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Date: Thu, 27 Feb 2020 23:53:13 -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
Cc: linux-kernel@...r.kernel.org, zhaohongjiang@...wei.com
Subject: Re: [PATCH v3 0/6] implement KASLR for powerpc/fsl_booke/64
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
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