Re: [PATCH v6 1/2] powerpc/64: Setup a paca before parsing device tree etc.

[Michael Ellerman <patch-notifications@ellerman.id.au>]

On Fri, 2020-03-20 at 03:21:15 UTC, Michael Ellerman wrote:
> From: Daniel Axtens <dja@axtens.net>
> 
> Currently we set up the paca after parsing the device tree for CPU
> features. Prior to that, r13 contains random data, which means there
> is random data in r13 while we're running the generic dt parsing code.
> 
> This random data varies depending on whether we boot through a vmlinux
> or a zImage: for the vmlinux case it's usually around zero, but for
> zImages we see random values like 912a72603d420015.
> 
> This is poor practice, and can also lead to difficult-to-debug
> crashes. For example, when kcov is enabled, the kcov instrumentation
> attempts to read preempt_count out of the current task, which goes via
> the paca. This then crashes in the zImage case.
> 
> Similarly stack protector can cause crashes if r13 is bogus, by
> reading from the stack canary in the paca.
> 
> To resolve this:
> 
>  - move the paca setup to before the CPU feature parsing.
> 
>  - because we no longer have access to CPU feature flags in paca
>  setup, change the HV feature test in the paca setup path to consider
>  the actual value of the MSR rather than the CPU feature.
> 
> Translations get switched on once we leave early_setup, so I think
> we'd already catch any other cases where the paca or task aren't set
> up.
> 
> Boot tested on a P9 guest and host.
> 
> Fixes: fb0b0a73b223 ("powerpc: Enable kcov")
> Fixes: 06ec27aea9fc ("powerpc/64: add stack protector support")
> Cc: stable@vger.kernel.org # v4.20+
> Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
> Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
> Signed-off-by: Daniel Axtens <dja@axtens.net>
> [mpe: Reword comments & change log a bit to mention stack protector]
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Series applied to powerpc next.

https://git.kernel.org/powerpc/c/d4a8e98621543d5798421eed177978bf2b3cdd11

cheers