powerpc: Fix stack protector crashes on CPU hotplug

[Michael Ellerman]

On Fri, 2018-10-19 at 05:59:27 UTC, Michael Ellerman wrote:
> Recently in commit 7241d26e8175 ("powerpc/64: properly initialise
> the stackprotector canary on SMP.") we fixed a crash with stack
> protector on SMP by initialising the stack canary in
> cpu_idle_thread_init().
> 
> But this can also causes crashes, when a CPU comes back online after
> being offline:
> 
>   Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: pnv_smp_cpu_kill_self+0x2a0/0x2b0
>   CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.19.0-rc3-gcc-7.3.1-00168-g4ffe713b7587 #94
>   Call Trace:
>     dump_stack+0xb0/0xf4 (unreliable)
>     panic+0x144/0x328
>     __stack_chk_fail+0x2c/0x30
>     pnv_smp_cpu_kill_self+0x2a0/0x2b0
>     cpu_die+0x48/0x70
>     arch_cpu_idle_dead+0x20/0x40
>     do_idle+0x274/0x390
>     cpu_startup_entry+0x38/0x50
>     start_secondary+0x5e4/0x600
>     start_secondary_prolog+0x10/0x14
> 
> Looking at the stack we see that the canary value in the stack frame
> doesn't match the canary in the task/paca. That is because we have
> reinitialised the task/paca value, but then the CPU coming online has
> returned into a function using the old canary value. That causes the
> comparison to fail.
> 
> Instead we can call boot_init_stack_canary() from start_secondary()
> which never returns. This is essentially what the generic code does in
> cpu_startup_entry() under #ifdef X86, we should make that non-x86
> specific in a future patch.
> 
> Fixes: 7241d26e8175 ("powerpc/64: properly initialise the stackprotector canary on SMP.")
> Reported-by: Joel Stanley <joel at jms.id.au>
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
> Reviewed-by: Christophe Leroy <christophe.leroy at c-s.fr>

Applied to powerpc next.

https://git.kernel.org/powerpc/c/b6aeddea74b08518289fc86545297c

cheers