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

Wen Yang
2020-12-17T02:27:12Z

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@linux.alibaba.com>
> Cc: Pavel Emelyanov <xemul@openvz.org>
> Cc: Oleg Nesterov <oleg@tv-sign.ru>
> Cc: Sukadev Bhattiprolu <sukadev@us.ibm.com>
> Cc: Paul Menage <menage@google.com>
> Cc: "Eric W. Biederman" <ebiederm@xmission.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: <stable@vger.kernel.org>
>

Hi Greg,

Could you kindly give some suggestions?

Thanks,