Re: [kernel-hardening] RE: [PATCH] printk: introduce kptr_restrict level 3

[Jann Horn]

On Thu, Oct 06, 2016 at 04:05:53PM +0200, Jann Horn wrote:
> On Thu, Oct 06, 2016 at 01:47:47PM +0000, Roberts, William C wrote:
> > > -----Original Message-----
> > > From: Christoph Hellwig [mailto:hch@xxxxxxxxxxxxx]
> > > Sent: Thursday, October 6, 2016 9:32 AM
> > > To: Roberts, William C <william.c.roberts@xxxxxxxxx>
> > > Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx; corbet@xxxxxxx; linux-
> > > doc@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > > Subject: Re: [PATCH] printk: introduce kptr_restrict level 3
> > > 
> > > On Wed, Oct 05, 2016 at 02:04:46PM -0400, william.c.roberts@xxxxxxxxx wrote:
> > > > From: William Roberts <william.c.roberts@xxxxxxxxx>
> > > >
> > > > Some out-of-tree modules do not use %pK and just use %p, as it's the
> > > > common C paradigm for printing pointers. Because of this,
> > > > kptr_restrict has no affect on the output and thus, no way to contain
> > > > the kernel address leak.
> > > 
> > > So what?  We a) don't care about out of tree modules and b) you could just triviall
> > > fix them up if you care.
> > 
> > Out of tree modules still affect core kernel security. I would also bet money, that somewhere
> > In-tree someone has put a %p when they wanted a %pK. So this method is just quite error
> > prone. We currently have a blacklist approach versus whitelist.
> 
> grep says you have a point:
> 
> $ grep -IR 'seq_printf.*%p[^FfSsBRrhbMmIiEUVKNadCDgG].*&'
> drivers/dma/qcom/hidma_dbg.c:	seq_printf(s, "dev_trca=%p\n", &dmadev->dev_trca);
> drivers/dma/qcom/hidma_dbg.c:	seq_printf(s, "dev_evca=%p\n", &dmadev->dev_evca);
> 
> $ grep -IR 'pr_info.*%p[^FfSsBRrhbMmIiEUVKNadCDgG].*&'
> drivers/misc/lkdtm_heap.c:	pr_info("Allocated memory %p-%p\n", base, &base[offset * 2]);
> 
> $ grep -IR 'pr_err.*%p[^FfSsBRrhbMmIiEUVKNadCDgG].*&'
> drivers/net/ethernet/qlogic/qlge/qlge_dbg.c:	pr_err("rx_ring->cqicb = %p\n", &rx_ring->cqicb);

Actually, I think I missed something here.

We don't really want to censor pointers in dmesg, right? dmesg can
contain pointers by design - and in particular, when an oops
happens, the register dump will reveal pointers. dmesg should just
be restricted using dmesg_restrict.

(What is annoying, though, is that e.g. Debian's default rsyslog
config sends all KERN_EMERG messages to all active PTYs by default,
meaning that on a system with such a config, making the kernel oops
can be used to leak some ASLR information.)

(And why does __die() in arch/x86/kernel/dumpstack.c use KERN_EMERG
for CONFIG_X86_32, but KERN_ALERT for !CONFIG_X86_32?)

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