Re: [syzbot] KASAN: use-after-free Read in srcu_invoke_callbacks

Paul E. McKenney
2022-01-14T15:27:55Z

On Fri, Jan 14, 2022 at 10:38:42PM +0800, Boqun Feng wrote:
> Hi,
>
> On Tue, Jan 11, 2022 at 11:05:00AM -0800, Paul E. McKenney wrote:
> [...]
> > > > The buggy address belongs to the object at ffff8880189b5c70
> > > > which belongs to the cache request_queue_srcu of size 3816
>
> This cache name drew my attention when I was trying to look into this,
> because I couldn't find it in v5.16, later on I realized the UAF was
> found in linux-next and the commit introducing the cache was merged into
> mainline if 5.17 merge window:
>
> 704b914f15fb blk-mq: move srcu from blk_mq_hw_ctx to request_queue
>
> I think the UAF is actually a bug introduced by the commit, because in
> that commit srcu structure was moved from blk_mq_hw_ctx to
> request_queue, and therefore the cleanup_srcu_struct() should be moved
> from blk_mq_hw_sysfs_release() to blk_release_queue(), however the above
> commit only deleted the one in blk_mq_hw_sysfs_release() but didn't add
> a new one in blk_release_queue(). As a result when a request queue is
> freed, the srcu structure is not fully clean up, therefore the UAF.
>
> IOW, something like below (untested) should fix this. Copy the auther
> and block maintainers.

One question for the author and block maintainers... Why not simply have
a single srcu_struct for all of the queues? Or is there some situation
where you need one queue's reader to avoid blocking other queues' SRCU
grace periods?

Thanx, Paul

> Regards,
> Boqun
>
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index e20eadfcf5c8..10f59416376e 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -812,6 +812,10 @@ static void blk_release_queue(struct kobject *kobj)
> bioset_exit(&q->bio_split);
>
> ida_simple_remove(&blk_queue_ida, q->id);
> +
> + if (test_bit(QUEUE_FLAG_HAS_SRCU, &q->queue_flags)) {
> + cleanup_srcu_struct(q->srcu);
> + }
> call_rcu(&q->rcu_head, blk_free_queue_rcu);
> }
>
>
> > > > The buggy address is located 3320 bytes inside of
> > > > 3816-byte region [ffff8880189b5c70, ffff8880189b6b58)
> > > > The buggy address belongs to the page:
> > > > page:ffffea0000626c00 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x189b0
> > > > head:ffffea0000626c00 order:3 compound_mapcount:0 compound_pincount:0
> > > > flags: 0xfff00000010200(slab|head|node=0|zone=1|lastcpupid=0x7ff)
> > > > raw: 00fff00000010200 0000000000000000 dead000000000001 ffff888012787140
> > > > raw: 0000000000000000 0000000080080008 00000001ffffffff 0000000000000000
> > > > page dumped because: kasan: bad access detected
> > > > page_owner tracks the page as allocated
> > > > page last allocated via order 3, migratetype Unmovable, gfp_mask 0x1d20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC|__GFP_HARDWALL), pid 14963, ts 196854875504, free_ts 196839673140
> > > > prep_new_page mm/page_alloc.c:2434 [inline]
> > > > get_page_from_freelist+0xa72/0x2f40 mm/page_alloc.c:4165
> > > > __alloc_pages+0x1b2/0x500 mm/page_alloc.c:5381
> > > > alloc_pages+0x1aa/0x310 mm/mempolicy.c:2271
> > > > alloc_slab_page mm/slub.c:1799 [inline]
> > > > allocate_slab mm/slub.c:1944 [inline]
> > > > new_slab+0x28d/0x380 mm/slub.c:2004
> > > > ___slab_alloc+0x6be/0xd60 mm/slub.c:3018
> > > > __slab_alloc.constprop.0+0x4d/0xa0 mm/slub.c:3105
> > > > slab_alloc_node mm/slub.c:3196 [inline]
> > > > kmem_cache_alloc_node+0x122/0x3e0 mm/slub.c:3266
> >
> > Again blk_alloc_queue().
> >
> > > > blk_alloc_queue+0x5ad/0x870 block/blk-core.c:446
> > > > blk_mq_init_queue_data block/blk-mq.c:3878 [inline]
> > > > __blk_mq_alloc_disk+0x8c/0x1c0 block/blk-mq.c:3902
> > > > nbd_dev_add+0x3b2/0xcd0 drivers/block/nbd.c:1765
> > > > nbd_genl_connect+0x11f3/0x1930 drivers/block/nbd.c:1948
> > > > genl_family_rcv_msg_doit+0x228/0x320 net/netlink/genetlink.c:731
> > > > genl_family_rcv_msg net/netlink/genetlink.c:775 [inline]
> > > > genl_rcv_msg+0x328/0x580 net/netlink/genetlink.c:792
> > > > netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2494
> > > > genl_rcv+0x24/0x40 net/netlink/genetlink.c:803
> > > > netlink_unicast_kernel net/netlink/af_netlink.c:1317 [inline]
> > > > netlink_unicast+0x533/0x7d0 net/netlink/af_netlink.c:1343
> > > > page last free stack trace:
> > > > reset_page_owner include/linux/page_owner.h:24 [inline]
> > > > free_pages_prepare mm/page_alloc.c:1352 [inline]
> > > > free_pcp_prepare+0x414/0xb60 mm/page_alloc.c:1404
> > > > free_unref_page_prepare mm/page_alloc.c:3325 [inline]
> > > > free_unref_page+0x19/0x690 mm/page_alloc.c:3404
> > > > __unfreeze_partials+0x17c/0x1a0 mm/slub.c:2536
> > > > qlink_free mm/kasan/quarantine.c:157 [inline]
> > > > qlist_free_all+0x6a/0x170 mm/kasan/quarantine.c:176
> > > > kasan_quarantine_reduce+0x180/0x200 mm/kasan/quarantine.c:283
> > > > __kasan_slab_alloc+0xa2/0xc0 mm/kasan/common.c:446
> > > > kasan_slab_alloc include/linux/kasan.h:260 [inline]
> > > > slab_post_alloc_hook mm/slab.h:738 [inline]
> > > > slab_alloc_node mm/slub.c:3230 [inline]
> > > > slab_alloc mm/slub.c:3238 [inline]
> > > > __kmalloc+0x1e7/0x340 mm/slub.c:4420
> > > > kmalloc include/linux/slab.h:586 [inline]
> > > > tomoyo_realpath_from_path+0xc3/0x620 security/tomoyo/realpath.c:254
> > > > tomoyo_get_realpath security/tomoyo/file.c:151 [inline]
> > > > tomoyo_path_perm+0x21b/0x400 security/tomoyo/file.c:822
> > > > security_inode_getattr+0xcf/0x140 security/security.c:1326
> > > > vfs_getattr fs/stat.c:157 [inline]
> > > > vfs_statx+0x164/0x390 fs/stat.c:225
> > > > vfs_fstatat fs/stat.c:243 [inline]
> > > > __do_sys_newfstatat+0x96/0x120 fs/stat.c:412
> > > > do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > > > do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> > > > entry_SYSCALL_64_after_hwframe+0x44/0xae
> > > >
> > > > Memory state around the buggy address:
> > > > ffff8880189b6800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > > > ffff8880189b6880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > > > >ffff8880189b6900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > > > ^
> > > > ffff8880189b6980: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > > > ffff8880189b6a00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > > > ==================================================================
> > >
> > > To fix the uaf, add completion to sdp and wait for sdp->work to complete
> > > even after work flush while cleaning ssp up.
> >
> > Good catch!!!
> >
> > First, restating the usage rules to help explain the failure scenario:
> >
> > ------------------------------------------------------------------------
> >
> > You are supposed to wait until all SRCU callbacks for a given srcu_struct
> > have been invoked before invoking cleanup_srcu_struct(). There is an
> > srcu_barrier() to help with this, but for it to do you any good you have
> > to have made sure that you stopped calling call_srcu() before invoking
> > srcu_barrier().
> >
> > Failing to follow this rule, that is, invoking cleanup_srcu_struct()
> > before all the corresponding SRCU callbacks have been invoked, could
> > indeed result in the mess above, and much worse.
> >
> > There are checks in cleanup_srcu_struct() for this sort of thing,
> > namely:
> >
> > if (WARN_ON(rcu_segcblist_n_cbs(&sdp->srcu_cblist)))
> >
> > However, such checks cannot be perfect. For example, the
> > cleanup_srcu_struct() might be invoked just a bit too early,
> > after the last callback was invoked (hence the above WARN_ON() not
> > triggering) and just before the call to rcu_seq_snap() at the end of
> > srcu_invoke_callbacks() (hence KASAN being unhappy with that call to
> > srcu_seq_snap()).
> >
> > So:
> >
> > 1. Make sure that you stop invoking call_srcu() before starting the
> > process of freeing that srcu_struct. In addition, you of course
> > need to make sure that you have stopped passing the srcu_struct
> > in question to anything else.
> >
> > 2. Once all CPUs agree that you have stopped invoking call_srcu(),
> > invoke srcu_barrier() to ensure that all callbacks have been
> > invoked. Keep in mind that srcu_barrier() is only guaranteed
> > to wait for callbacks queued by calls to call_srcu() that happen
> > -before- that call to srcu_barrier(). This is one of those cases
> > where being almost correct has nothing to do with being correct.
> >
> > 3. Then, and only then, is it safe to invoke cleanup_srcu_struct().
> >
> > Of course, if you never use call_srcu() on a given srcu_struct,
> > you don't need to invoke srcu_barrier().
> >
> > ------------------------------------------------------------------------
> >
> > Except that srcu_invoke_callbacks() accesses the various srcu_struct
> > fields after the last rcu_barrier() callback was invoked. Which
> > means that the following sequence of events can happen:
> >
> > o Task A invokes the last call_srcu().
> >
> > o Later, Task B invokes rcu_barrier().
> >
> > o The last rcu_barrier() callback is invoked by CPU 0 from
> > within srcu_invoke_callbacks(), which awakens Task B.
> >
> > o Task B invokes cleanup_srcu_struct(), which is perfectly legal
> > given that Task B followed the rules called out above.
> >
> > o But now CPU 0 continues executing srcu_invoke_callbacks(),
> > which accesses srcu_struct fields and related structures
> > that were freed by cleanup_srcu_struct().
> >
> > As both you and syzbot say, use after free bug.
> >
> > > Only for thoughts now.
> > >
> > > Hillf
> > >
> > > +++ y/kernel/rcu/srcutree.c
> > > @@ -74,6 +74,12 @@ do { \
> > > #define spin_unlock_irqrestore_rcu_node(p, flags) \
> > > spin_unlock_irqrestore(&ACCESS_PRIVATE(p, lock), flags) \
> > >
> > > +static inline void init_completion_done(struct completion *x)
> > > +{
> > > + x->done = 1;
> > > + init_swait_queue_head(&x->wait);
> >
> > This is small enough that it should be inlined.
> >
> > But it turns out that we don't really need it.
> >
> > > +}
> > > +
> > > /*
> > > * Initialize SRCU combining tree. Note that statically allocated
> > > * srcu_struct structures might already have srcu_read_lock() and
> > > @@ -150,6 +156,7 @@ static void init_srcu_struct_nodes(struc
> > > INIT_WORK(&sdp->work, srcu_invoke_callbacks);
> > > timer_setup(&sdp->delay_work, srcu_delay_timer, 0);
> > > sdp->ssp = ssp;
> > > + init_completion_done(&sdp->work_done);
> > > sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);
> > > }
> > > }
> > > @@ -378,6 +385,7 @@ void cleanup_srcu_struct(struct srcu_str
> > >
> > > del_timer_sync(&sdp->delay_work);
> > > flush_work(&sdp->work);
> > > + wait_for_completion(&sdp->work_done);
> >
> > With the changes described below, this happens once rather than once
> > per CPU. The trick is to make use of the ordering that is already
> > provided by the atomic_dec_and_test(&ssp->srcu_barrier_cpu_cnt) in the
> > rcu_barrier() function's callback function, that is, srcu_barrier_cb(),
> > but by moving this to the end of srcu_barrier().
> >
> >
> > > if (WARN_ON(rcu_segcblist_n_cbs(&sdp->srcu_cblist)))
> > > return; /* Forgot srcu_barrier(), so just leak it! */
> > > }
> > > @@ -1248,6 +1256,7 @@ static void srcu_invoke_callbacks(struct
> > > struct srcu_struct *ssp;
> > >
> > > sdp = container_of(work, struct srcu_data, work);
> > > + reinit_completion(&sdp->work_done);
> >
> > We can avoid this by reusing the existing ->srcu_barrier_completion.
> >
> > > ssp = sdp->ssp;
> > > rcu_cblist_init(&ready_cbs);
> > > @@ -1257,7 +1266,7 @@ static void srcu_invoke_callbacks(struct
> > > if (sdp->srcu_cblist_invoking ||
> > > !rcu_segcblist_ready_cbs(&sdp->srcu_cblist)) {
> > > spin_unlock_irq_rcu_node(sdp);
> > > - return; /* Someone else on the job or nothing to do. */
> > > + goto out; /* Someone else on the job or nothing to do. */
> > > }
> > >
> > > /* We are on the job! Extract and invoke ready callbacks. */
> > > @@ -1287,6 +1296,9 @@ static void srcu_invoke_callbacks(struct
> > > spin_unlock_irq_rcu_node(sdp);
> > > if (more)
> > > srcu_schedule_cbs_sdp(sdp, 0);
> > > + else
> > > +out:
> > > + complete(&sdp->work_done);
> >
> > This is too ugly to live. ;-)
> >
> > And it should also be executed in srcu_barrier(). This can be done using
> > a flag that is cleared early on in rcu_barrier() and then set by the last
> > rcu_barrier() callback (the same one that does the complete() invocation).
> >
> > Plus we really do want to execute complete() in the case where there are
> > callbacks in order to increase the probability of cleanup_srcu_struct()
> > finding SRCU-abuse bugs.
> >
> > > }
> > >
> > > /*
> > > +++ y/include/linux/srcutree.h
> > > @@ -40,6 +40,7 @@ struct srcu_data {
> > > /* ->srcu_data_have_cbs[]. */
> > > int cpu;
> > > struct srcu_struct *ssp;
> > > + struct completion work_done;
> >
> > All we really need to add here is a flag that allows srcu_barrier_cb()
> > to signal srcu_invoke_callbacks().
> >
> > OK, I didn't do a really good job of describing what I am after, so here
> > is a patch with your Reported-by and that of syzbot.
> >
> > Thoughts?
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit b01a1d25f93633c89f782ec5d3830beeb3892030
> > Author: Paul E. McKenney <paulmck@kernel.org>
> > Date: Tue Jan 11 10:30:07 2022 -0800
> >
> > srcu: Prevent srcu_barrier()->cleanup_srcu_struct() use-after-free
> >
> > Currently, srcu_barrier() will return immediately after its last
> > srcu_barrier_cb() has been invoked, thus guaranteeing that all previous
> > callbacks have been completely invoked, as required. Unfortunately,
> > this is insufficient in the case where the call to srcu_barrier()
> > is immediately followed by a call to cleanup_srcu_struct(). In this
> > case, srcu_invoke_callbacks() will access the srcu_struct and related
> > structures after invoking the callbacks, and these structures might have
> > already have been freed by cleanup_srcu_struct().
> >
> > This can happen as follows:
> >
> > o Task A invokes the last call_srcu().
> >
> > o Later, Task B invokes rcu_barrier().
> >
> > o The last rcu_barrier() callback is invoked by CPU 0 from
> > within srcu_invoke_callbacks(), which awakens Task B.
> >
> > o Task B invokes cleanup_srcu_struct(), which is perfectly legal
> > assuming that Task B waited until the last call_srcu()
> > completed before invoking rcu_barrier().
> >
> > o But now CPU 0 continues executing srcu_invoke_callbacks(),
> > which accesses srcu_struct fields and related structures
> > that were freed by cleanup_srcu_struct().
> >
> > This commit fixes this by moving the complete() call that awakens
> > srcu_barrier() from the the srcu_barrier_cb() callback to the end of
> > srcu_invoke_callbacks(). This requires introducing a per-srcu_data
> > structure ->srcu_barrier_cb_done flag that allows srcu_barrier_cb()
> > to tell srcu_invoke_callbacks() that a wakeup might be required.
> >
> > Link: https://lore.kernel.org/all/00000000000081b56205d54c6667@google.com/
> > Reported-by: syzbot+4f789823c1abc5accf13@syzkaller.appspotmail.com
> > Reported-by: Hillf Danton <hdanton@sina.com>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> >
> > diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> > index cb1f4351e8baa..339dd55fbea5c 100644
> > --- a/include/linux/srcutree.h
> > +++ b/include/linux/srcutree.h
> > @@ -32,6 +32,7 @@ struct srcu_data {
> > unsigned long srcu_gp_seq_needed; /* Furthest future GP needed. */
> > unsigned long srcu_gp_seq_needed_exp; /* Furthest future exp GP. */
> > bool srcu_cblist_invoking; /* Invoking these CBs? */
> > + bool srcu_barrier_cb_done; /* This CPU's srcu_rcu_barrier_cb() done? */
> > struct timer_list delay_work; /* Delay for CB invoking */
> > struct work_struct work; /* Context for CB invoking. */
> > struct rcu_head srcu_barrier_head; /* For srcu_barrier() use. */
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 6833d88871816..128034f858897 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -1088,13 +1088,9 @@ EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
> > */
> > static void srcu_barrier_cb(struct rcu_head *rhp)
> > {
> > - struct srcu_data *sdp;
> > - struct srcu_struct *ssp;
> > + struct srcu_data *sdp = container_of(rhp, struct srcu_data, srcu_barrier_head);
> >
> > - sdp = container_of(rhp, struct srcu_data, srcu_barrier_head);
> > - ssp = sdp->ssp;
> > - if (atomic_dec_and_test(&ssp->srcu_barrier_cpu_cnt))
> > - complete(&ssp->srcu_barrier_completion);
> > + sdp->srcu_barrier_cb_done = true;
> > }
> >
> > /**
> > @@ -1287,6 +1283,15 @@ static void srcu_invoke_callbacks(struct work_struct *work)
> > spin_unlock_irq_rcu_node(sdp);
> > if (more)
> > srcu_schedule_cbs_sdp(sdp, 0);
> > +
> > + // Do complete() to wake up srcu_barrier() here to ensure no
> > + // use-after-free should cleanup_srcu_struct() be invoked immediately
> > + // after the return from srcu_barrier().
> > + if (sdp->srcu_barrier_cb_done) {
> > + sdp->srcu_barrier_cb_done = false;
> > + if (atomic_dec_and_test(&ssp->srcu_barrier_cpu_cnt))
> > + complete(&ssp->srcu_barrier_completion);
> > + }
> > }
> >
> > /*