linux.git/fs/namespace.c, branch v4.18 Linux kernel source tree fix __legitimize_mnt()/mntput() race 2018-08-09T21:51:32+00:00 Al Viro viro@zeniv.linux.org.uk 2018-08-09T21:51:32+00:00 119e1ef80ecfe0d1deb6378d4ab41f5b71519de1 __legitimize_mnt() has two problems - one is that in case of success the check of mount_lock is not ordered wrt preceding increment of refcount, making it possible to have successful __legitimize_mnt() on one CPU just before the otherwise final mntpu() on another, with __legitimize_mnt() not seeing mntput() taking the lock and mntput() not seeing the increment done by __legitimize_mnt(). Solved by a pair of barriers. Another is that failure of __legitimize_mnt() on the second read_seqretry() leaves us with reference that'll need to be dropped by caller; however, if that races with final mntput() we can end up with caller dropping rcu_read_lock() and doing mntput() to release that reference - with the first mntput() having freed the damn thing just as rcu_read_lock() had been dropped. Solution: in "do mntput() yourself" failure case grab mount_lock, check if MNT_DOOMED has been set by racing final mntput() that has missed our increment and if it has - undo the increment and treat that as "failure, caller doesn't need to drop anything" case. It's not easy to hit - the final mntput() has to come right after the first read_seqretry() in __legitimize_mnt() *and* manage to miss the increment done by __legitimize_mnt() before the second read_seqretry() in there. The things that are almost impossible to hit on bare hardware are not impossible on SMP KVM, though... Reported-by: Oleg Nesterov <oleg@redhat.com> Fixes: 48a066e72d97 ("RCU'd vsfmounts") Cc: stable@vger.kernel.org Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> 
__legitimize_mnt() has two problems - one is that in case of success
the check of mount_lock is not ordered wrt preceding increment of
refcount, making it possible to have successful __legitimize_mnt()
on one CPU just before the otherwise final mntpu() on another,
with __legitimize_mnt() not seeing mntput() taking the lock and
mntput() not seeing the increment done by __legitimize_mnt().
Solved by a pair of barriers.

Another is that failure of __legitimize_mnt() on the second
read_seqretry() leaves us with reference that'll need to be
dropped by caller; however, if that races with final mntput()
we can end up with caller dropping rcu_read_lock() and doing
mntput() to release that reference - with the first mntput()
having freed the damn thing just as rcu_read_lock() had been
dropped.  Solution: in "do mntput() yourself" failure case
grab mount_lock, check if MNT_DOOMED has been set by racing
final mntput() that has missed our increment and if it has -
undo the increment and treat that as "failure, caller doesn't
need to drop anything" case.

It's not easy to hit - the final mntput() has to come right
after the first read_seqretry() in __legitimize_mnt() *and*
manage to miss the increment done by __legitimize_mnt() before
the second read_seqretry() in there.  The things that are almost
impossible to hit on bare hardware are not impossible on SMP
KVM, though...

Reported-by: Oleg Nesterov <oleg@redhat.com>
Fixes: 48a066e72d97 ("RCU'd vsfmounts")
Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
fix mntput/mntput race 2018-08-09T21:21:17+00:00 Al Viro viro@zeniv.linux.org.uk 2018-08-09T21:21:17+00:00 9ea0a46ca2c318fcc449c1e6b62a7230a17888f1 mntput_no_expire() does the calculation of total refcount under mount_lock; unfortunately, the decrement (as well as all increments) are done outside of it, leading to false positives in the "are we dropping the last reference" test. Consider the following situation: * mnt is a lazy-umounted mount, kept alive by two opened files. One of those files gets closed. Total refcount of mnt is 2. On CPU 42 mntput(mnt) (called from __fput()) drops one reference, decrementing component * After it has looked at component #0, the process on CPU 0 does mntget(), incrementing component #0, gets preempted and gets to run again - on CPU 69. There it does mntput(), which drops the reference (component #69) and proceeds to spin on mount_lock. * On CPU 42 our first mntput() finishes counting. It observes the decrement of component #69, but not the increment of component #0. As the result, the total it gets is not 1 as it should've been - it's 0. At which point we decide that vfsmount needs to be killed and proceed to free it and shut the filesystem down. However, there's still another opened file on that filesystem, with reference to (now freed) vfsmount, etc. and we are screwed. It's not a wide race, but it can be reproduced with artificial slowdown of the mnt_get_count() loop, and it should be easier to hit on SMP KVM setups. Fix consists of moving the refcount decrement under mount_lock; the tricky part is that we want (and can) keep the fast case (i.e. mount that still has non-NULL ->mnt_ns) entirely out of mount_lock. All places that zero mnt->mnt_ns are dropping some reference to mnt and they call synchronize_rcu() before that mntput(). IOW, if mntput() observes (under rcu_read_lock()) a non-NULL ->mnt_ns, it is guaranteed that there is another reference yet to be dropped. Reported-by: Jann Horn <jannh@google.com> Tested-by: Jann Horn <jannh@google.com> Fixes: 48a066e72d97 ("RCU'd vsfmounts") Cc: stable@vger.kernel.org Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> 
mntput_no_expire() does the calculation of total refcount under mount_lock;
unfortunately, the decrement (as well as all increments) are done outside
of it, leading to false positives in the "are we dropping the last reference"
test.  Consider the following situation:
	* mnt is a lazy-umounted mount, kept alive by two opened files.  One
of those files gets closed.  Total refcount of mnt is 2.  On CPU 42
mntput(mnt) (called from __fput()) drops one reference, decrementing component
	* After it has looked at component #0, the process on CPU 0 does
mntget(), incrementing component #0, gets preempted and gets to run again -
on CPU 69.  There it does mntput(), which drops the reference (component #69)
and proceeds to spin on mount_lock.
	* On CPU 42 our first mntput() finishes counting.  It observes the
decrement of component #69, but not the increment of component #0.  As the
result, the total it gets is not 1 as it should've been - it's 0.  At which
point we decide that vfsmount needs to be killed and proceed to free it and
shut the filesystem down.  However, there's still another opened file
on that filesystem, with reference to (now freed) vfsmount, etc. and we are
screwed.

It's not a wide race, but it can be reproduced with artificial slowdown of
the mnt_get_count() loop, and it should be easier to hit on SMP KVM setups.

Fix consists of moving the refcount decrement under mount_lock; the tricky
part is that we want (and can) keep the fast case (i.e. mount that still
has non-NULL ->mnt_ns) entirely out of mount_lock.  All places that zero
mnt->mnt_ns are dropping some reference to mnt and they call synchronize_rcu()
before that mntput().  IOW, if mntput() observes (under rcu_read_lock())
a non-NULL ->mnt_ns, it is guaranteed that there is another reference yet to
be dropped.

Reported-by: Jann Horn <jannh@google.com>
Tested-by: Jann Horn <jannh@google.com>
Fixes: 48a066e72d97 ("RCU'd vsfmounts")
Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
fs: Allow superblock owner to access do_remount_sb() 2018-05-24T17:02:25+00:00 Eric W. Biederman ebiederm@xmission.com 2017-09-18T22:58:08+00:00 bc6155d1326092f4c29fe05a32b614249620d88e Superblock level remounts are currently restricted to global CAP_SYS_ADMIN, as is the path for changing the root mount to read only on umount. Loosen both of these permission checks to also allow CAP_SYS_ADMIN in any namespace which is privileged towards the userns which originally mounted the filesystem. Signed-off-by: Seth Forshee <seth.forshee@canonical.com> Acked-by: "Eric W. Biederman" <ebiederm@xmission.com> Acked-by: Serge Hallyn <serge@hallyn.com> Acked-by: Christian Brauner <christian@brauner.io> Signed-off-by: Eric W. Biederman <ebiederm@xmission.com> 
Superblock level remounts are currently restricted to global
CAP_SYS_ADMIN, as is the path for changing the root mount to
read only on umount. Loosen both of these permission checks to
also allow CAP_SYS_ADMIN in any namespace which is privileged
towards the userns which originally mounted the filesystem.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Serge Hallyn <serge@hallyn.com>
Acked-by: Christian Brauner <christian@brauner.io>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
vfs: Undo an overly zealous MS_RDONLY -> SB_RDONLY conversion 2018-04-20T16:59:33+00:00 David Howells dhowells@redhat.com 2018-04-20T12:35:02+00:00 a9e5b73288cf1595ac2e05cf1acd1924ceea05fa In do_mount() when the MS_* flags are being converted to MNT_* flags, MS_RDONLY got accidentally convered to SB_RDONLY. Undo this change. Fixes: e462ec50cb5f ("VFS: Differentiate mount flags (MS_*) from internal superblock flags") Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> 
In do_mount() when the MS_* flags are being converted to MNT_* flags,
MS_RDONLY got accidentally convered to SB_RDONLY.

Undo this change.

Fixes: e462ec50cb5f ("VFS: Differentiate mount flags (MS_*) from internal superblock flags")
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Don't leak MNT_INTERNAL away from internal mounts 2018-04-20T03:52:15+00:00 Al Viro viro@zeniv.linux.org.uk 2018-04-20T02:03:08+00:00 16a34adb9392b2fe4195267475ab5b472e55292c We want it only for the stuff created by SB_KERNMOUNT mounts, *not* for their copies. As it is, creating a deep stack of bindings of /proc/*/ns/* somewhere in a new namespace and exiting yields a stack overflow. Cc: stable@kernel.org Reported-by: Alexander Aring <aring@mojatatu.com> Bisected-by: Kirill Tkhai <ktkhai@virtuozzo.com> Tested-by: Kirill Tkhai <ktkhai@virtuozzo.com> Tested-by: Alexander Aring <aring@mojatatu.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> 
We want it only for the stuff created by SB_KERNMOUNT mounts, *not* for
their copies.  As it is, creating a deep stack of bindings of /proc/*/ns/*
somewhere in a new namespace and exiting yields a stack overflow.

Cc: stable@kernel.org
Reported-by: Alexander Aring <aring@mojatatu.com>
Bisected-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Tested-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Tested-by: Alexander Aring <aring@mojatatu.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
fs: add ksys_umount() helper; remove in-kernel call to sys_umount() 2018-04-02T18:15:48+00:00 Dominik Brodowski linux@dominikbrodowski.net 2018-03-11T10:34:40+00:00 3a18ef5c1b3935cb05888fc37964321f7bd6231d Using this helper allows us to avoid the in-kernel call to the sys_umount() syscall. The ksys_ prefix denotes that this function is meant as a drop-in replacement for the syscall. In particular, it uses the same calling convention as ksys_umount(). In the near future, the only fs-external caller of ksys_umount() should be converted to call do_umount() directly. Then, ksys_umount() can be moved within sys_umount() again. This patch is part of a series which removes in-kernel calls to syscalls. On this basis, the syscall entry path can be streamlined. For details, see http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net Cc: Alexander Viro <viro@zeniv.linux.org.uk> Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net> 
Using this helper allows us to avoid the in-kernel call to the sys_umount()
syscall. The ksys_ prefix denotes that this function is meant as a drop-in
replacement for the syscall. In particular, it uses the same calling
convention as ksys_umount().

In the near future, the only fs-external caller of ksys_umount() should be
converted to call do_umount() directly. Then, ksys_umount() can be moved
within sys_umount() again.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
fs: add ksys_mount() helper; remove in-kernel calls to sys_mount() 2018-04-02T18:15:48+00:00 Dominik Brodowski linux@dominikbrodowski.net 2018-03-11T10:34:39+00:00 312db1aa1dc7bff133d95c92efcc5e42b57cefa6 Using this helper allows us to avoid the in-kernel calls to the sys_mount() syscall. The ksys_ prefix denotes that this function is meant as a drop-in replacement for the syscall. In particular, it uses the same calling convention as sys_mount(). In the near future, all callers of ksys_mount() should be converted to call do_mount() directly. This patch is part of a series which removes in-kernel calls to syscalls. On this basis, the syscall entry path can be streamlined. For details, see http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net Cc: Alexander Viro <viro@zeniv.linux.org.uk> Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net> 
Using this helper allows us to avoid the in-kernel calls to the sys_mount()
syscall. The ksys_ prefix denotes that this function is meant as a drop-in
replacement for the syscall. In particular, it uses the same calling
convention as sys_mount().

In the near future, all callers of ksys_mount() should be converted to call
do_mount() directly.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
VFS: Handle lazytime in do_mount() 2017-12-10T01:16:33+00:00 Markus Trippelsdorf markus@trippelsdorf.de 2017-10-11T05:01:31+00:00 d7ee946942bdd12394809305e3df05aa4c8b7b8f Since commit e462ec50cb5fa ("VFS: Differentiate mount flags (MS_*) from internal superblock flags") the lazytime mount option doesn't get passed on anymore. Fix the issue by handling the option in do_mount(). Reviewed-by: Lukas Czerner <lczerner@redhat.com> Signed-off-by: Markus Trippelsdorf <markus@trippelsdorf.de> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> 
Since commit e462ec50cb5fa ("VFS: Differentiate mount flags (MS_*) from
internal superblock flags") the lazytime mount option doesn't get passed
on anymore.

Fix the issue by handling the option in do_mount().

Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
locking/atomics: COCCINELLE/treewide: Convert trivial ACCESS_ONCE() patterns to READ_ONCE()/WRITE_ONCE() 2017-10-25T09:01:08+00:00 Mark Rutland mark.rutland@arm.com 2017-10-23T21:07:29+00:00 6aa7de059173a986114ac43b8f50b297a86f09a8 Please do not apply this to mainline directly, instead please re-run the coccinelle script shown below and apply its output. For several reasons, it is desirable to use {READ,WRITE}_ONCE() in preference to ACCESS_ONCE(), and new code is expected to use one of the former. So far, there's been no reason to change most existing uses of ACCESS_ONCE(), as these aren't harmful, and changing them results in churn. However, for some features, the read/write distinction is critical to correct operation. To distinguish these cases, separate read/write accessors must be used. This patch migrates (most) remaining ACCESS_ONCE() instances to {READ,WRITE}_ONCE(), using the following coccinelle script: ---- // Convert trivial ACCESS_ONCE() uses to equivalent READ_ONCE() and // WRITE_ONCE() // $ make coccicheck COCCI=/home/mark/once.cocci SPFLAGS="--include-headers" MODE=patch virtual patch @ depends on patch @ expression E1, E2; @@ - ACCESS_ONCE(E1) = E2 + WRITE_ONCE(E1, E2) @ depends on patch @ expression E; @@ - ACCESS_ONCE(E) + READ_ONCE(E) ---- Signed-off-by: Mark Rutland <mark.rutland@arm.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: davem@davemloft.net Cc: linux-arch@vger.kernel.org Cc: mpe@ellerman.id.au Cc: shuah@kernel.org Cc: snitzer@redhat.com Cc: thor.thayer@linux.intel.com Cc: tj@kernel.org Cc: viro@zeniv.linux.org.uk Cc: will.deacon@arm.com Link: http://lkml.kernel.org/r/1508792849-3115-19-git-send-email-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org> 
Please do not apply this to mainline directly, instead please re-run the
coccinelle script shown below and apply its output.

For several reasons, it is desirable to use {READ,WRITE}_ONCE() in
preference to ACCESS_ONCE(), and new code is expected to use one of the
former. So far, there's been no reason to change most existing uses of
ACCESS_ONCE(), as these aren't harmful, and changing them results in
churn.

However, for some features, the read/write distinction is critical to
correct operation. To distinguish these cases, separate read/write
accessors must be used. This patch migrates (most) remaining
ACCESS_ONCE() instances to {READ,WRITE}_ONCE(), using the following
coccinelle script:

----
// Convert trivial ACCESS_ONCE() uses to equivalent READ_ONCE() and
// WRITE_ONCE()

// $ make coccicheck COCCI=/home/mark/once.cocci SPFLAGS="--include-headers" MODE=patch

virtual patch

@ depends on patch @
expression E1, E2;
@@

- ACCESS_ONCE(E1) = E2
+ WRITE_ONCE(E1, E2)

@ depends on patch @
expression E;
@@

- ACCESS_ONCE(E)
+ READ_ONCE(E)
----

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: davem@davemloft.net
Cc: linux-arch@vger.kernel.org
Cc: mpe@ellerman.id.au
Cc: shuah@kernel.org
Cc: snitzer@redhat.com
Cc: thor.thayer@linux.intel.com
Cc: tj@kernel.org
Cc: viro@zeniv.linux.org.uk
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/1508792849-3115-19-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
vfs: fix mounting a filesystem with i_version 2017-10-17T06:22:07+00:00 Mimi Zohar zohar@linux.vnet.ibm.com 2017-10-08T04:28:21+00:00 917086ff231f614e6705927d8fe3eb6aa74b21bf The mount i_version flag is not enabled in the new sb_flags. This patch adds the missing SB_I_VERSION flag. Fixes: e462ec5 "VFS: Differentiate mount flags (MS_*) from internal superblock flags" Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> 
The mount i_version flag is not enabled in the new sb_flags.  This patch
adds the missing SB_I_VERSION flag.

Fixes: e462ec5 "VFS: Differentiate mount flags (MS_*) from internal
       superblock flags"
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>