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

Greg Kroah-Hartman
2020-12-31T09:22:00Z

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@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?

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