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Subject[v2 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct
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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 and env_start|end.

And, commit ddf1d398e517e660207e2c807f76a90df543a217 ("prctl: take mmap
sem for writing to protect against others") changed down_read to
down_write to avoid write race condition in prctl_set_mm(). Since we
already have dedicated lock to protect them, it is safe to change back
to down_read.

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@openvz.org>
---
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 | 14 ++++++++++----
mm/init-mm.c | 1 +
5 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 9298324..e0282b6 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -242,12 +242,12 @@ 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 fd1af6b..3be4588 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -413,6 +413,8 @@ struct mm_struct {
unsigned long def_flags;
unsigned long start_code, end_code, start_data, end_data;
unsigned long start_brk, brk, start_stack;
+
+ spinlock_t arg_lock; /* protect concurrent access to arg_* and env_* */
unsigned long arg_start, arg_end, env_start, env_end;

unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */
diff --git a/kernel/fork.c b/kernel/fork.c
index e5d9d40..6540ae7 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -898,6 +898,7 @@ 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 f2289de..17bddd2 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1959,7 +1959,7 @@ 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
@@ -1980,10 +1980,13 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
mm->start_brk = prctl_map.start_brk;
mm->brk = prctl_map.brk;
mm->start_stack = prctl_map.start_stack;
+
+ spin_lock(&mm->arg_lock);
mm->arg_start = prctl_map.arg_start;
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
@@ -1996,7 +1999,7 @@ 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 */
@@ -2063,7 +2066,7 @@ static int prctl_set_mm(int opt, unsigned long addr,

error = -EINVAL;

- down_write(&mm->mmap_sem);
+ down_read(&mm->mmap_sem);
vma = find_vma(mm, addr);

prctl_map.start_code = mm->start_code;
@@ -2149,14 +2152,17 @@ static int prctl_set_mm(int opt, unsigned long addr,
mm->start_brk = prctl_map.start_brk;
mm->brk = prctl_map.brk;
mm->start_stack = prctl_map.start_stack;
+
+ spin_lock(&mm->arg_lock);
mm->arg_start = prctl_map.arg_start;
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);

error = 0;
out:
- up_write(&mm->mmap_sem);
+ up_read(&mm->mmap_sem);
return error;
}

diff --git a/mm/init-mm.c b/mm/init-mm.c
index f94d5d1..66cce4c 100644
--- a/mm/init-mm.c
+++ b/mm/init-mm.c
@@ -23,6 +23,7 @@ struct mm_struct init_mm = {
.mmap_sem = __RWSEM_INITIALIZER(init_mm.mmap_sem),
.page_table_lock = __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
.mmlist = LIST_HEAD_INIT(init_mm.mmlist),
+ .arg_lock = __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
.user_ns = &init_user_ns,
INIT_MM_CONTEXT(init_mm)
};
--
1.8.3.1
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