Re: [PATCH] mm, vmstat: Make sure mutex is a global static

[Vlastimil Babka <vbabka@suse.cz>]

On 11/07/2017 10:38 PM, Kees Cook wrote:
> The mutex in sysctl_vm_numa_stat_handler() needs to be a global static, not
> a stack variable, otherwise it doesn't serve any purpose. Also, reading the
> file with CONFIG_LOCKDEP=y will complain:

Oops, good catch.

> [   63.258593] INFO: trying to register non-static key.
> [   63.259113] the code is fine but needs lockdep annotation.
> [   63.259596] turning off the locking correctness validator.
> [   63.260073] CPU: 1 PID: 4102 Comm: perl Not tainted 4.14.0-rc8-next-20171107+ #419
> [   63.260769] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
> [   63.261570] Call Trace:
> [   63.261783]  dump_stack+0x5f/0x86
> [   63.262062]  register_lock_class+0xe4/0x550
> [   63.262408]  ? __lock_acquire+0x308/0x1170
> [   63.262746]  __lock_acquire+0x7e/0x1170
> [   63.263063]  lock_acquire+0x9d/0x1d0
> [   63.263363]  ? sysctl_vm_numa_stat_handler+0x8f/0x2d0
> [   63.263777]  ? sysctl_vm_numa_stat_handler+0x8f/0x2d0
> [   63.264192]  __mutex_lock+0xb8/0x9a0
> [   63.264488]  ? sysctl_vm_numa_stat_handler+0x8f/0x2d0
> [   63.264942]  ? sysctl_vm_numa_stat_handler+0x8f/0x2d0
> [   63.265398]  ? sysctl_vm_numa_stat_handler+0x8f/0x2d0
> [   63.265840]  sysctl_vm_numa_stat_handler+0x8f/0x2d0
> [   63.266270]  proc_sys_call_handler+0xe3/0x100
> [   63.266655]  __vfs_read+0x33/0x1b0
> [   63.266957]  vfs_read+0xa6/0x150
> [   63.267244]  SyS_read+0x55/0xc0
> [   63.267525]  do_syscall_64+0x56/0x140
> [   63.267850]  entry_SYSCALL64_slow_path+0x25/0x25
> 
> Fixes: 920d5f77d1a25 ("mm, sysctl: make NUMA stats configurable")

Note that this hash is specific to particular next-$DATE as mmotm is
reimported each day.

> Cc: Jesper Dangaard Brouer <brouer@redhat.com>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Ying Huang <ying.huang@intel.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  mm/vmstat.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index e0593434fd58..40b2db6db6b1 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -72,11 +72,12 @@ static void invalid_numa_statistics(void)
>  	zero_global_numa_counters();
>  }
>  
> +static DEFINE_MUTEX(vm_numa_stat_lock);
> +
>  int sysctl_vm_numa_stat_handler(struct ctl_table *table, int write,
>  		void __user *buffer, size_t *length, loff_t *ppos)
>  {
>  	int ret, oldval;
> -	DEFINE_MUTEX(vm_numa_stat_lock);

Yeah it was Michal who suggested scoping the mutex here instead of
global scope, but I think he didn't mean to remove the 'static'
qualifier, and we both missed that in the review :(
So the scope under sysctl_vm_numa_stat_handler() should be okay, just
with the 'static' added.

>  
>  	mutex_lock(&vm_numa_stat_lock);
>  	if (write)
>