Re: [PATCH V2] x86: mce: fix kernel panic when check_interval is changed

[Seunghun Han]

Hi, Borislav.

I made new patch according to your advice.
The patch is here, https://lkml.org/lkml/2018/2/28/1230.

If you have any advice about it, please let me know.

Best regards.

Seunghun.

2018-03-01 5:31 GMT+09:00 Seunghun Han <kkamagui@xxxxxxxxx>:
> I am Seunghun Han and a senior security researcher at National Security
> Research Institute of South Korea.
>
> I found a security issue which can make kernel panic in userspace. After
> analyzing the issue carefully, I found that MCE driver in the kernel has a
> problem which can be occurred in SMP environment.
>
> The check_interval file in
> /sys/devices/system/machinecheck/machinecheck<cpu number> directory is a
> global timer value for MCE polling. If it is changed by one CPU, MCE driver
> in kernel calls mce_restart() function and broadcasts the event to other
> CPUs to delete and restart MCE polling timer.
>
> The __mcheck_cpu_init_timer() function which is called by mce_restart()
> function initializes the mce_timer variable, and the "lock" in mce_timer is
> also reinitialized. If more than one CPU write a specific value to
> check_interval file concurrently, one can initialize the "lock" in mce_timer
> while the others are handling "lock" in mce_timer. This problem causes some
> synchronization errors such as kernel panic and kernel hang.
>
> It is a security problem because the attacker can make kernel panic by
> writing a value to the check_interval file in userspace, and it can be used
> for Denial-of-Service (DoS) attack.
>
> To fix this problem, I added a mce_sysfs_mutex to serialize requests.
>
> Signed-off-by: Seunghun Han <kkamagui@xxxxxxxxx>
> ---
> Changes since v1: add mce_sysfs_mutex according to review result.
>
>  arch/x86/kernel/cpu/mcheck/mce.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
> index 706584681a4c..bee0795a3b8c 100644
> --- a/arch/x86/kernel/cpu/mcheck/mce.c
> +++ b/arch/x86/kernel/cpu/mcheck/mce.c
> @@ -55,6 +55,7 @@
>  #include "mce-internal.h"
>
>  static DEFINE_MUTEX(mce_log_mutex);
> +static DEFINE_MUTEX(mce_sysfs_mutex);
>
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/mce.h>
> @@ -2045,8 +2046,11 @@ static void mce_enable_ce(void *all)
>                 return;
>         cmci_reenable();
>         cmci_recheck();
> -       if (all)
> +       if (all) {
> +               mutex_lock(&mce_sysfs_mutex);
>                 __mcheck_cpu_init_timer();
> +               mutex_unlock(&mce_sysfs_mutex);
> +       }
>  }
>
>  static struct bus_type mce_subsys = {
> @@ -2132,8 +2136,14 @@ static ssize_t store_int_with_restart(struct device *s,
>                                       struct device_attribute *attr,
>                                       const char *buf, size_t size)
>  {
> +       unsigned long old_check_interval = check_interval;
>         ssize_t ret = device_store_int(s, attr, buf, size);
> +
> +       if (check_interval == old_check_interval)
> +               return ret;
> +       mutex_lock(&mce_sysfs_mutex);
>         mce_restart();
> +       mutex_unlock(&mce_sysfs_mutex);
>         return ret;
>  }
>
> --
> 2.16.2
>