_for_each_entry(insn, &file->insns, list)
> 0.03 : 404312: mov 0x38(%rsp),%rax
> 0.00 : 404317: cmp %rbp,%rax
> 0.00 : 40431a: jne 404334 <cmd_check+0x5b4>
> 0.00 : 40431c: jmpq 4045ba <cmd_check+0x83a>
> 0.00 : 404321: nopl 0x0(%rax)
> 6.14 : 404328: mov (%rax),%rax
> 0.00 : 40432b: cmp %rbp,%rax
> 2.02 : 40432e: je 4045ba <cmd_check+0x83a>
> : if (insn->sec == sec && insn->offset == offset)
> 0.55 : 404334: cmp %r12,0x10(%rax)
> 87.91 : 404338: jne 404328 <cmd_check+0x5a8>
> 0.00 : 40433a: cmp %r13,0x18(%rax)
> 3.36 : 40433e: jne 404328 <cmd_check+0x5a8>
> : get_jump_destinations():
> : * later in validate_functions().
> : */
> : continue;
> : }
> :
> : insn->jump_dest = find_instruction(file, dest_sec, dest_off);
> 0.00 : 404340: mov %rax,0x48(%rbx)
> 0.00 : 404344: jmpq 4042b0 <cmd_check+0x530>
> 0.00 : 404349: nopl 0x0(%rax)
> : fprintf():
> :
> : # ifdef __va_arg_pack
> : __fortify_function int
> : fprintf (FILE *__restrict __stream, const char *__restrict __fmt, ...)
> : {
> : return __fprintf_chk (__stream, __USE_FORTIFY_LEVEL - 1, __fmt,
>
> that looks like a linear list search? That would suck with thousands of entries.
>
> (If this is non-trivial to improve then we can delay this optimization to a later
> patch.)

Yeah, I agree that the linear list search isn't good. I still need to
think about the data structures a bit, so if it's ok with you, I'll fix
it with a later patch.

> 4)
>
> I think the various 'STACKTOOL' flags in the kernel source are a bit of a misnomer
> - it's not the tool we want to name but the actual property of the code.
>
> So instead of:
>
> STACKTOOL_IGNORE_FUNC(__bpf_prog_run);
>
> we should do something like:
>
> STACK_FRAME_NON_STANDARD(__bpf_prog_run);
>
> see how much more expressive it is? It becomes a function attribute independent of
> the tooling that makes use of it.

Ok, STACK_FRAME_NON_STANDARD sounds fine to me.

> Similarly, for the highest level 'don't check these directories' makefile flags,
> I'd suggest, instead of using this rather opaque, tool dependent naming:
>
> STACKTOOL := n
>
> something more specific, like:
>
> OBJECT_FILES_NON_STANDARD := y
>
> which makes it clearer what's going on: these are special object files that are
> not the typical kernel object files.
>
> Stacktool (or objtool) would be one consumer of this annotation.
>
> I think Boris made similar observations in past reviews.

Sounds reasonable. With this approach we could probably eventually get
rid of KASAN_SANITIZE.

I'll change it to "OBJECT_FILES_NON_STANDARD := y" if there are no
objections.

Note there are also per-object ignores like:

STACKTOOL_head_$(BITS).o := n

I can similarly change that to something like:

OBJECT_FILES_NON_STANDARD_head_$(BITS).o := n

> 5)
>
> Likewise, I think the CONFIG_STACK_VALIDATION=y Kconfig flag does not express that
> we do exception table checks as well - and it does not express all the other
> things we may check in object files in the future.
>
> Something like CONFIG_CHECK_OBJECT_FILES=y would express it, and the help text
> would list all the things the tool is able to checks for at the moment.

Hm, I'm not really sure about this. Yes, the tool could potentially do
other types of checks, but is it necessary to lump them all together
into a single config option? It does have subcommands after all ;-)

The exception table check reported above is very basic and doesn't serve
any useful purpose other than supporting the goal of validating the
stack.

However, I don't feel strong enough about it to hold up the merge any
longer, so I'll plan to make the change unless I hear back from you.

> Please send followup iterations of the series against the tip:x86/debug tree (or
> tip:master), to keep the size of the series down.

Will do, thanks!

--
Josh