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commitd90a10e244upstream. 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:08991e83b7CC: stable@vger.kernel.org Signed-off-by: Robert Kolchmeyer <rkolchmeyer@google.com> Signed-off-by: Jan Kara <jack@suse.cz> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.