Re: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly

Borislav Petkov
2015-11-25T09:55:15Z

On Fri, Nov 20, 2015 at 06:36:48PM +0900, Hidehiro Kawai wrote:
> Currently, panic() and crash_kexec() can be called at the same time.
> For example (x86 case):
>
> CPU 0:
> oops_end()
> crash_kexec()
> mutex_trylock() // acquired
> nmi_shootdown_cpus() // stop other cpus
>
> CPU 1:
> panic()
> crash_kexec()
> mutex_trylock() // failed to acquire
> smp_send_stop() // stop other cpus
> infinite loop
>
> If CPU 1 calls smp_send_stop() before nmi_shootdown_cpus(), kdump
> fails.
>
> In another case:
>
> CPU 0:
> oops_end()
> crash_kexec()
> mutex_trylock() // acquired
> <NMI>
> io_check_error()
> panic()
> crash_kexec()
> mutex_trylock() // failed to acquire
> infinite loop
>
> Clearly, this is an undesirable result.
>
> To fix this problem, this patch changes crash_kexec() to exclude
> others by using atomic_t panic_cpu.
>
> V5:
> - Add missing dummy __crash_kexec() for !CONFIG_KEXEC_CORE case
> - Replace atomic_xchg() with atomic_set() in crash_kexec() because
> it is used as a release operation and there is no need of memory
> barrier effect. This change also removes an unused value warning
>
> V4:
> - Use new __crash_kexec(), no exclusion check version of crash_kexec(),
> instead of checking if panic_cpu is the current cpu or not
>
> V2:
> - Use atomic_cmpxchg() instead of spin_trylock() on panic_lock
> to exclude concurrent accesses
> - Don't introduce no-lock version of crash_kexec()
>
> Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
> Cc: Eric Biederman <ebiederm@xmission.com>
> Cc: Vivek Goyal <vgoyal@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> ---
> include/linux/kexec.h | 2 ++
> kernel/kexec_core.c | 26 +++++++++++++++++++++++++-
> kernel/panic.c | 4 ++--
> 3 files changed, 29 insertions(+), 3 deletions(-)

...

> +void crash_kexec(struct pt_regs *regs)
> +{
> + int old_cpu, this_cpu;
> +
> + /*
> + * Only one CPU is allowed to execute the crash_kexec() code as with
> + * panic(). Otherwise parallel calls of panic() and crash_kexec()
> + * may stop each other. To exclude them, we use panic_cpu here too.
> + */
> + this_cpu = raw_smp_processor_id();
> + old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
> + if (old_cpu == -1) {
> + /* This is the 1st CPU which comes here, so go ahead. */
> + __crash_kexec(regs);
> +
> + /*
> + * Reset panic_cpu to allow another panic()/crash_kexec()
> + * call.

So can we make __crash_kexec() return error values?

* failed to grab kexec_mutex -> reset panic_cpu

* no kexec_crash_image -> no need to reset it, all future crash_kexec()
calls won't work so no need to run into that path anymore. However, this could
be problematic if we want the other CPUs to panic. Do we care?

* machine_kexec successful -> doesn't matter

Thanks.

--
Regards/Gruss,
Boris.

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