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Message-ID: <5879F762.32059.37092152@pageexec.freemail.hu>
Date: Sat, 14 Jan 2017 11:03:14 +0100
From: "PaX Team" <pageexec@...email.hu>
To: kernel-hardening@...ts.openwall.com, Kees Cook <keescook@...omium.org>
CC: Emese Revfy <re.emese@...il.com>,
"AKASHI, Takahiro" <takahiro.akashi@...aro.org>,
Mark Rutland <mark.rutland@....com>,
park jinbum <jinb.park7@...il.com>,
Daniel Micay <danielmicay@...il.com>, linux-kernel@...r.kernel.org,
spender@...ecurity.net
Subject: Re: [PATCH] gcc-plugins: Add structleak for more stack initialization
On 13 Jan 2017 at 14:02, Kees Cook wrote:
> This plugin detects any structures that contain __user attributes and
> makes sure it is being fulling initialized so that a specific class of
> information exposure is eliminated. (For example, the exposure of siginfo
> in CVE-2013-2141 would have been blocked by this plugin.)
why the conditional? the plugin was specifically written to block that bug
and block it did ;).
> +config GCC_PLUGIN_STRUCTLEAK
> + bool "Force initialization of variables containing userspace addresses"
> + depends on GCC_PLUGINS
> + help
> + This plugin zero-initializes any structures that containing a
> + __user attribute. This can prevent some classes of information
> + exposures.
i see that you completely ditched the description in PaX, is there a reason
for it? your text isn't correct as is because
- the __user attribute (which is an implementation choice, see below) doesn't
apply to structures but pointers only (as it does for sparse IIRC)
- a structure is a type, but the plugin initializes variables, not types
(the latter makes little sense)
- the plugin doesn't initialize 'any structures' (well, variables), only locals
and only at function scope (subject to further evolution as discussed earlier).
> +config GCC_PLUGIN_STRUCTLEAK_VERBOSE
> + bool "Report initialized variables"
> + depends on GCC_PLUGIN_STRUCTLEAK
> + depends on !COMPILE_TEST
> + help
> + This option will cause a warning to be printed each time the
> + structleak plugin finds a variable it thinks needs to be
> + initialized. Since not all existing initializers are detected
> + by the plugin, this can produce false positive warnings.
there are no false positives, a variable either has a constructor or it does not ;)
> +/* unused C type flag in all versions 4.5-6 */
> +#define TYPE_USERSPACE(TYPE) TYPE_LANG_FLAG_5(TYPE)
FYI, this is a sort of abuse/hack of tree flags and should not be implemented this
way in the upstream kernel as it's a finite resource and needs careful verification
against all supported gcc versions (these flags are meant for language fronteds, i
kinda got lucky to have a few of them unusued but it's not a robust future-proof
approach). instead an attribute should be used to mark these types. whether that
can/should be __user itself is a good question since that's another hack where the
plugin 'hijacks' a sparse address space atttribute (for which gcc 4.6+ has its own
facilities and that the checker gcc plugin makes use of thus it's not compatible
with structleak as is).
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