xen: fix UAF in dmabuf_exp_from_pages()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: xen: fix UAF in dmabuf_exp_from_pages() [dma_buf_fd() fixes; no preferences regarding the tree it goes through - up to xen folks] As soon as we'd inserted a file reference into descriptor table, another thread could close it. That's fine for the case when all we are doing is returning that descriptor to userland (it's a race, but it's a userland race and there's nothing the kernel can do about it). However, if we follow fd_install() with any kind of access to objects that would be destroyed on close (be it the struct file itself or anything destroyed by its ->release()), we have a UAF. dma_buf_fd() is a combination of reserving a descriptor and fd_install(). gntdev dmabuf_exp_from_pages() calls it and then proceeds to access the objects destroyed on close - starting with gntdev_dmabuf itself. Fix that by doing reserving descriptor before anything else and do fd_install() only when everything had been set up.

References

• https://git.kernel.org/stable/c/e5907885260401bba300d4d18d79875c05b82651
• https://git.kernel.org/stable/c/3edfd2353f301bfffd5ee41066e37320a59ccc2d
• https://git.kernel.org/stable/c/d59d49af4aeed9a81e673e37c26c6a3bacf1a181
• https://git.kernel.org/stable/c/532c8b51b3a8676cbf533a291f8156774f30ea87