Re: [PATCH] powerpc: Fix stack protector crashes on CPU hotplug

[Abdul Haleem <abdhalee@linux.vnet.ibm.com>]

On Fri, 2018-10-19 at 16:59 +1100, 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@jms.id.au>
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> ---
>  arch/powerpc/kernel/smp.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 8e3a5da24d59..951c476faffc 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -61,6 +61,7 @@
>  #include <asm/asm-prototypes.h>
>  #include <asm/cpu_has_feature.h>
>  #include <asm/ftrace.h>
> +#include <asm/stackprotector.h>
> 
>  #ifdef DEBUG
>  #include <asm/udbg.h>
> @@ -1014,16 +1015,9 @@ static void cpu_idle_thread_init(unsigned int cpu, struct task_struct *idle)
>  {
>  	struct thread_info *ti = task_thread_info(idle);
> 
> -#ifdef CONFIG_STACKPROTECTOR
> -	idle->stack_canary = get_random_canary();
> -#endif
> -
>  #ifdef CONFIG_PPC64
>  	paca_ptrs[cpu]->__current = idle;
>  	paca_ptrs[cpu]->kstack = (unsigned long)ti + THREAD_SIZE - STACK_FRAME_OVERHEAD;
> -#ifdef CONFIG_STACKPROTECTOR
> -	paca_ptrs[cpu]->canary = idle->stack_canary;
> -#endif
>  #endif
>  	ti->cpu = cpu;
>  	secondary_ti = current_set[cpu] = ti;
> @@ -1316,6 +1310,8 @@ void start_secondary(void *unused)
>  	notify_cpu_starting(cpu);
>  	set_cpu_online(cpu, true);
> 
> +	boot_init_stack_canary();
> +
>  	local_irq_enable();
> 
>  	/* We can enable ftrace for secondary cpus now */

Tested-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>

-- 
Regard's

Abdul Haleem
IBM Linux Technology Centre