epoll: be better about file lifetimes

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: epoll: be better about file lifetimes epoll can call out to vfs_poll() with a file pointer that may race with the last 'fput()'. That would make f_count go down to zero, and while the ep->mtx locking means that the resulting file pointer tear-down will be blocked until the poll returns, it means that f_count is already dead, and any use of it won't actually get a reference to the file any more: it's dead regardless. Make sure we have a valid ref on the file pointer before we call down to vfs_poll() from the epoll routines.

References

• https://git.kernel.org/stable/c/cbfd1088e24ec4c1199756a37cb8e4cd0a4b016e
• https://git.kernel.org/stable/c/559214eb4e5c3d05e69428af2fae2691ba1eb784
• https://git.kernel.org/stable/c/4f65f4defe4e23659275ce5153541cd4f76ce2d2
• https://git.kernel.org/stable/c/16e3182f6322575eb7c12e728ad3c7986a189d5d
• https://git.kernel.org/stable/c/4efaa5acf0a1d2b5947f98abb3acf8bfd966422b