linux-kernel - Re: [PATCH 3/6] x86/dumpstack: make printk_stack

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Date:   Thu, 25 Aug 2016 16:07:16 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Nilay Vaish <nilayvaish@...il.com>
Subject: Re: [PATCH 3/6] x86/dumpstack: make printk_stack_address() more
 generally useful

On Thu, Aug 25, 2016 at 04:41:29PM -0400, Kees Cook wrote:
> On Thu, Aug 25, 2016 at 1:49 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > On Wed, Aug 24, 2016 at 02:37:07PM -0500, Josh Poimboeuf wrote:
> >> On Wed, Aug 24, 2016 at 02:37:21PM -0400, Linus Torvalds wrote:
> >> > On Wed, Aug 24, 2016 at 2:22 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> >> > >
> >> > > I actively disable KASLR on my dev box and feed these hex numbers into
> >> > > addr2line -ie vmlinux to find where in the function we are.
> >> > >
> >> > > Having the option to make %pB generate them works for me.
> >> >
> >> > Yeah, considering that this is the only place this is used, changing
> >> > %pB sounds quite reasonable.
> >>
> >> There's now another use of '%pB' in proc_pid_stack() in the tip tree: I
> >> changed it to '%pB' from '%pS'.  But I think the modified '%pB' would
> >> work there as well.
> >>
> >> > We could perhaps make %pB show the hex numbers and address (so pB
> >> > would expand to "[<hex>] symbolname".if
> >> >
> >> >  (a) not randomizing (so the hex numbers _may_ be useful)
> >> >
> >> >  (b) kptr_restrict is 0 (so the hex numbers are "safe" in the dmesg)
> >> >
> >> > and fall back to just the symbolic name if either of those aren't true?
> >>
> >> Do we really need to check for both?  '%pK' only checks kptr_restrict.
> >> I'd think we should be consistent with that.  And maybe there are some
> >> scenarios where the actual text addresses provide useful debug
> >> information if KASLR is enabled and kptr_restrict is zero.
> >
> > So I was looking at implementing this, and I noticed that '%pK' prints
> > "pK-error" if it's called from interrupt context when kptr_restrict==1.
> > Because checking CAP_SYSLOG would be meaningless in that case.
> >
> > I don't really understand the point of the "pK-error" thing.  Any reason
> > why we can't print zero, i.e., just degrade the kptr_restrict from 1 to
> > 2 in an interrupt?
> >
> > That would make the '%pK' code simpler and usable from interrupt
> > context.  Also it would make its behavior consistent with the proposed
> > '%pB' changes, and the kptr_restrict code could be shared between '%pK'
> > and '%pB'.
> >
> > Kess (or others), any objections if I make that change?

Ahem, Kees, sorry :-)

> I don't mind this becoming "0" on error. I suspect the rationale was
> to make it a discoverable condition and to avoid confusion.
> 
> As far as expanding the usage, I'm still in favor, though there is
> work planned to make kptr_restrict go away in favor of having
> blacklisted destination buffers, etc. I'm hoping to have this as part
> of the continuing usercopy hardening work.
> 
> Regardless, aren't these values being written to dmesg buffer?
> Traditionally we've not bothered censoring values that go there, as
> "dmesg_restrict" exists to protect those contents.

Ah, the plot thickens.  I didn't know about 'dmesg_restrict'.  So I
guess we don't have to restrict the stack dump addresses after all,
since the entire dmesg buffer is protected by syslog()?

If so, I'm thinking that expanding '%pB' wouldn't be worthwhile after
all, because its two users would have two different requirements for
printing the address: /proc/<pid>/stack needs to use kptr_restrict but
the unwinder doesn't.

In which case I think the current code is fine.

-- 
Josh