Re: [PATCH v3 0/6] implement KASLR for powerpc/fsl_booke/64

From: Scott Wood <oss@buserror.net>
To: Jason Yan <yanaijie@huawei.com>, Daniel Axtens <dja@axtens.net>,
	mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org,
	diana.craciun@nxp.com, christophe.leroy@c-s.fr,
	benh@kernel.crashing.org, paulus@samba.org, npiggin@gmail.com,
	keescook@chromium.org, kernel-hardening@lists.openwall.com
Cc: linux-kernel@vger.kernel.org, zhaohongjiang@huawei.com
Subject: Re: [PATCH v3 0/6] implement KASLR for powerpc/fsl_booke/64
Date: Thu, 27 Feb 2020 23:53:13 -0600	[thread overview]
Message-ID: <e8cd8f287934954cfa07dcf76ac73492e2d49a5b.camel@buserror.net> (raw)
In-Reply-To: <8171d326-5138-4f5c-cff6-ad3ee606f0c2@huawei.com>

On Wed, 2020-02-26 at 16:18 +0800, Jason Yan wrote:
> Hi Daniel,
> 
> 在 2020/2/26 15:16, Daniel Axtens 写道:
> > Hi Jason,
> > 
> > > This is a try to implement KASLR for Freescale BookE64 which is based on
> > > my earlier implementation for Freescale BookE32:
> > > https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=131718
> > > 
> > > The implementation for Freescale BookE64 is similar as BookE32. One
> > > difference is that Freescale BookE64 set up a TLB mapping of 1G during
> > > booting. Another difference is that ppc64 needs the kernel to be
> > > 64K-aligned. So we can randomize the kernel in this 1G mapping and make
> > > it 64K-aligned. This can save some code to creat another TLB map at
> > > early boot. The disadvantage is that we only have about 1G/64K = 16384
> > > slots to put the kernel in.
> > > 
> > >      KERNELBASE
> > > 
> > >            64K                     |--> kernel <--|
> > >             |                      |              |
> > >          +--+--+--+    +--+--+--+--+--+--+--+--+--+    +--+--+
> > >          |  |  |  |....|  |  |  |  |  |  |  |  |  |....|  |  |
> > >          +--+--+--+    +--+--+--+--+--+--+--+--+--+    +--+--+
> > >          |                         |                        1G
> > >          |----->   offset    <-----|
> > > 
> > >                                kernstart_virt_addr
> > > 
> > > I'm not sure if the slot numbers is enough or the design has any
> > > defects. If you have some better ideas, I would be happy to hear that.
> > > 
> > > Thank you all.
> > > 
> > 
> > Are you making any attempt to hide kernel address leaks in this series?
> 
> Yes.
> 
> > I've just been looking at the stackdump code just now, and it directly
> > prints link registers and stack pointers, which is probably enough to
> > determine the kernel base address:
> > 
> >                    SPs:               LRs:             %pS pointer
> > [    0.424506] [c0000000de403970] [c000000001fc0458] dump_stack+0xfc/0x154
> > (unreliable)
> > [    0.424593] [c0000000de4039c0] [c000000000267eec] panic+0x258/0x5ac
> > [    0.424659] [c0000000de403a60] [c0000000024d7a00]
> > mount_block_root+0x634/0x7c0
> > [    0.424734] [c0000000de403be0] [c0000000024d8100]
> > prepare_namespace+0x1ec/0x23c
> > [    0.424811] [c0000000de403c60] [c0000000024d7010]
> > kernel_init_freeable+0x804/0x880
> > 
> > git grep \\\"REG\\\" arch/powerpc shows a few other uses like this, all
> > in process.c or in xmon.
> > 
> 
> Thanks for reminding this.
> 
> > Maybe replacing the REG format string in KASLR mode would be sufficient?
> > 
> 
> Most archs have removed the address printing when dumping stack. Do we 
> really have to print this?
> 
> If we have to do this, maybe we can use "%pK" so that they will be 
> hidden from unprivileged users.

I've found the addresses to be useful, especially if I had a way to dump the
stack data itself.  Wouldn't the register dump also be likely to give away the
addresses?

I don't see any debug setting for %pK (or %p) to always print the actual
address (closest is kptr_restrict=1 but that only works in certain
contexts)... from looking at the code it seems it hashes even if kaslr is
entirely disabled?  Or am I missing something?

-Scott