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

[Scott Wood]

On Wed, 2020-02-26 at 18:16 +1100, Daniel Axtens wrote:
> 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?
> 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.
> 
> Maybe replacing the REG format string in KASLR mode would be sufficient?

Whatever we decide to do here, it's not book3e-specific so it should be
considered separately from these patches.

-Scott