Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix race setting file private on concurrent lseek using same fd

When doing concurrent lseek(2) system calls against the same file descriptor, using multiple threads belonging to the same process, we have a short time window where a race happens and can result in a memory leak.

The race happens like this:

1) A program opens a file descriptor for a file and then spawns two threads (with the pthreads library for example), lets call them task A and task B;

2) Task A calls lseek with SEEK_DATA or SEEK_HOLE and ends up at file.c:find_desired_extent() while holding a read lock on the inode;

3) At the start of find_desired_extent(), it extracts the file's private_data pointer into a local variable named 'private', which has a value of NULL;

4) Task B also calls lseek with SEEK_DATA or SEEK_HOLE, locks the inode in shared mode and enters file.c:find_desired_extent(), where it also extracts file->private_data into its local variable 'private', which has a NULL value;

5) Because it saw a NULL file private, task A allocates a private structure and assigns to the file structure;

6) Task B also saw a NULL file private so it also allocates its own file private and then assigns it to the same file structure, since both tasks are using the same file descriptor.

At this point we leak the private structure allocated by task A.

Besides the memory leak, there's also the detai…