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Subjectkernfs: possible deadlock between of->mutex and mmap_sem
Hi all,

I've stumbled on the following while fuzzing with trinity inside a KVM tools running the latest
-next kernel.

We deal with files that have an mmap op by giving them a different locking class than the files
which don't due to mmap_sem nesting being different for those files.

We assume that for mmap supporting files, of->mutex will be nested inside mm->mmap_sem. However,
this is not always the case. Consider the following:

kernfs_fop_write()
copy_from_user()
might_fault()

might_fault() suggests that we may lock mm->mmap_sem, which causes a reverse lock nesting of
mm->mmap_sem inside of of->mutex.

I'll send a patch to fix it some time next week unless someone beats me to it :)


[ 1182.846501] ======================================================
[ 1182.847256] [ INFO: possible circular locking dependency detected ]
[ 1182.848111] 3.14.0-rc4-next-20140228-sasha-00011-g4077c67-dirty #26 Tainted: G W
[ 1182.849088] -------------------------------------------------------
[ 1182.849927] trinity-c236/10658 is trying to acquire lock:
[ 1182.850094] (&of->mutex#2){+.+.+.}, at: [<fs/kernfs/file.c:487>] kernfs_fop_mmap+0x54/0x120
[ 1182.850094]
[ 1182.850094] but task is already holding lock:
[ 1182.850094] (&mm->mmap_sem){++++++}, at: [<mm/util.c:397>] vm_mmap_pgoff+0x6e/0xe0
[ 1182.850094]
[ 1182.850094] which lock already depends on the new lock.
[ 1182.850094]
[ 1182.850094]
[ 1182.850094] the existing dependency chain (in reverse order) is:
[ 1182.850094]
-> #1 (&mm->mmap_sem){++++++}:
[ 1182.856968] [<kernel/locking/lockdep.c:1945 kernel/locking/lockdep.c:2131>]
validate_chain+0x6c5/0x7b0
[ 1182.856968] [<kernel/locking/lockdep.c:3182>] __lock_acquire+0x4cd/0x5a0
[ 1182.856968] [<arch/x86/include/asm/current.h:14 kernel/locking/lockdep.c:3602>]
lock_acquire+0x182/0x1d0
[ 1182.856968] [<mm/memory.c:4188>] might_fault+0x7e/0xb0
[ 1182.860975] [<arch/x86/include/asm/uaccess.h:713 fs/kernfs/file.c:291>]
kernfs_fop_write+0xd8/0x190
[ 1182.860975] [<fs/read_write.c:473>] vfs_write+0xe3/0x1d0
[ 1182.860975] [<fs/read_write.c:523 fs/read_write.c:515>] SyS_write+0x5d/0xa0
[ 1182.860975] [<arch/x86/kernel/entry_64.S:749>] tracesys+0xdd/0xe2
[ 1182.860975]
-> #0 (&of->mutex#2){+.+.+.}:
[ 1182.860975] [<kernel/locking/lockdep.c:1840>] check_prev_add+0x13f/0x560
[ 1182.860975] [<kernel/locking/lockdep.c:1945 kernel/locking/lockdep.c:2131>]
validate_chain+0x6c5/0x7b0
[ 1182.860975] [<kernel/locking/lockdep.c:3182>] __lock_acquire+0x4cd/0x5a0
[ 1182.860975] [<arch/x86/include/asm/current.h:14 kernel/locking/lockdep.c:3602>]
lock_acquire+0x182/0x1d0
[ 1182.860975] [<kernel/locking/mutex.c:470 kernel/locking/mutex.c:571>]
mutex_lock_nested+0x6a/0x510
[ 1182.860975] [<fs/kernfs/file.c:487>] kernfs_fop_mmap+0x54/0x120
[ 1182.860975] [<mm/mmap.c:1573>] mmap_region+0x310/0x5c0
[ 1182.860975] [<mm/mmap.c:1365>] do_mmap_pgoff+0x385/0x430
[ 1182.860975] [<mm/util.c:399>] vm_mmap_pgoff+0x8f/0xe0
[ 1182.860975] [<mm/mmap.c:1416 mm/mmap.c:1374>] SyS_mmap_pgoff+0x1b0/0x210
[ 1182.860975] [<arch/x86/kernel/sys_x86_64.c:72>] SyS_mmap+0x1d/0x20
[ 1182.860975] [<arch/x86/kernel/entry_64.S:749>] tracesys+0xdd/0xe2
[ 1182.860975]
[ 1182.860975] other info that might help us debug this:
[ 1182.860975]
[ 1182.860975] Possible unsafe locking scenario:
[ 1182.860975]
[ 1182.860975] CPU0 CPU1
[ 1182.860975] ---- ----
[ 1182.860975] lock(&mm->mmap_sem);
[ 1182.860975] lock(&of->mutex#2);
[ 1182.860975] lock(&mm->mmap_sem);
[ 1182.860975] lock(&of->mutex#2);
[ 1182.860975]
[ 1182.860975] *** DEADLOCK ***
[ 1182.860975]
[ 1182.860975] 1 lock held by trinity-c236/10658:
[ 1182.860975] #0: (&mm->mmap_sem){++++++}, at: [<mm/util.c:397>] vm_mmap_pgoff+0x6e/0xe0
[ 1182.860975]
[ 1182.860975] stack backtrace:
[ 1182.860975] CPU: 2 PID: 10658 Comm: trinity-c236 Tainted: G W
3.14.0-rc4-next-20140228-sasha-00011-g4077c67-dirty #26
[ 1182.860975] 0000000000000000 ffff88011911fa48 ffffffff8438e945 0000000000000000
[ 1182.860975] 0000000000000000 ffff88011911fa98 ffffffff811a0109 ffff88011911fab8
[ 1182.860975] ffff88011911fab8 ffff88011911fa98 ffff880119128cc0 ffff880119128cf8
[ 1182.860975] Call Trace:
[ 1182.860975] [<lib/dump_stack.c:52>] dump_stack+0x52/0x7f
[ 1182.860975] [<kernel/locking/lockdep.c:1213>] print_circular_bug+0x129/0x160
[ 1182.860975] [<kernel/locking/lockdep.c:1840>] check_prev_add+0x13f/0x560
[ 1182.860975] [<include/linux/spinlock.h:343 mm/slub.c:1933>] ? deactivate_slab+0x511/0x550
[ 1182.860975] [<kernel/locking/lockdep.c:1945 kernel/locking/lockdep.c:2131>]
validate_chain+0x6c5/0x7b0
[ 1182.860975] [<kernel/locking/lockdep.c:3182>] __lock_acquire+0x4cd/0x5a0
[ 1182.860975] [<mm/mmap.c:1552>] ? mmap_region+0x24a/0x5c0
[ 1182.860975] [<arch/x86/include/asm/current.h:14 kernel/locking/lockdep.c:3602>]
lock_acquire+0x182/0x1d0
[ 1182.860975] [<fs/kernfs/file.c:487>] ? kernfs_fop_mmap+0x54/0x120
[ 1182.860975] [<kernel/locking/mutex.c:470 kernel/locking/mutex.c:571>] mutex_lock_nested+0x6a/0x510
[ 1182.860975] [<fs/kernfs/file.c:487>] ? kernfs_fop_mmap+0x54/0x120
[ 1182.860975] [<kernel/sched/core.c:2477>] ? get_parent_ip+0x11/0x50
[ 1182.860975] [<fs/kernfs/file.c:487>] ? kernfs_fop_mmap+0x54/0x120
[ 1182.860975] [<fs/kernfs/file.c:487>] kernfs_fop_mmap+0x54/0x120
[ 1182.860975] [<mm/mmap.c:1573>] mmap_region+0x310/0x5c0
[ 1182.860975] [<mm/mmap.c:1365>] do_mmap_pgoff+0x385/0x430
[ 1182.860975] [<mm/util.c:397>] ? vm_mmap_pgoff+0x6e/0xe0
[ 1182.860975] [<mm/util.c:399>] vm_mmap_pgoff+0x8f/0xe0
[ 1182.860975] [<kernel/rcu/update.c:97>] ? __rcu_read_unlock+0x44/0xb0
[ 1182.860975] [<fs/file.c:641>] ? dup_fd+0x3c0/0x3c0
[ 1182.860975] [<mm/mmap.c:1416 mm/mmap.c:1374>] SyS_mmap_pgoff+0x1b0/0x210
[ 1182.860975] [<arch/x86/kernel/sys_x86_64.c:72>] SyS_mmap+0x1d/0x20
[ 1182.860975] [<arch/x86/kernel/entry_64.S:749>] tracesys+0xdd/0xe2


Thanks,
Sasha


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