|| ||Ingo Molnar <mingo-AT-elte.hu> |
|| ||Dan Rosenberg <drosenberg-AT-vsecurity.com> |
|| ||Re: [PATCH v5] kptr_restrict for hiding kernel pointers |
|| ||Wed, 22 Dec 2010 22:26:09 +0100|
|| ||linux-kernel-AT-vger.kernel.org, netdev-AT-vger.kernel.org,
eric.dumazet-AT-gmail.com, tgraf-AT-infradead.org, eugeneteo-AT-kernel.org,
|| ||Article, Thread
* Dan Rosenberg <firstname.lastname@example.org> wrote:
> On Wed, 2010-12-22 at 18:13 +0100, Ingo Molnar wrote:
> > * Dan Rosenberg <email@example.com> wrote:
> > > + case 'K':
> > > + /*
> > > + * %pK cannot be used in IRQ context because its test
> > > + * for CAP_SYSLOG would be meaningless.
> > > + */
> > > + if (in_irq() || in_serving_softirq() || in_nmi())
> > > + WARN_ONCE(1, "%%pK used in interrupt context.\n");
> > Hm, that bit looks possibly broken - some useful warning in irq context could print
> > a pointer into the syslog and this would generate a second warning? That probably
> > would crash as it recurses back into the printk code?
> I don't see a reason to ever use %pK to print to the syslog, since
> reading it is now optionally protected with dmesg_restrict, and
> stripping pointers from the syslog will cripple any post-mortem
> debugging for everyone. I understand the desire to prevent things from
> breaking even if it's used incorrectly, but I'm not really convinced
> that this would break anything even in this scenario. The WARN_ONCE
> will prevent any unbounded recursion. I'm just not clear on how this
> could cause a crash.
It's a simple QOI issue. We simply do not add kernel facilities that can produce a
stack overflow, memory corruption and triple fault if a rare debug statement
triggers in an IRQ context by accident:
printk(KERN_WARN "driver bar: bug foo in function %pK\n");
> > Instead a warning could be inserted into the generated output instead, for
> > example 'pK-error' (carefully staying within pointer length limits).
> If it's used in IRQ context and its output needs to be read by a
> userspace utility using %p to parse, this will break it.
Didnt you just say that it should not be used from IRQ context? There wont be any
user-space tool to read it - it's a simple robustness change: the warning as you
implemented it can crash the system. I suggested an implementation that would emit
the warning in a more robust way.
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