Re: [PATCH 00/10] Cover letter: fix a race in release_task when flushing the dentry

[Wen Yang]

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@xxxxxxxxxxxxxxxxx>
> Cc: Pavel Emelyanov <xemul@xxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Sukadev Bhattiprolu <sukadev@xxxxxxxxxx>
> Cc: Paul Menage <menage@xxxxxxxxxx>
> Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>

Hi Greg,

Could you kindly give some suggestions?

Thanks,