protect the fetch of ->fd[fd] in do_dup2() from mispredictions

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: protect the fetch of ->fd[fd] in do_dup2() from mispredictions both callers have verified that fd is not greater than ->max_fds; however, misprediction might end up with tofree = fdt->fd[fd]; being speculatively executed. That's wrong for the same reasons why it's wrong in close_fd()/file_close_fd_locked(); the same solution applies - array_index_nospec(fd, fdt->max_fds) could differ from fd only in case of speculative execution on mispredicted path.

References

• https://git.kernel.org/stable/c/ed42e8ff509d2a61c6642d1825032072dab79f26
• https://git.kernel.org/stable/c/41a6c31df77bd8e050136b0a200b537da9e1084a
• https://git.kernel.org/stable/c/08775b3d6ed117cf4518754ec7300ee42b6a5368
• https://git.kernel.org/stable/c/3f480493550b6a23d3a65d095d6569d4a7f56a0f
• https://git.kernel.org/stable/c/5db999fff545b924b24c9afd368ef5c17279b176
• https://git.kernel.org/stable/c/da72e783afd27d9f487836b2e6738146c0edd149
• https://git.kernel.org/stable/c/1171ceccabfd596ca370c5d2cbb47d110c3f2fe1
• https://git.kernel.org/stable/c/8aa37bde1a7b645816cda8b80df4753ecf172bf1