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SubjectRe: [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@linux.alibaba.com>
> Cc: Alexey Dobriyan <adobriyan@gmail.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Mateusz Guzik <mguzik@redhat.com>
> Cc: Cyrill Gorcunov <gorcunov@gmail.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@suse.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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