linux-kernel - Re: [PATCH v4 0/3] Hardening perf subsystem

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Date: Sat, 15 Jun 2024 18:09:07 +0200
From: Martin Uecker <uecker@...raz.at>
To: Peter Zijlstra <peterz@...radead.org>, Kees Cook <kees@...nel.org>
Cc: Erick Archer <erick.archer@...look.com>, Ingo Molnar <mingo@...hat.com>,
  Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim
 <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>, Alexander
 Shishkin <alexander.shishkin@...ux.intel.com>,  Jiri Olsa
 <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>, Adrian Hunter
 <adrian.hunter@...el.com>,  "Liang, Kan" <kan.liang@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,  Dave
 Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>, Nathan Chancellor
 <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, Bill
 Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 Christophe JAILLET <christophe.jaillet@...adoo.fr>,  Matthew Wilcox
 <mawilcox@...rosoft.com>, x86@...nel.org, linux-perf-users@...r.kernel.org,
  linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org, 
 llvm@...ts.linux.dev
Subject: Re: [PATCH v4 0/3] Hardening perf subsystem

Am Freitag, dem 14.06.2024 um 12:17 +0200 schrieb Peter Zijlstra:
> On Wed, Jun 12, 2024 at 04:23:31PM -0700, Kees Cook wrote:
> > On Thu, Jun 13, 2024 at 12:08:21AM +0200, Peter Zijlstra wrote:
> > > On Wed, Jun 12, 2024 at 12:01:19PM -0700, Kees Cook wrote:
> > > > I'm happy to take patches. And for this bikeshed, this would be better
> > > > named under the size_*() helpers which are trying to keep size_t
> > > > calculations from overflowing (by saturating). i.e.:
> > > > 
> > > > 	size_add_mult(sizeof(*p), sizeof(*p->member), num)
> > > 
> > > Fine I suppose, but what if we want something not size_t? Are we waiting
> > > for the type system extension?
> > 
> > Because of C's implicit promotion/truncation, we can't do anything
> > sanely with return values of arbitrary type size; we have to capture the
> > lvalue type somehow so the checking can happen without C doing silent
> > garbage.

What is the specific problem here?

> 
> So sizeof() returns the native (built-in) size_t, right? If that type
> the nooverflow qualifier on, then:
> 
> 	sizeof(*p) + num*sizeof(p->foo[0])
> 
> should all get the nooverflow semantics right? Because size_t is
> effectively 'nooverflow unsigned long' the multiplication should promote
> 'num' to some 'long'.
> 
> Now, I've re-read the rules and I don't see qualifiers mentioned, so
> can't we state that the overflow/nooverflow qualifiers are to be
> preserved on (implicit) promotion and when nooverflow and overflow are
> combined the 'safe' nooverflow takes precedence?
> 
> I mean, when we're adding qualifiers we can make up rules about them
> too, right?

It should probably be a type attribute.

> 
> If 'people' don't want to adorn the built-in size_t, we can always do
> something like:
> 
> #define sizeof(x) ((nooverflow unsigned long)(sizeof(x)))
> 
> and 'fix' it ourselves.

This is likely a stupid question, but making it signed 
wouldn't work?   Or is a signed size_t too small  
or some architectures? Or would this change break too much?

Martin

> 
> > > But none of that is showing me generated asm for the various cases. As
> > > such, I don't consider myself informed enough.
> > 
> > Gotcha. For the compile-time stuff it's all just looking at
> > known-at-compile-time sizes. So for something like this, we get a
> > __compiletime_warning() emitted:
> > 
> > 	const char src[] = "Hello there";
> > 	char dst[10];
> > 
> > 	strscpy(dst, src); /* Compiler yells since src is bigger than dst. */
> > 
> > For run-time checks it's basically just using the regular WARN()
> > infrastructure with __builtin_dynamic_object_size(). Here's a simplified
> > userspace example with assert():
> > 
> > https://godbolt.org/z/zMrKnMxn5
> > 
> > The kernel's FORTIFY_SOURCE is much more complex in how it does the
> > checking, how it does the reporting (for helping people figure out what's
> > gone weird), etc.
> 
> Thanks, I'll go have a look at that.