This is the sixth patchset to fix the use-after-free problem in lockd
which we originally discussed back in October. The main problem is
detailed in the last patch of the series. Along the way, Christoph
Hellwig mentioned that it would be advantageous to convert lockd to use
the kthread API. This patch set first makes that change and then patches
it to actually fix the use after free problem. It also fixes a couple of
minor bugs in the current lockd implementation.
Most of the changes from the last patchset were ones suggested by
Neil Brown and are:
+ fix a preexisting bug that would cause a NULL pointer dereference if
the later kmallocs failed in svc_prepare_thread
+ additional comments to explain the rationale behind nlmsvc_ref
increments and decrements
+ removed module_get/put from lockd(). It should no longer be necessary
and isn't safe
+ sanity checks in lockd_down have been changed to BUG() calls. They
should never happen and if they do, then something is very wrong.
I've done some basic testing and everything seems to work as expected.
I've also tested this against the reproducer that I have for the
use-after-free problem and this does fix it. I've tried to make this
cleanly bisectable, but have only really tested the final result.
Many thanks to Trond Myklebust, Chuck Lever, Neil Brown and Christoph
Hellwig for their guidance on this.
Signed-off-by: Jeff Layton <[email protected]>
Needed since the plan is to not have a svc_create_thread helper and to
have current users of that function just call kthread_run directly.
Signed-off-by: Jeff Layton <[email protected]>
---
net/sunrpc/svcsock.c | 1 +
1 files changed, 1 insertions(+), 0 deletions(-)
diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
index 057c870..f2bef16 100644
--- a/net/sunrpc/svcsock.c
+++ b/net/sunrpc/svcsock.c
@@ -1407,6 +1407,7 @@ svc_sock_update_bufs(struct svc_serv *serv)
}
spin_unlock_bh(&serv->sv_lock);
}
+EXPORT_SYMBOL(svc_sock_update_bufs);
/*
* Receive the next request on any socket. This code is carefully
--
1.5.3.3
Move the initialzation in __svc_create_thread that happens prior to
thread creation to a new function. Export the function to allow
services to have better control over the svc_rqst structs.
Also rearrange the rqstp initialization to prevent NULL pointer
dereferences in svc_exit_thread in case allocations fail.
Signed-off-by: Jeff Layton <[email protected]>
---
include/linux/sunrpc/svc.h | 2 +
net/sunrpc/svc.c | 59 +++++++++++++++++++++++++++++++------------
2 files changed, 44 insertions(+), 17 deletions(-)
diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index 8531a70..5f07300 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -382,6 +382,8 @@ struct svc_procedure {
*/
struct svc_serv * svc_create(struct svc_program *, unsigned int,
void (*shutdown)(struct svc_serv*));
+struct svc_rqst *svc_prepare_thread(struct svc_serv *serv,
+ struct svc_pool *pool);
int svc_create_thread(svc_thread_fn, struct svc_serv *);
void svc_exit_thread(struct svc_rqst *);
struct svc_serv * svc_create_pooled(struct svc_program *, unsigned int,
diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index fca17d0..f9636bf 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -538,31 +538,17 @@ svc_release_buffer(struct svc_rqst *rqstp)
put_page(rqstp->rq_pages[i]);
}
-/*
- * Create a thread in the given pool. Caller must hold BKL.
- * On a NUMA or SMP machine, with a multi-pool serv, the thread
- * will be restricted to run on the cpus belonging to the pool.
- */
-static int
-__svc_create_thread(svc_thread_fn func, struct svc_serv *serv,
- struct svc_pool *pool)
+struct svc_rqst *
+svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool)
{
struct svc_rqst *rqstp;
- int error = -ENOMEM;
- int have_oldmask = 0;
- cpumask_t oldmask;
rqstp = kzalloc(sizeof(*rqstp), GFP_KERNEL);
if (!rqstp)
- goto out;
+ goto out_enomem;
init_waitqueue_head(&rqstp->rq_wait);
- if (!(rqstp->rq_argp = kmalloc(serv->sv_xdrsize, GFP_KERNEL))
- || !(rqstp->rq_resp = kmalloc(serv->sv_xdrsize, GFP_KERNEL))
- || !svc_init_buffer(rqstp, serv->sv_max_mesg))
- goto out_thread;
-
serv->sv_nrthreads++;
spin_lock_bh(&pool->sp_lock);
pool->sp_nrthreads++;
@@ -571,6 +557,45 @@ __svc_create_thread(svc_thread_fn func, struct svc_serv *serv,
rqstp->rq_server = serv;
rqstp->rq_pool = pool;
+ rqstp->rq_argp = kmalloc(serv->sv_xdrsize, GFP_KERNEL);
+ if (!rqstp->rq_argp)
+ goto out_thread;
+
+ rqstp->rq_resp = kmalloc(serv->sv_xdrsize, GFP_KERNEL);
+ if (!rqstp->rq_resp)
+ goto out_thread;
+
+ if (!svc_init_buffer(rqstp, serv->sv_max_mesg))
+ goto out_thread;
+
+ return rqstp;
+out_thread:
+ svc_exit_thread(rqstp);
+out_enomem:
+ return ERR_PTR(-ENOMEM);
+}
+EXPORT_SYMBOL(svc_prepare_thread);
+
+/*
+ * Create a thread in the given pool. Caller must hold BKL.
+ * On a NUMA or SMP machine, with a multi-pool serv, the thread
+ * will be restricted to run on the cpus belonging to the pool.
+ */
+static int
+__svc_create_thread(svc_thread_fn func, struct svc_serv *serv,
+ struct svc_pool *pool)
+{
+ struct svc_rqst *rqstp;
+ int error = -ENOMEM;
+ int have_oldmask = 0;
+ cpumask_t oldmask;
+
+ rqstp = svc_prepare_thread(serv, pool);
+ if (IS_ERR(rqstp)) {
+ error = PTR_ERR(rqstp);
+ goto out;
+ }
+
if (serv->sv_nrpools > 1)
have_oldmask = svc_pool_map_set_cpumask(pool->sp_id, &oldmask);
--
1.5.3.3
lockd makes itself freezable, but never calls try_to_freeze(). Have it
call try_to_freeze() within the main loop.
Signed-off-by: Jeff Layton <[email protected]>
---
fs/lockd/svc.c | 3 +++
1 files changed, 3 insertions(+), 0 deletions(-)
diff --git a/fs/lockd/svc.c b/fs/lockd/svc.c
index 0f4148a..03a83a0 100644
--- a/fs/lockd/svc.c
+++ b/fs/lockd/svc.c
@@ -155,6 +155,9 @@ lockd(struct svc_rqst *rqstp)
long timeout = MAX_SCHEDULE_TIMEOUT;
char buf[RPC_MAX_ADDRBUFLEN];
+ if (try_to_freeze())
+ continue;
+
if (signalled()) {
flush_signals(current);
if (nlmsvc_ops) {
--
1.5.3.3
Have lockd_up start lockd using kthread_run. With this change,
lockd_down now blocks until lockd actually exits, so there's no longer
need for the waitqueue code at the end of lockd_down. This also means
that only one lockd can be running at a time which simplifies the code
within lockd's main loop.
Signed-off-by: Jeff Layton <[email protected]>
---
fs/lockd/svc.c | 93 ++++++++++++++++++++++++--------------------------------
1 files changed, 40 insertions(+), 53 deletions(-)
diff --git a/fs/lockd/svc.c b/fs/lockd/svc.c
index 03a83a0..0777a4e 100644
--- a/fs/lockd/svc.c
+++ b/fs/lockd/svc.c
@@ -25,6 +25,7 @@
#include <linux/smp.h>
#include <linux/smp_lock.h>
#include <linux/mutex.h>
+#include <linux/kthread.h>
#include <linux/freezer.h>
#include <linux/sunrpc/types.h>
@@ -48,13 +49,12 @@ EXPORT_SYMBOL(nlmsvc_ops);
static DEFINE_MUTEX(nlmsvc_mutex);
static unsigned int nlmsvc_users;
-static pid_t nlmsvc_pid;
+static struct task_struct *nlmsvc_task;
static struct svc_serv *nlmsvc_serv;
int nlmsvc_grace_period;
unsigned long nlmsvc_timeout;
static DECLARE_COMPLETION(lockd_start_done);
-static DECLARE_WAIT_QUEUE_HEAD(lockd_exit);
/*
* These can be set at insmod time (useful for NFS as root filesystem),
@@ -111,10 +111,11 @@ static inline void clear_grace_period(void)
/*
* This is the lockd kernel thread
*/
-static void
-lockd(struct svc_rqst *rqstp)
+static int
+lockd(void *vrqstp)
{
int err = 0;
+ struct svc_rqst *rqstp = vrqstp;
unsigned long grace_period_expire;
/* Lock module and set up kernel thread */
@@ -122,17 +123,14 @@ lockd(struct svc_rqst *rqstp)
* be holding a reference to this module, so it
* is safe to just claim another reference
*/
- __module_get(THIS_MODULE);
lock_kernel();
/*
* Let our maker know we're running.
*/
- nlmsvc_pid = current->pid;
nlmsvc_serv = rqstp->rq_server;
complete(&lockd_start_done);
- daemonize("lockd");
set_freezable();
/* Process request with signals blocked, but allow SIGKILL. */
@@ -151,7 +149,7 @@ lockd(struct svc_rqst *rqstp)
* NFS mount or NFS daemon has gone away, and we've been sent a
* signal, or else another process has taken over our job.
*/
- while ((nlmsvc_users || !signalled()) && nlmsvc_pid == current->pid) {
+ while (!kthread_should_stop()) {
long timeout = MAX_SCHEDULE_TIMEOUT;
char buf[RPC_MAX_ADDRBUFLEN];
@@ -199,27 +197,18 @@ lockd(struct svc_rqst *rqstp)
flush_signals(current);
- /*
- * Check whether there's a new lockd process before
- * shutting down the hosts and clearing the slot.
- */
- if (!nlmsvc_pid || current->pid == nlmsvc_pid) {
- if (nlmsvc_ops)
- nlmsvc_invalidate_all();
- nlm_shutdown_hosts();
- nlmsvc_pid = 0;
- nlmsvc_serv = NULL;
- } else
- printk(KERN_DEBUG
- "lockd: new process, skipping host shutdown\n");
- wake_up(&lockd_exit);
+ if (nlmsvc_ops)
+ nlmsvc_invalidate_all();
+ nlm_shutdown_hosts();
+ nlmsvc_task = NULL;
+ nlmsvc_serv = NULL;
/* Exit the RPC thread */
svc_exit_thread(rqstp);
/* Release module */
unlock_kernel();
- module_put_and_exit(0);
+ return 0;
}
@@ -269,14 +258,15 @@ static int make_socks(struct svc_serv *serv, int proto)
int
lockd_up(int proto) /* Maybe add a 'family' option when IPv6 is supported ?? */
{
- struct svc_serv * serv;
- int error = 0;
+ struct svc_serv *serv;
+ struct svc_rqst *rqstp;
+ int error = 0;
mutex_lock(&nlmsvc_mutex);
/*
* Check whether we're already up and running.
*/
- if (nlmsvc_pid) {
+ if (nlmsvc_task) {
if (proto)
error = make_socks(nlmsvc_serv, proto);
goto out;
@@ -303,11 +293,24 @@ lockd_up(int proto) /* Maybe add a 'family' option when IPv6 is supported ?? */
/*
* Create the kernel thread and wait for it to start.
*/
+ rqstp = svc_prepare_thread(serv, &serv->sv_pools[0]);
+ if (IS_ERR(rqstp)) {
+ error = PTR_ERR(rqstp);
+ printk(KERN_WARNING
+ "lockd_up: svc_rqst allocation failed, error=%d\n",
+ error);
+ goto destroy_and_out;
+ }
+
+ svc_sock_update_bufs(serv);
init_completion(&lockd_start_done);
- error = svc_create_thread(lockd, serv);
- if (error) {
+ nlmsvc_task = kthread_run(lockd, rqstp, serv->sv_name);
+ if (IS_ERR(nlmsvc_task)) {
+ error = PTR_ERR(nlmsvc_task);
+ nlmsvc_task = NULL;
printk(KERN_WARNING
- "lockd_up: create thread failed, error=%d\n", error);
+ "lockd_up: kthread_run failed, error=%d\n", error);
+ svc_exit_thread(rqstp);
goto destroy_and_out;
}
wait_for_completion(&lockd_start_done);
@@ -332,37 +335,21 @@ EXPORT_SYMBOL(lockd_up);
void
lockd_down(void)
{
- static int warned;
-
mutex_lock(&nlmsvc_mutex);
if (nlmsvc_users) {
if (--nlmsvc_users)
goto out;
- } else
- printk(KERN_WARNING "lockd_down: no users! pid=%d\n", nlmsvc_pid);
-
- if (!nlmsvc_pid) {
- if (warned++ == 0)
- printk(KERN_WARNING "lockd_down: no lockd running.\n");
- goto out;
+ } else {
+ printk(KERN_ERR "lockd_down: no users! task=%p\n",
+ nlmsvc_task);
+ BUG();
}
- warned = 0;
- kill_proc(nlmsvc_pid, SIGKILL, 1);
- /*
- * Wait for the lockd process to exit, but since we're holding
- * the lockd semaphore, we can't wait around forever ...
- */
- clear_thread_flag(TIF_SIGPENDING);
- interruptible_sleep_on_timeout(&lockd_exit, HZ);
- if (nlmsvc_pid) {
- printk(KERN_WARNING
- "lockd_down: lockd failed to exit, clearing pid\n");
- nlmsvc_pid = 0;
+ if (!nlmsvc_task) {
+ printk(KERN_ERR "lockd_down: no lockd running.\n");
+ BUG();
}
- spin_lock_irq(¤t->sighand->siglock);
- recalc_sigpending();
- spin_unlock_irq(¤t->sighand->siglock);
+ kthread_stop(nlmsvc_task);
out:
mutex_unlock(&nlmsvc_mutex);
}
--
1.5.3.3
lockd_start_done is a global var that can be reused if lockd is
restarted, but it's never reinitialized. On all but the first use,
wait_for_completion isn't actually waiting on it since it has
already completed once.
Signed-off-by: Jeff Layton <[email protected]>
---
fs/lockd/svc.c | 1 +
1 files changed, 1 insertions(+), 0 deletions(-)
diff --git a/fs/lockd/svc.c b/fs/lockd/svc.c
index 82e2192..0f4148a 100644
--- a/fs/lockd/svc.c
+++ b/fs/lockd/svc.c
@@ -300,6 +300,7 @@ lockd_up(int proto) /* Maybe add a 'family' option when IPv6 is supported ?? */
/*
* Create the kernel thread and wait for it to start.
*/
+ init_completion(&lockd_start_done);
error = svc_create_thread(lockd, serv);
if (error) {
printk(KERN_WARNING
--
1.5.3.3
...and only have lockd exit when the last reference is dropped.
The problem is this:
When a lock that a client is blocking on comes free, lockd does this in
nlmsvc_grant_blocked():
nlm_async_call(block->b_call, NLMPROC_GRANTED_MSG, &nlmsvc_grant_ops);
the callback from this call is nlmsvc_grant_callback(). That function
does this at the end to wake up lockd:
svc_wake_up(block->b_daemon);
However there is no guarantee that lockd will be up when this happens.
If someone shuts down or restarts lockd before the async call completes,
then the b_daemon pointer will point to freed memory and the kernel may
oops.
I first noticed this on older kernels and had mistakenly thought that
newer kernels weren't susceptible, but that's not correct. There's a bit
of a race to make sure that the nlm_host is bound when the async call is
done, but I can now reproduce this at will on current kernels.
This patch is based on Trond's suggestion to add a new reference counter
to lockd, and only allows lockd to go down when it reaches 0. With this
change we can't use kthread_stop here. nlmsvc_unlink_block is called by
lockd and a kthread can't call kthread_stop on itself. So the patch
changes lockd to check the refcount itself and to return if it goes to
0. We do the checking and exit while holding the nlmsvc_mutex to make
sure that a new lockd is not started until the old one is down.
Signed-off-by: Jeff Layton <[email protected]>
---
fs/lockd/svc.c | 50 +++++++++++++++++++++++++++++++++++++------
fs/lockd/svclock.c | 8 +++++++
include/linux/lockd/lockd.h | 1 +
3 files changed, 52 insertions(+), 7 deletions(-)
diff --git a/fs/lockd/svc.c b/fs/lockd/svc.c
index 0777a4e..b1918e9 100644
--- a/fs/lockd/svc.c
+++ b/fs/lockd/svc.c
@@ -51,6 +51,7 @@ static DEFINE_MUTEX(nlmsvc_mutex);
static unsigned int nlmsvc_users;
static struct task_struct *nlmsvc_task;
static struct svc_serv *nlmsvc_serv;
+atomic_t nlmsvc_ref = ATOMIC_INIT(0);
int nlmsvc_grace_period;
unsigned long nlmsvc_timeout;
@@ -133,7 +134,10 @@ lockd(void *vrqstp)
set_freezable();
- /* Process request with signals blocked, but allow SIGKILL. */
+ /*
+ * Process request with signals blocked, but allow SIGKILL which
+ * signifies that lockd should drop all of its locks.
+ */
allow_signal(SIGKILL);
dprintk("NFS locking service started (ver " LOCKD_VERSION ").\n");
@@ -146,15 +150,19 @@ lockd(void *vrqstp)
/*
* The main request loop. We don't terminate until the last
- * NFS mount or NFS daemon has gone away, and we've been sent a
- * signal, or else another process has taken over our job.
+ * NFS mount or NFS daemon has gone away, and the nlm_blocked
+ * list is empty. The nlmsvc_mutex ensures that we prevent a
+ * new lockd from being started before the old one is down.
*/
- while (!kthread_should_stop()) {
+ mutex_lock(&nlmsvc_mutex);
+ while (atomic_read(&nlmsvc_ref) != 0) {
long timeout = MAX_SCHEDULE_TIMEOUT;
char buf[RPC_MAX_ADDRBUFLEN];
+ mutex_unlock(&nlmsvc_mutex);
+
if (try_to_freeze())
- continue;
+ goto again;
if (signalled()) {
flush_signals(current);
@@ -181,11 +189,12 @@ lockd(void *vrqstp)
*/
err = svc_recv(rqstp, timeout);
if (err == -EAGAIN || err == -EINTR)
- continue;
+ goto again;
if (err < 0) {
printk(KERN_WARNING
"lockd: terminating on error %d\n",
-err);
+ mutex_lock(&nlmsvc_mutex);
break;
}
@@ -193,8 +202,15 @@ lockd(void *vrqstp)
svc_print_addr(rqstp, buf, sizeof(buf)));
svc_process(rqstp);
+again:
+ mutex_lock(&nlmsvc_mutex);
}
+ /*
+ * at this point lockd is committed to going down. We hold the
+ * nlmsvc_mutex until just before exit to prevent a new one
+ * from starting before it's down.
+ */
flush_signals(current);
if (nlmsvc_ops)
@@ -202,6 +218,7 @@ lockd(void *vrqstp)
nlm_shutdown_hosts();
nlmsvc_task = NULL;
nlmsvc_serv = NULL;
+ mutex_unlock(&nlmsvc_mutex);
/* Exit the RPC thread */
svc_exit_thread(rqstp);
@@ -263,6 +280,11 @@ lockd_up(int proto) /* Maybe add a 'family' option when IPv6 is supported ?? */
int error = 0;
mutex_lock(&nlmsvc_mutex);
+
+ /* first lockd_up caller takes a nlmsvc_ref */
+ if (!nlmsvc_users)
+ atomic_inc(&nlmsvc_ref);
+
/*
* Check whether we're already up and running.
*/
@@ -322,6 +344,9 @@ lockd_up(int proto) /* Maybe add a 'family' option when IPv6 is supported ?? */
destroy_and_out:
svc_destroy(serv);
out:
+ /* if there was an error and this is the first user, drop reference */
+ if (!nlmsvc_users && error)
+ atomic_dec(&nlmsvc_ref);
if (!error)
nlmsvc_users++;
mutex_unlock(&nlmsvc_mutex);
@@ -349,7 +374,18 @@ lockd_down(void)
printk(KERN_ERR "lockd_down: no lockd running.\n");
BUG();
}
- kthread_stop(nlmsvc_task);
+ if (!atomic_dec_and_test(&nlmsvc_ref))
+ printk(KERN_WARNING "lockd_down: lockd is waiting for "
+ "outstanding requests to complete before exiting.\n");
+
+ /*
+ * Sending a signal is necessary here. If we get to this point and
+ * nlm_blocked isn't empty then lockd may be held hostage by clients
+ * that are still blocking. Sending the signal makes sure that lockd
+ * invalidates all of its locks so that it's just waiting on RPC
+ * callbacks to complete
+ */
+ kill_proc(nlmsvc_task->pid, SIGKILL, 1);
out:
mutex_unlock(&nlmsvc_mutex);
}
diff --git a/fs/lockd/svclock.c b/fs/lockd/svclock.c
index d120ec3..8333315 100644
--- a/fs/lockd/svclock.c
+++ b/fs/lockd/svclock.c
@@ -61,6 +61,11 @@ nlmsvc_insert_block(struct nlm_block *block, unsigned long when)
struct list_head *pos;
dprintk("lockd: nlmsvc_insert_block(%p, %ld)\n", block, when);
+
+ /* take a lockd reference when first lock goes on nlm_blocked */
+ if (list_empty(&nlm_blocked))
+ atomic_inc(&nlmsvc_ref);
+
if (list_empty(&block->b_list)) {
kref_get(&block->b_count);
} else {
@@ -239,6 +244,9 @@ static int nlmsvc_unlink_block(struct nlm_block *block)
/* Remove block from list */
status = posix_unblock_lock(block->b_file->f_file, &block->b_call->a_args.lock.fl);
nlmsvc_remove_block(block);
+ /* drop lockd reference when last lock is removed from nlm_blocked */
+ if (list_empty(&nlm_blocked))
+ atomic_dec(&nlmsvc_ref);
return status;
}
diff --git a/include/linux/lockd/lockd.h b/include/linux/lockd/lockd.h
index e2d1ce3..7389553 100644
--- a/include/linux/lockd/lockd.h
+++ b/include/linux/lockd/lockd.h
@@ -154,6 +154,7 @@ extern struct svc_procedure nlmsvc_procedures4[];
extern int nlmsvc_grace_period;
extern unsigned long nlmsvc_timeout;
extern int nsm_use_hostnames;
+extern atomic_t nlmsvc_ref;
/*
* Lockd client functions
--
1.5.3.3
On Fri, Mar 14, 2008 at 10:44:31PM -0500, Mike Snitzer wrote:
> On Sun, Jan 13, 2008 at 1:17 PM, Christoph Hellwig <[email protected]> wrote:
> > Btw, lockd() takes BKL just after starting up and only implicitly drops
> > it when blocking. This seems very dangerous to me and badly wants
> > updating to some real locking scheme..
>
> Can you elaborate on what is meant by lockd "blocking"? Blocking in
> svc_recv() or during a SETLKW or ???
Blocking in kernel context means sleeping aka scheduling away. So in the
sentence above that means BKL is dropped once lockd sleeps on a
syncronization primitive the first time.