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Date: Mon, 16 Jan 2017 15:24:25 +0000
From: Mark Rutland <mark.rutland@....com>
To: PaX Team <pageexec@...email.hu>
Cc: kernel-hardening@...ts.openwall.com, Kees Cook <keescook@...omium.org>,
Emese Revfy <re.emese@...il.com>,
"AKASHI, Takahiro" <takahiro.akashi@...aro.org>,
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
Hi,
On Sat, Jan 14, 2017 at 11:03:14AM +0100, PaX Team wrote:
> On 13 Jan 2017 at 14:02, Kees Cook wrote:
>
> > +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 ;)
... or it has no constructor, but is clobbered by a memset before it is
possibly copied. ;)
For example:
arch/arm64/kernel/fpsimd.c: In function 'do_fpsimd_exc':
arch/arm64/kernel/fpsimd.c:106:12: note: userspace variable will be forcibly initialized
siginfo_t info;
... where the code looks like:
void do_fpsimd_exc(unsigned int esr, struct pt_regs *regs)
{
siginfo_t info;
unsigned int si_code = 0;
if (esr & FPEXC_IOF)
si_code = FPE_FLTINV;
else if (esr & FPEXC_DZF)
si_code = FPE_FLTDIV;
else if (esr & FPEXC_OFF)
si_code = FPE_FLTOVF;
else if (esr & FPEXC_UFF)
si_code = FPE_FLTUND;
else if (esr & FPEXC_IXF)
si_code = FPE_FLTRES;
memset(&info, 0, sizeof(info));
info.si_signo = SIGFPE;
info.si_code = si_code;
info.si_addr = (void __user *)instruction_pointer(regs);
send_sig_info(SIGFPE, &info, current);
}
... so it's clear to a human that info is initialised prior to use,
though not by an explicit field initializer.
> > +/* 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).
To me, it seems that the __user annotation can only be an indicator of
an issue by chance. We have structures with __user pointers in structs
that will never be copied to userspace, and conversely we have structs
that don't contain a __user field, but will be copied to userspace.
Maybe it happens that structs in more complex systems are more likely to
contain some __user pointer. Was that part of the rationale?
I wonder if there's any analysis we can do of data passing into
copy_to_user() and friends. I guess we can't follow the data flow across
compilation units, but we might be able to follow it well enough if we
added a new attribute that described whether data was to be copied to
userspace.
Thanks,
Mark.
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