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

[Christophe LEROY]

Le 19/10/2018 à 07:59, Michael Ellerman a écrit :
> 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.

This shall not be done on arches that uses a global canary, so I think 
it has to be kept arch specific. Indeed I think x86 should move it to 
cpu_bringup_and_idle(). I'll propose them a patch for that.

Christophe

> 
> 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>



> ---
>   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 */
>