This bug was erroneously marked for verification in bionic; verification is not required and verification-needed-bionic is being removed.
** Tags removed: verification-needed-bionic ** Tags added: kernel-fixup-verification-needed-bionic verification-done-bionic -- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux-azure in Ubuntu. https://bugs.launchpad.net/bugs/1765564 Title: fsnotify: Fix fsnotify_mark_connector race Status in Linux: Incomplete Status in linux package in Ubuntu: Fix Released Status in linux-azure package in Ubuntu: Fix Released Status in linux source package in Xenial: Invalid Status in linux-azure source package in Xenial: Fix Released Status in linux source package in Artful: Fix Released Status in linux-azure source package in Artful: Invalid Status in linux source package in Bionic: Fix Released Status in linux-azure source package in Bionic: Fix Released Bug description: On Azure we have had sporadic cases of soft lockups in fsnotify that may very well be mitigated by the following fix. The LKML thread is "kernel panics with 4.14.X". This should be applied to 4.13 and 4.15 versions of the linux-azure kernel, and possibly the 4.15 generic kernel in bionic as well. ----- fsnotify() acquires a reference to a fsnotify_mark_connector through the SRCU-protected pointer to_tell->i_fsnotify_marks. However, it appears that no precautions are taken in fsnotify_put_mark() to ensure that fsnotify() drops its reference to this fsnotify_mark_connector before assigning a value to its 'destroy_next' field. This can result in fsnotify_put_mark() assigning a value to a connector's 'destroy_next' field right before fsnotify() tries to traverse the linked list referenced by the connector's 'list' field. Since these two fields are members of the same union, this behavior results in a kernel panic. This issue is resolved by moving the connector's 'destroy_next' field into the object pointer union. This should work since the object pointer access is protected by both a spinlock and the value of the 'flags' field, and the 'flags' field is cleared while holding the spinlock in fsnotify_put_mark() before 'destroy_next' is updated. It shouldn't be possible for another thread to accidentally read from the object pointer after the 'destroy_next' field is updated. The offending behavior here is extremely unlikely; since fsnotify_put_mark() removes references to a connector (specifically, it ensures that the connector is unreachable from the inode it was formerly attached to) before updating its 'destroy_next' field, a sizeable chunk of code in fsnotify_put_mark() has to execute in the short window between when fsnotify() acquires the connector reference and saves the value of its 'list' field. On the HEAD kernel, I've only been able to reproduce this by inserting a udelay(1) in fsnotify(). However, I've been able to reproduce this issue without inserting a udelay(1) anywhere on older unmodified release kernels, so I believe it's worth fixing at HEAD. References: https://bugzilla.kernel.org/show_bug.cgi?id=199437 Fixes: 08991e83b7286635167bab40927665a90fb00d81 CC: sta...@vger.kernel.org Signed-off-by: Robert Kolchmeyer <rkolchme...@google.com> Signed-off-by: Jan Kara <j...@suse.cz> To manage notifications about this bug go to: https://bugs.launchpad.net/linux/+bug/1765564/+subscriptions -- Mailing list: https://launchpad.net/~kernel-packages Post to : email@example.com Unsubscribe : https://launchpad.net/~kernel-packages More help : https://help.launchpad.net/ListHelp