Date:   Wed, 18 Apr 2018 10:05:55 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     adobriyan@...il.com, willy@...radead.org, mguzik@...hat.com,
        gorcunov@...il.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and
 env_start|end in mm_struct

On Sun 15-04-18 02:24:51, Yang Shi wrote:
> mmap_sem is on the hot path of kernel, and it very contended, but it is
> abused too. It is used to protect arg_start|end and evn_start|end when
> reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make
> sense since those proc files just expect to read 4 values atomically and
> not related to VM, they could be set to arbitrary values by C/R.
> 
> And, the mmap_sem contention may cause unexpected issue like below:
> 
> INFO: task ps:14018 blocked for more than 120 seconds.
>        Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
> message.
>  ps              D    0 14018      1 0x00000004
>   ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
>   ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
>   00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
>  Call Trace:
>   [<ffffffff817154d0>] ? __schedule+0x250/0x730
>   [<ffffffff817159e6>] schedule+0x36/0x80
>   [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
>   [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
>   [<ffffffff81717db0>] down_read+0x20/0x40
>   [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
>   [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
>   [<ffffffff81241d87>] __vfs_read+0x37/0x150
>   [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
>   [<ffffffff81242266>] vfs_read+0x96/0x130
>   [<ffffffff812437b5>] SyS_read+0x55/0xc0
>   [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5
> 
> Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock
> for them to mitigate the abuse of mmap_sem.
> 
> So, introduce a new spinlock in mm_struct to protect the concurrent
> access to arg_start|end, env_start|end and others, as well as replace
> write map_sem to read to protect the race condition between prctl and
> sys_brk which might break check_data_rlimit(), and makes prctl more
> friendly to other VM operations.
> 
> This patch just eliminates the abuse of mmap_sem, but it can't resolve the
> above hung task warning completely since the later access_remote_vm() call
> needs acquire mmap_sem. The mmap_sem scalability issue will be solved in the
> future.
> 
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
> Cc: Alexey Dobriyan <adobriyan@...il.com>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Mateusz Guzik <mguzik@...hat.com>
> Cc: Cyrill Gorcunov <gorcunov@...il.com>

Yes, looks good to me. As mentioned in other emails prctl_set_mm_map
really deserves a comment explaining why we are doing the down_read

What about something like the following?
"
arg_lock protects concurent updates but we still need mmap_sem for read
to exclude races with do_brk.
"
Acked-by: Michal Hocko <mhocko@...e.com>

> ---
> v3 --> v4:
> * Protected values update with down_read + spin_lock to prevent from race
>   condition between prctl and sys_brk and made prctl more friendly to VM
>   operations per Michal's suggestion
> 
> v2 --> v3:
> * Restored down_write in prctl syscall
> * Elaborate the limitation of this patch suggested by Michal
> * Protect those fields by the new lock except brk and start_brk per Michal's
>   suggestion
> * Based off Cyrill's non PR_SET_MM_MAP oprations deprecation patch
>   (https://lkml.org/lkml/2018/4/5/541)
> 
> v1 --> v2:
> * Use spinlock instead of rwlock per Mattew's suggestion
> * Replace down_write to down_read in prctl_set_mm (see commit log for details)
>  fs/proc/base.c           | 8 ++++----
>  include/linux/mm_types.h | 2 ++
>  kernel/fork.c            | 1 +
>  kernel/sys.c             | 6 ++++--
>  mm/init-mm.c             | 1 +
>  5 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index eafa39a..3551757 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -239,12 +239,12 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
>  		goto out_mmput;
>  	}
>  
> -	down_read(&mm->mmap_sem);
> +	spin_lock(&mm->arg_lock);
>  	arg_start = mm->arg_start;
>  	arg_end = mm->arg_end;
>  	env_start = mm->env_start;
>  	env_end = mm->env_end;
> -	up_read(&mm->mmap_sem);
> +	spin_unlock(&mm->arg_lock);
>  
>  	BUG_ON(arg_start > arg_end);
>  	BUG_ON(env_start > env_end);
> @@ -929,10 +929,10 @@ static ssize_t environ_read(struct file *file, char __user *buf,
>  	if (!mmget_not_zero(mm))
>  		goto free;
>  
> -	down_read(&mm->mmap_sem);
> +	spin_lock(&mm->arg_lock);
>  	env_start = mm->env_start;
>  	env_end = mm->env_end;
> -	up_read(&mm->mmap_sem);
> +	spin_unlock(&mm->arg_lock);
>  
>  	while (count > 0) {
>  		size_t this_len, max_len;
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 2161234..49dd59e 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -413,6 +413,8 @@ struct mm_struct {
>  	unsigned long exec_vm;		/* VM_EXEC & ~VM_WRITE & ~VM_STACK */
>  	unsigned long stack_vm;		/* VM_STACK */
>  	unsigned long def_flags;
> +
> +	spinlock_t arg_lock; /* protect the below fields */
>  	unsigned long start_code, end_code, start_data, end_data;
>  	unsigned long start_brk, brk, start_stack;
>  	unsigned long arg_start, arg_end, env_start, env_end;
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 242c8c9..295f903 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -900,6 +900,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>  	mm->pinned_vm = 0;
>  	memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
>  	spin_lock_init(&mm->page_table_lock);
> +	spin_lock_init(&mm->arg_lock);
>  	mm_init_cpumask(mm);
>  	mm_init_aio(mm);
>  	mm_init_owner(mm, p);
> diff --git a/kernel/sys.c b/kernel/sys.c
> index f16725e..0cc5a1c 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2011,7 +2011,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>  			return error;
>  	}
>  
> -	down_write(&mm->mmap_sem);
> +	down_read(&mm->mmap_sem);
>  
>  	/*
>  	 * We don't validate if these members are pointing to
> @@ -2025,6 +2025,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>  	 *    to any problem in kernel itself
>  	 */
>  
> +	spin_lock(&mm->arg_lock);
>  	mm->start_code	= prctl_map.start_code;
>  	mm->end_code	= prctl_map.end_code;
>  	mm->start_data	= prctl_map.start_data;
> @@ -2036,6 +2037,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>  	mm->arg_end	= prctl_map.arg_end;
>  	mm->env_start	= prctl_map.env_start;
>  	mm->env_end	= prctl_map.env_end;
> +	spin_unlock(&mm->arg_lock);
>  
>  	/*
>  	 * Note this update of @saved_auxv is lockless thus
> @@ -2048,7 +2050,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>  	if (prctl_map.auxv_size)
>  		memcpy(mm->saved_auxv, user_auxv, sizeof(user_auxv));
>  
> -	up_write(&mm->mmap_sem);
> +	up_read(&mm->mmap_sem);
>  	return 0;
>  }
>  #endif /* CONFIG_CHECKPOINT_RESTORE */
> diff --git a/mm/init-mm.c b/mm/init-mm.c
> index f94d5d1..f0179c9 100644
> --- a/mm/init-mm.c
> +++ b/mm/init-mm.c
> @@ -22,6 +22,7 @@ struct mm_struct init_mm = {
>  	.mm_count	= ATOMIC_INIT(1),
>  	.mmap_sem	= __RWSEM_INITIALIZER(init_mm.mmap_sem),
>  	.page_table_lock =  __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
> +	.arg_lock	=  __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
>  	.mmlist		= LIST_HEAD_INIT(init_mm.mmlist),
>  	.user_ns	= &init_user_ns,
>  	INIT_MM_CONTEXT(init_mm)
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs

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