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.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/file.c b/fs/file.c
index a3b72aa64f1164380fdaa6524081a4ac956831da..a11e59b5d6026a2e2b195815582f3b48e5b4e331 100644 (file)
--- a/fs/file.c
+++ b/fs/file.c
@@ -1248,6 +1248,7 @@ __releases(&files->file_lock)
         * tables and this condition does not arise without those.
         */
        fdt = files_fdtable(files);
+       fd = array_index_nospec(fd, fdt->max_fds);
        tofree = fdt->fd[fd];
        if (!tofree && fd_is_open(fd, fdt))
                goto Ebusy;