Date:	Mon, 21 Sep 2015 13:00:04 +0200
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Jeff Layton <jlayton@...chiereds.net>
Cc:	Bruce Fields <bfields@...ldses.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Andrey Konovalov <andreyknvl@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	ktsan@...glegroups.com, Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] fs: fix data races on inode->i_flctx

On Mon, Sep 21, 2015 at 12:56 PM, Jeff Layton <jlayton@...chiereds.net> wrote:
> On Mon, 21 Sep 2015 12:53:40 +0200
> Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
>> On Mon, Sep 21, 2015 at 12:50 PM, Jeff Layton <jlayton@...chiereds.net> wrote:
>> > On Mon, 21 Sep 2015 09:43:06 +0200
>> > Dmitry Vyukov <dvyukov@...gle.com> wrote:
>> >
>> >> locks_get_lock_context() uses cmpxchg() to install i_flctx.
>> >> cmpxchg() is a release operation which is correct. But it uses
>> >> a plain load to load i_flctx. This is incorrect. Subsequent loads
>> >> from i_flctx can hoist above the load of i_flctx pointer itself
>> >> and observe uninitialized garbage there. This in turn can lead
>> >> to corruption of ctx->flc_lock and other members.
>> >>
>> >
>> > I don't get this bit. The i_flctx field is initialized to zero when the
>> > inode is allocated, and we only assign it with cmpxchg(). How would you
>> > ever see uninitialized garbage in there? It should either be NULL or a
>> > valid pointer, no?
>>
>> This is not about i_flctx pointer, this is about i_flctx object
>> contents (pointee).
>> Readers can see a non-NULL pointer, but read garbage from *i_flctx.
>>
>
> Again, I don't get it though. The cmpxchg happens after we've
> initialized the structure. Given that there are implicit full memory
> barriers before and after the cmpxchg(), how do you end up with another
> task seeing the pointer before it was ever initialized?
>
> The store should only happen after the initialization has occurred, and
> the loads in the other tasks shouldn't be able to see the results of
> that store until then. No?

If a reader looks at things in the opposite order, then it does not
matter how you store them. Reader still can observe them in the wrong
order.
Memory barriers is always a game of two. Writer needs to do stores in
the correct order and reader must do reads in the correct order. A
single memory barrier cannot possible make any sense.


>> > If that'st the case, then most of the places where you're adding
>> > smp_load_acquire are places that can tolerate seeing NULL there. e.g.
>> > you have racing LOCK_EX/LOCK_UN flock() calls in different tasks or
>> > something.
>> >
>> >> Documentation/memory-barriers.txt explicitly requires to use
>> >> a barrier in such context:
>> >> "A load-load control dependency requires a full read memory barrier".
>> >>
>> >
>> > The same file also says:
>> >
>> > "Any atomic operation that modifies some state in memory and returns information
>> > about the state (old or new) implies an SMP-conditional general memory barrier
>> > (smp_mb()) on each side of the actual operation (with the exception of
>> > explicit lock operations, described later).  These include:
>> >
>> > ...
>> > /* when succeeds */
>> > cmpxchg();
>> > "
>> >
>> > If there is an implied smp_mb() before and after the cmpxchg(), how
>> > could the CPU hoist anything before that point?
>> >
>> > Note that I'm not saying that you're wrong, I just want to make sure I
>> > fully understand the problem before we go trying to fix it.
>> >
>> >> Use smp_load_acquire() in locks_get_lock_context() and in bunch
>> >> of other functions that can proceed concurrently with
>> >> locks_get_lock_context().
>> >>
>> >> The data race was found with KernelThreadSanitizer (KTSAN).
>> >>
>> >> Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
>> >> ---
>> >> For the record, here is the KTSAN report:
>> >>
>> >> ThreadSanitizer: data-race in _raw_spin_lock
>> >>
>> >> Read at 0xffff8800bae50da0 of size 1 by thread 3060 on CPU 2:
>> >>  [<     inline     >] __raw_spin_lock include/linux/spinlock_api_smp.h:158
>> >>  [<ffffffff81ee37d0>] _raw_spin_lock+0x50/0x70 kernel/locking/spinlock.c:151
>> >>  [<     inline     >] spin_lock include/linux/spinlock.h:312
>> >>  [<ffffffff812e1187>] flock_lock_inode+0xb7/0x440 fs/locks.c:895
>> >>  [<ffffffff812e16e6>] flock_lock_inode_wait+0x46/0x110 fs/locks.c:1871
>> >>  [<ffffffff812e2814>] SyS_flock+0x224/0x23
>> >>
>> >> Previous write at 0xffff8800bae50da0 of size 8 by thread 3063 on CPU 8:
>> >>  [<ffffffff81248628>] kmem_cache_alloc+0xd8/0x2f0 mm/slab.c:3420
>> >>  [<ffffffff812ddba0>] locks_get_lock_context+0x60/0x140 fs/locks.c:213
>> >>  [<ffffffff812e112a>] flock_lock_inode+0x5a/0x440 fs/locks.c:882
>> >>  [<ffffffff812e16e6>] flock_lock_inode_wait+0x46/0x110 fs/locks.c:1871
>> >>  [<     inline     >] flock_lock_file_wait include/linux/fs.h:1219
>> >>  [<     inline     >] SYSC_flock fs/locks.c:1941
>> >>  [<ffffffff812e2814>] SyS_flock+0x224/0x230 fs/locks.c:1904
>> >>  [<ffffffff81ee3e11>] entry_SYSCALL_64_fastpath+0x31/0x95 arch/x86/entry/entry_64.S:188
>> >> ---
>> >>  fs/locks.c | 63 +++++++++++++++++++++++++++++++++++---------------------------
>> >>  1 file changed, 36 insertions(+), 27 deletions(-)
>> >>
>> >> diff --git a/fs/locks.c b/fs/locks.c
>> >> index 2a54c80..316e474 100644
>> >> --- a/fs/locks.c
>> >> +++ b/fs/locks.c
>> >> @@ -205,28 +205,32 @@ static struct kmem_cache *filelock_cache __read_mostly;
>> >>  static struct file_lock_context *
>> >>  locks_get_lock_context(struct inode *inode, int type)
>> >>  {
>> >> -     struct file_lock_context *new;
>> >> +     struct file_lock_context *ctx;
>> >>
>> >> -     if (likely(inode->i_flctx) || type == F_UNLCK)
>> >> +     /* paired with cmpxchg() below */
>> >> +     ctx = smp_load_acquire(&inode->i_flctx);
>> >> +     if (likely(ctx) || type == F_UNLCK)
>> >>               goto out;
>> >>
>> >> -     new = kmem_cache_alloc(flctx_cache, GFP_KERNEL);
>> >> -     if (!new)
>> >> +     ctx = kmem_cache_alloc(flctx_cache, GFP_KERNEL);
>> >> +     if (!ctx)
>> >>               goto out;
>> >>
>> >> -     spin_lock_init(&new->flc_lock);
>> >> -     INIT_LIST_HEAD(&new->flc_flock);
>> >> -     INIT_LIST_HEAD(&new->flc_posix);
>> >> -     INIT_LIST_HEAD(&new->flc_lease);
>> >> +     spin_lock_init(&ctx->flc_lock);
>> >> +     INIT_LIST_HEAD(&ctx->flc_flock);
>> >> +     INIT_LIST_HEAD(&ctx->flc_posix);
>> >> +     INIT_LIST_HEAD(&ctx->flc_lease);
>> >>
>> >>       /*
>> >>        * Assign the pointer if it's not already assigned. If it is, then
>> >>        * free the context we just allocated.
>> >>        */
>> >> -     if (cmpxchg(&inode->i_flctx, NULL, new))
>> >> -             kmem_cache_free(flctx_cache, new);
>> >> +     if (cmpxchg(&inode->i_flctx, NULL, ctx)) {
>> >> +             kmem_cache_free(flctx_cache, ctx);
>> >> +             ctx = smp_load_acquire(&inode->i_flctx);
>> >> +     }
>> >>  out:
>> >> -     return inode->i_flctx;
>> >> +     return ctx;
>> >>  }
>> >>
>> >>  void
>> >> @@ -762,7 +766,7 @@ posix_test_lock(struct file *filp, struct file_lock *fl)
>> >>       struct file_lock_context *ctx;
>> >>       struct inode *inode = file_inode(filp);
>> >>
>> >> -     ctx = inode->i_flctx;
>> >> +     ctx = smp_load_acquire(&inode->i_flctx);
>> >>       if (!ctx || list_empty_careful(&ctx->flc_posix)) {
>> >>               fl->fl_type = F_UNLCK;
>> >>               return;
>> >> @@ -1203,7 +1207,7 @@ int locks_mandatory_locked(struct file *file)
>> >>       struct file_lock_context *ctx;
>> >>       struct file_lock *fl;
>> >>
>> >> -     ctx = inode->i_flctx;
>> >> +     ctx = smp_load_acquire(&inode->i_flctx);
>> >>       if (!ctx || list_empty_careful(&ctx->flc_posix))
>> >>               return 0;
>> >>
>> >> @@ -1388,7 +1392,7 @@ any_leases_conflict(struct inode *inode, struct file_lock *breaker)
>> >>  int __break_lease(struct inode *inode, unsigned int mode, unsigned int type)
>> >>  {
>> >>       int error = 0;
>> >> -     struct file_lock_context *ctx = inode->i_flctx;
>> >> +     struct file_lock_context *ctx;
>> >>       struct file_lock *new_fl, *fl, *tmp;
>> >>       unsigned long break_time;
>> >>       int want_write = (mode & O_ACCMODE) != O_RDONLY;
>> >> @@ -1400,6 +1404,7 @@ int __break_lease(struct inode *inode, unsigned int mode, unsigned int type)
>> >>       new_fl->fl_flags = type;
>> >>
>> >>       /* typically we will check that ctx is non-NULL before calling */
>> >> +     ctx = smp_load_acquire(&inode->i_flctx);
>> >>       if (!ctx) {
>> >>               WARN_ON_ONCE(1);
>> >>               return error;
>> >> @@ -1494,9 +1499,10 @@ EXPORT_SYMBOL(__break_lease);
>> >>  void lease_get_mtime(struct inode *inode, struct timespec *time)
>> >>  {
>> >>       bool has_lease = false;
>> >> -     struct file_lock_context *ctx = inode->i_flctx;
>> >> +     struct file_lock_context *ctx;
>> >>       struct file_lock *fl;
>> >>
>> >> +     ctx = smp_load_acquire(&inode->i_flctx);
>> >>       if (ctx && !list_empty_careful(&ctx->flc_lease)) {
>> >>               spin_lock(&ctx->flc_lock);
>> >>               if (!list_empty(&ctx->flc_lease)) {
>> >> @@ -1543,10 +1549,11 @@ int fcntl_getlease(struct file *filp)
>> >>  {
>> >>       struct file_lock *fl;
>> >>       struct inode *inode = file_inode(filp);
>> >> -     struct file_lock_context *ctx = inode->i_flctx;
>> >> +     struct file_lock_context *ctx;
>> >>       int type = F_UNLCK;
>> >>       LIST_HEAD(dispose);
>> >>
>> >> +     ctx = smp_load_acquire(&inode->i_flctx);
>> >>       if (ctx && !list_empty_careful(&ctx->flc_lease)) {
>> >>               spin_lock(&ctx->flc_lock);
>> >>               time_out_leases(file_inode(filp), &dispose);
>> >> @@ -1713,9 +1720,10 @@ static int generic_delete_lease(struct file *filp, void *owner)
>> >>       struct file_lock *fl, *victim = NULL;
>> >>       struct dentry *dentry = filp->f_path.dentry;
>> >>       struct inode *inode = dentry->d_inode;
>> >> -     struct file_lock_context *ctx = inode->i_flctx;
>> >> +     struct file_lock_context *ctx;
>> >>       LIST_HEAD(dispose);
>> >>
>> >> +     ctx = smp_load_acquire(&inode->i_flctx);
>> >>       if (!ctx) {
>> >>               trace_generic_delete_lease(inode, NULL);
>> >>               return error;
>> >> @@ -2359,13 +2367,14 @@ out:
>> >>  void locks_remove_posix(struct file *filp, fl_owner_t owner)
>> >>  {
>> >>       struct file_lock lock;
>> >> -     struct file_lock_context *ctx = file_inode(filp)->i_flctx;
>> >> +     struct file_lock_context *ctx;
>> >>
>> >>       /*
>> >>        * If there are no locks held on this file, we don't need to call
>> >>        * posix_lock_file().  Another process could be setting a lock on this
>> >>        * file at the same time, but we wouldn't remove that lock anyway.
>> >>        */
>> >> +     ctx =  smp_load_acquire(&file_inode(filp)->i_flctx);
>> >>       if (!ctx || list_empty(&ctx->flc_posix))
>> >>               return;
>> >>
>> >> @@ -2389,7 +2398,7 @@ EXPORT_SYMBOL(locks_remove_posix);
>> >>
>> >>  /* The i_flctx must be valid when calling into here */
>> >>  static void
>> >> -locks_remove_flock(struct file *filp)
>> >> +locks_remove_flock(struct file *filp, struct file_lock_context *flctx)
>> >>  {
>> >>       struct file_lock fl = {
>> >>               .fl_owner = filp,
>> >> @@ -2400,7 +2409,6 @@ locks_remove_flock(struct file *filp)
>> >>               .fl_end = OFFSET_MAX,
>> >>       };
>> >>       struct inode *inode = file_inode(filp);
>> >> -     struct file_lock_context *flctx = inode->i_flctx;
>> >>
>> >>       if (list_empty(&flctx->flc_flock))
>> >>               return;
>> >> @@ -2416,10 +2424,8 @@ locks_remove_flock(struct file *filp)
>> >>
>> >>  /* The i_flctx must be valid when calling into here */
>> >>  static void
>> >> -locks_remove_lease(struct file *filp)
>> >> +locks_remove_lease(struct file *filp, struct file_lock_context *ctx)
>> >>  {
>> >> -     struct inode *inode = file_inode(filp);
>> >> -     struct file_lock_context *ctx = inode->i_flctx;
>> >>       struct file_lock *fl, *tmp;
>> >>       LIST_HEAD(dispose);
>> >>
>> >> @@ -2439,17 +2445,20 @@ locks_remove_lease(struct file *filp)
>> >>   */
>> >>  void locks_remove_file(struct file *filp)
>> >>  {
>> >> -     if (!file_inode(filp)->i_flctx)
>> >> +     struct file_lock_context *ctx;
>> >> +
>> >> +     ctx = smp_load_acquire(&file_inode(filp)->i_flctx);
>> >> +     if (!ctx)
>> >>               return;
>> >>
>> >>       /* remove any OFD locks */
>> >>       locks_remove_posix(filp, filp);
>> >>
>> >>       /* remove flock locks */
>> >> -     locks_remove_flock(filp);
>> >> +     locks_remove_flock(filp, ctx);
>> >>
>> >>       /* remove any leases */
>> >> -     locks_remove_lease(filp);
>> >> +     locks_remove_lease(filp, ctx);
>> >>  }
>> >>
>> >>  /**
>> >> @@ -2616,7 +2625,7 @@ void show_fd_locks(struct seq_file *f,
>> >>       struct file_lock_context *ctx;
>> >>       int id = 0;
>> >>
>> >> -     ctx = inode->i_flctx;
>> >> +     ctx = smp_load_acquire(&inode->i_flctx);
>> >>       if (!ctx)
>> >>               return;
>> >>
>> >
>> >
>> > --
>> > Jeff Layton <jlayton@...chiereds.net>
>>
>>
>>
>
>
> --
> Jeff Layton <jlayton@...chiereds.net>



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