linux-kernel - Re: [PATCH v3 0/6] implement KASLR for powerpc/fsl

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Message-ID: <7f608c18250c509ff091990d4bb460846fae11a0.camel@buserror.net>
Date:   Mon, 02 Mar 2020 02:47:13 -0600
From:   Scott Wood <oss@...error.net>
To:     Jason Yan <yanaijie@...wei.com>, Daniel Axtens <dja@...ens.net>,
        mpe@...erman.id.au, linuxppc-dev@...ts.ozlabs.org,
        diana.craciun@....com, christophe.leroy@....fr,
        benh@...nel.crashing.org, paulus@...ba.org, npiggin@...il.com,
        keescook@...omium.org, kernel-hardening@...ts.openwall.com,
        me@...in.cc
Cc:     linux-kernel@...r.kernel.org, zhaohongjiang@...wei.com
Subject: Re: [PATCH v3 0/6] implement KASLR for powerpc/fsl_booke/64

On Mon, 2020-03-02 at 15:12 +0800, Jason Yan wrote:
> 
> 在 2020/3/2 11:24, Scott Wood 写道:
> > On Mon, 2020-03-02 at 10:17 +0800, Jason Yan wrote:
> > > 
> > > 在 2020/3/1 6:54, Scott Wood 写道:
> > > > On Sat, 2020-02-29 at 15:27 +0800, Jason Yan wrote:
> > > > > 
> > > > > Turnning to %p may not be a good idea in this situation. So
> > > > > for the REG logs printed when dumping stack, we can disable it when
> > > > > KASLR is open. For the REG logs in other places like show_regs(),
> > > > > only
> > > > > privileged can trigger it, and they are not combind with a symbol,
> > > > > so
> > > > > I think it's ok to keep them.
> > > > > 
> > > > > diff --git a/arch/powerpc/kernel/process.c
> > > > > b/arch/powerpc/kernel/process.c
> > > > > index fad50db9dcf2..659c51f0739a 100644
> > > > > --- a/arch/powerpc/kernel/process.c
> > > > > +++ b/arch/powerpc/kernel/process.c
> > > > > @@ -2068,7 +2068,10 @@ void show_stack(struct task_struct *tsk,
> > > > > unsigned
> > > > > long *stack)
> > > > >                    newsp = stack[0];
> > > > >                    ip = stack[STACK_FRAME_LR_SAVE];
> > > > >                    if (!firstframe || ip != lr) {
> > > > > -                       printk("["REG"] ["REG"] %pS", sp, ip, (void
> > > > > *)ip);
> > > > > +                       if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
> > > > > +                               printk("%pS", (void *)ip);
> > > > > +                       else
> > > > > +                               printk("["REG"] ["REG"] %pS", sp,
> > > > > ip,
> > > > > (void *)ip);
> > > > 
> > > > This doesn't deal with "nokaslr" on the kernel command line.  It also
> > > > doesn't
> > > > seem like something that every callsite should have to opencode,
> > > > versus
> > > > having
> > > > an appropriate format specifier behaves as I described above (and I
> > > > still
> > > > don't see why that format specifier should not be "%p").
> > > > 
> > > 
> > > Actually I still do not understand why we should print the raw value
> > > here. When KALLSYMS is enabled we have symbol name  and  offset like
> > > put_cred_rcu+0x108/0x110, and when KALLSYMS is disabled we have the raw
> > > address.
> > 
> > I'm more concerned about the stack address for wading through a raw stack
> > dump
> > (to find function call arguments, etc).  The return address does help
> > confirm
> > that I'm on the right stack frame though, and also makes looking up a line
> > number slightly easier than having to look up a symbol address and then
> > add
> > the offset (at least for non-module addresses).
> > 
> > As a random aside, the mismatch between Linux printing a hex offset and
> > GDB
> > using decimal in disassembly is annoying...
> > 
> 
> OK, I will send a RFC patch to add a new format specifier such as "%pk" 
> or change the exsiting "%pK" to print raw value of addresses when KASLR 
> is disabled and print hash value of addresses when KASLR is enabled. 
> Let's see what the printk guys would say :)

I'm not sure that a new format specifier is needed versus changing the
behavior of "%p", and "%pK" definitely doesn't seem suitable given that it's
intended to be more restricted than "%p" (see commit ef0010a30935de4).  The
question is whether there is a legitimate reason to hash in the absence of
kaslr.

-Scott