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Date:   Thu, 31 Dec 2020 10:22:41 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Wen Yang <wenyang@...ux.alibaba.com>
Cc:     Sasha Levin <sashal@...nel.org>,
        Xunlei Pang <xlpang@...ux.alibaba.com>,
        linux-kernel@...r.kernel.org, Pavel Emelyanov <xemul@...nvz.org>,
        Oleg Nesterov <oleg@...sign.ru>,
        Sukadev Bhattiprolu <sukadev@...ibm.com>,
        Paul Menage <menage@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>, stable@...r.kernel.org
Subject: Re: [PATCH 00/10] Cover letter: fix a race in release_task when
 flushing the dentry

On Thu, Dec 17, 2020 at 10:26:23AM +0800, Wen Yang wrote:
> 
> 
> 在 2020/12/4 上午2:31, Wen Yang 写道:
> > The dentries such as /proc/<pid>/ns/ have the DCACHE_OP_DELETE flag, they
> > should be deleted when the process exits.
> > 
> > Suppose the following race appears:
> > 
> > release_task                 dput
> > -> proc_flush_task
> >                               -> dentry->d_op->d_delete(dentry)
> > -> __exit_signal
> >                               -> dentry->d_lockref.count--  and return.
> > 
> > In the proc_flush_task(), if another process is using this dentry, it will
> > not be deleted. At the same time, in dput(), d_op->d_delete() can be executed
> > before __exit_signal(pid has not been hashed), d_delete returns false, so
> > this dentry still cannot be deleted.
> > 
> > This dentry will always be cached (although its count is 0 and the
> > DCACHE_OP_DELETE flag is set), its parent denry will also be cached too, and
> > these dentries can only be deleted when drop_caches is manually triggered.
> > 
> > This will result in wasted memory. What's more troublesome is that these
> > dentries reference pid, according to the commit f333c700c610 ("pidns: Add a
> > limit on the number of pid namespaces"), if the pid cannot be released, it
> > may result in the inability to create a new pid_ns.
> > 
> > This problem occurred in our cluster environment (Linux 4.9 LTS).
> > We could reproduce it by manually constructing a test program + adding some
> > debugging switches in the kernel:
> > * A test program to open the directory (/proc/<pid>/ns) [1]
> > * Adding some debugging switches to the kernel, adding a delay between
> >     proc_flush_task and __exit_signal in release_task() [2]
> > 
> > The test process is as follows:
> > 
> > A, terminal #1
> > 
> > Turn on the debug switch:
> > echo 1> /proc/sys/vm/dentry_debug_trace
> > 
> > Execute the following unshare command:
> > sudo unshare --pid --fork --mount-proc bash
> > 
> > 
> > B, terminal #2
> > 
> > Find the pid of the unshare process:
> > 
> > # pstree -p | grep unshare
> >             | `-sshd(716)---bash(718)--sudo(816)---unshare(817)---bash(818)
> > 
> > 
> > Find the corresponding dentry:
> > # dmesg | grep pid=818
> > [70.424722] XXX proc_pid_instantiate:3119 pid=818 tid=818 entry=818/ffff8802c7b670e8
> > 
> > 
> > C, terminal #3
> > 
> > Execute the opendir program, it will always open the /proc/818/ns/ directory:
> > 
> > # ./a.out /proc/818/ns/
> > pid: 876
> > .
> > ..
> > net
> > uts
> > ipc
> > pid
> > user
> > mnt
> > cgroup
> > 
> > D, go back to terminal #2
> > 
> > Turn on the debugging switches to construct the race:
> > # echo 818> /proc/sys/vm/dentry_debug_pid
> > # echo 1> /proc/sys/vm/dentry_debug_delay
> > 
> > Kill the unshare process (pid 818). Since the debugging switches have been
> > turned on, it will get stuck in release_task():
> > # kill -9 818
> > 
> > Then kill the process that opened the /proc/818/ns/ directory:
> > # kill -9 876
> > 
> > Then turn off these debugging switches to allow the 818 process to exit:
> > # echo 0> /proc/sys/vm/dentry_debug_delay
> > # echo 0> /proc/sys/vm/dentry_debug_pid
> > 
> > Checking the dmesg, we will find that the dentry(/proc/818/ns) ’s count is 0,
> > and the flag is 2800cc (#define DCACHE_OP_DELETE 0x00000008), but it is still
> > cached:
> > # dmesg | grep ffff8802a3999548
> > …
> > [565.559156] XXX dput:853 dentry=ns/ffff8802bea7b528, flag=2800cc, cnt=0, inode=ffff8802b38c2010, pdentry=818/ffff8802c7b670e8, pflag=20008c, pcnt=1, pinode=ffff8802c7812010, keywords: be cached
> > 
> > 
> > It could also be verified via the crash tool:
> > 
> > crash> dentry.d_flags,d_iname,d_inode,d_lockref -x  ffff8802bea7b528
> >    d_flags = 0x2800cc
> >    d_iname = "ns\000kkkkkkkkkkkkkkkkkkkkkkkkkkkk"
> >    d_inode = 0xffff8802b38c2010
> >    d_lockref = {
> >      {
> >        lock_count = 0x0,
> >        {
> >          lock = {
> >            {
> >              rlock = {
> >                raw_lock = {
> >                  {
> >                    val = {
> >                      counter = 0x0
> >                    },
> >                    {
> >                      locked = 0x0,
> >                      pending = 0x0
> >                    },
> >                    {
> >                      locked_pending = 0x0,
> >                      tail = 0x0
> >                    }
> >                  }
> >                }
> >              }
> >            }
> >          },
> >          count = 0x0
> >        }
> >      }
> >    }
> > crash> kmem  ffff8802bea7b528
> > CACHE             OBJSIZE  ALLOCATED     TOTAL  SLABS  SSIZE  NAME
> > ffff8802dd5f5900      192      23663     26130    871    16k  dentry
> >    SLAB              MEMORY            NODE  TOTAL  ALLOCATED  FREE
> >    ffffea000afa9e00  ffff8802bea78000     0     30         25     5
> >    FREE / [ALLOCATED]
> >    [ffff8802bea7b520]
> > 
> >        PAGE        PHYSICAL      MAPPING       INDEX CNT FLAGS
> > ffffea000afa9ec0 2bea7b000 dead000000000400        0  0 2fffff80000000
> > crash>
> > 
> > This series of patches is to fix this issue.
> > 
> > Regards,
> > Wen
> > 
> > Alexey Dobriyan (1):
> >    proc: use %u for pid printing and slightly less stack
> > 
> > Andreas Gruenbacher (1):
> >    proc: Pass file mode to proc_pid_make_inode
> > 
> > Christian Brauner (1):
> >    clone: add CLONE_PIDFD
> > 
> > Eric W. Biederman (6):
> >    proc: Better ownership of files for non-dumpable tasks in user
> >      namespaces
> >    proc: Rename in proc_inode rename sysctl_inodes sibling_inodes
> >    proc: Generalize proc_sys_prune_dcache into proc_prune_siblings_dcache
> >    proc: Clear the pieces of proc_inode that proc_evict_inode cares about
> >    proc: Use d_invalidate in proc_prune_siblings_dcache
> >    proc: Use a list of inodes to flush from proc
> > 
> > Joel Fernandes (Google) (1):
> >    pidfd: add polling support
> > 
> >   fs/proc/base.c             | 242 ++++++++++++++++++++-------------------------
> >   fs/proc/fd.c               |  20 +---
> >   fs/proc/inode.c            |  67 ++++++++++++-
> >   fs/proc/internal.h         |  22 ++---
> >   fs/proc/namespaces.c       |   3 +-
> >   fs/proc/proc_sysctl.c      |  45 ++-------
> >   fs/proc/self.c             |   6 +-
> >   fs/proc/thread_self.c      |   5 +-
> >   include/linux/pid.h        |   5 +
> >   include/linux/proc_fs.h    |   4 +-
> >   include/uapi/linux/sched.h |   1 +
> >   kernel/exit.c              |   5 +-
> >   kernel/fork.c              | 145 ++++++++++++++++++++++++++-
> >   kernel/pid.c               |   3 +
> >   kernel/signal.c            |  11 +++
> >   security/selinux/hooks.c   |   1 +
> >   16 files changed, 357 insertions(+), 228 deletions(-)
> > 
> > [1] A test program to open the directory (/proc/<pid>/ns)
> > #include <stdio.h>
> > #include <sys/types.h>
> > #include <dirent.h>
> > #include <errno.h>
> > 
> > int main(int argc, char *argv[])
> > {
> > 	DIR *dip;
> > 	struct dirent *dit;
> > 
> > 	if (argc < 2) {
> > 		printf("Usage :%s <directory>\n", argv[0]);
> > 		return -1;
> > 	}
> > 
> > 	if ((dip = opendir(argv[1])) == NULL) {
> > 		perror("opendir");
> > 		return -1;
> > 	}
> > 
> > 	printf("pid: %d\n", getpid());
> > 	while((dit = readdir (dip)) != NULL) {
> > 		printf("%s\n", dit->d_name);
> > 	}
> > 
> > 	while (1)
> > 		sleep (1);
> > 
> > 	return 0;
> > }
> > 
> > [2] Adding some debugging switches to the kernel, also adding a delay between
> >      proc_flush_task and __exit_signal in release_task():
> > 
> > diff --git a/fs/dcache.c b/fs/dcache.c
> > index 05bad55..fafad37 100644
> > --- a/fs/dcache.c
> > +++ b/fs/dcache.c
> > @@ -84,6 +84,9 @@
> >   int sysctl_vfs_cache_pressure __read_mostly = 100;
> >   EXPORT_SYMBOL_GPL(sysctl_vfs_cache_pressure);
> > 
> > +int sysctl_dentry_debug_trace __read_mostly = 0;
> > +EXPORT_SYMBOL_GPL(sysctl_dentry_debug_trace);
> > +
> >   __cacheline_aligned_in_smp DEFINE_SEQLOCK(rename_lock);
> > 
> >   EXPORT_SYMBOL(rename_lock);
> > @@ -758,6 +761,26 @@ static inline bool fast_dput(struct dentry *dentry)
> >   	return 0;
> >   }
> > 
> > +#define DENTRY_DEBUG_TRACE(dentry, keywords)                            \
> > +do {                                                                    \
> > +	if (sysctl_dentry_debug_trace)                                   \
> > +		printk("XXX %s:%d "                                      \
> > +                	"dentry=%s/%p, flag=%x, cnt=%d, inode=%p, "      \
> > +                	"pdentry=%s/%p, pflag=%x, pcnt=%d, pinode=%p, "  \
> > +			"keywords: %s\n",                                \
> > +			__func__, __LINE__,                              \
> > +			dentry->d_name.name,                             \
> > +			dentry,                                          \
> > +			dentry->d_flags,                                 \
> > +			dentry->d_lockref.count,                         \
> > +			dentry->d_inode,                                 \
> > +			dentry->d_parent->d_name.name,                   \
> > +			dentry->d_parent,                                \
> > +			dentry->d_parent->d_flags,                       \
> > +			dentry->d_parent->d_lockref.count,               \
> > +			dentry->d_parent->d_inode,                       \
> > +			keywords);                                       \
> > +} while (0)
> > 
> >   /*
> >    * This is dput
> > @@ -804,6 +827,8 @@ void dput(struct dentry *dentry)
> > 
> >   	WARN_ON(d_in_lookup(dentry));
> > 
> > +	DENTRY_DEBUG_TRACE(dentry, "be checked");
> > +
> >   	/* Unreachable? Get rid of it */
> >   	if (unlikely(d_unhashed(dentry)))
> >   		goto kill_it;
> > @@ -812,8 +837,10 @@ void dput(struct dentry *dentry)
> >   		goto kill_it;
> > 
> >   	if (unlikely(dentry->d_flags & DCACHE_OP_DELETE)) {
> > -		if (dentry->d_op->d_delete(dentry))
> > +		if (dentry->d_op->d_delete(dentry)) {
> > +			DENTRY_DEBUG_TRACE(dentry, "be killed");
> >   			goto kill_it;
> > +		}
> >   	}
> > 
> >   	if (!(dentry->d_flags & DCACHE_REFERENCED))
> > @@ -822,6 +849,9 @@ void dput(struct dentry *dentry)
> > 
> >   	dentry->d_lockref.count--;
> >   	spin_unlock(&dentry->d_lock);
> > +
> > +	DENTRY_DEBUG_TRACE(dentry, "be cached");
> > +
> >   	return;
> > 
> >   kill_it:
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index b9e4183..419a409 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -3090,6 +3090,8 @@ void proc_flush_task(struct task_struct *task)
> >   	}
> >   }
> > 
> > +extern int sysctl_dentry_debug_trace;
> > +
> >   static int proc_pid_instantiate(struct inode *dir,
> >   				   struct dentry * dentry,
> >   				   struct task_struct *task, const void *ptr)
> > @@ -3111,6 +3113,12 @@ static int proc_pid_instantiate(struct inode *dir,
> >   	d_set_d_op(dentry, &pid_dentry_operations);
> > 
> >   	d_add(dentry, inode);
> > +
> > +	if (sysctl_dentry_debug_trace)
> > +		printk("XXX %s:%d pid=%d tid=%d  entry=%s/%p\n",
> > +			__func__, __LINE__, task->pid, task->tgid,
> > +			dentry->d_name.name, dentry);
> > +
> >   	/* Close the race of the process dying before we return the dentry */
> >   	if (pid_revalidate(dentry, 0))
> >   		return 0;
> > diff --git a/kernel/exit.c b/kernel/exit.c
> > index 27f4168..2b3e1b6 100644
> > --- a/kernel/exit.c
> > +++ b/kernel/exit.c
> > @@ -55,6 +55,8 @@
> >   #include <linux/shm.h>
> >   #include <linux/kcov.h>
> > 
> > +#include <linux/delay.h>
> > +
> >   #include <asm/uaccess.h>
> >   #include <asm/unistd.h>
> >   #include <asm/pgtable.h>
> > @@ -164,6 +166,8 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
> >   	put_task_struct(tsk);
> >   }
> > 
> > +int sysctl_dentry_debug_delay __read_mostly = 0;
> > +int sysctl_dentry_debug_pid __read_mostly = 0;
> > 
> >   void release_task(struct task_struct *p)
> >   {
> > @@ -178,6 +182,11 @@ void release_task(struct task_struct *p)
> > 
> >   	proc_flush_task(p);
> > 
> > +	if (sysctl_dentry_debug_delay && p->pid == sysctl_dentry_debug_pid) {
> > +		while (sysctl_dentry_debug_delay)
> > +			mdelay(1);
> > +	}
> > +
> >   	write_lock_irq(&tasklist_lock);
> >   	ptrace_release_task(p);
> >   	__exit_signal(p);
> > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> > index 513e6da..27f1395 100644
> > --- a/kernel/sysctl.c
> > +++ b/kernel/sysctl.c
> > @@ -282,6 +282,10 @@ static int sysrq_sysctl_handler(struct ctl_table *table, int write,
> >   static int max_extfrag_threshold = 1000;
> >   #endif
> > 
> > +extern int sysctl_dentry_debug_trace;
> > +extern int sysctl_dentry_debug_delay;
> > +extern int sysctl_dentry_debug_pid;
> > +
> >   static struct ctl_table kern_table[] = {
> >   	{
> >   		.procname	= "sched_child_runs_first",
> > @@ -1498,6 +1502,30 @@ static int sysrq_sysctl_handler(struct ctl_table *table, int write,
> >   		.proc_handler	= proc_dointvec,
> >   		.extra1		= &zero,
> >   	},
> > +	{
> > +		.procname	= "dentry_debug_trace",
> > +		.data		= &sysctl_dentry_debug_trace,
> > +		.maxlen		= sizeof(sysctl_dentry_debug_trace),
> > +		.mode		= 0644,
> > +		.proc_handler	= proc_dointvec,
> > +		.extra1		= &zero,
> > +	},
> > +	{
> > +		.procname	= "dentry_debug_delay",
> > +		.data		= &sysctl_dentry_debug_delay,
> > +		.maxlen		= sizeof(sysctl_dentry_debug_delay),
> > +		.mode		= 0644,
> > +		.proc_handler	= proc_dointvec,
> > +		.extra1		= &zero,
> > +	},
> > +	{
> > +		.procname	= "dentry_debug_pid",
> > +		.data		= &sysctl_dentry_debug_pid,
> > +		.maxlen		= sizeof(sysctl_dentry_debug_pid),
> > +		.mode		= 0644,
> > +		.proc_handler	= proc_dointvec,
> > +		.extra1		= &zero,
> > +	},
> >   #ifdef HAVE_ARCH_PICK_MMAP_LAYOUT
> >   	{
> >   		.procname	= "legacy_va_layout",
> > 
> > 
> > Signed-off-by: Wen Yang <wenyang@...ux.alibaba.com>
> > Cc: Pavel Emelyanov <xemul@...nvz.org>
> > Cc: Oleg Nesterov <oleg@...sign.ru>
> > Cc: Sukadev Bhattiprolu <sukadev@...ibm.com>
> > Cc: Paul Menage <menage@...gle.com>
> > Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: <stable@...r.kernel.org>
> > 
> 
> Hi Greg,
> 
> Could you kindly give some suggestions?

I'm sorry, but I don't really understand what this patch series is for.

Why is there a patch in the 00/10 email?  What stable kernel(s) should
this be backported to?  And why can't you just use a newer kernel (like
4.19) if you are hitting this issue with much older ones?

confused,

greg k-h

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