[PATCH 4.19 126/196] protect the fetch of ->fd[fd] in do_dup2() from mispredictions

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4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

commit 8aa37bde1a7b645816cda8b80df4753ecf172bf1 upstream.

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@xxxxxxxxxxxxxxx
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/file.c |    1 +
 1 file changed, 1 insertion(+)

--- a/fs/file.c
+++ b/fs/file.c
@@ -879,6 +879,7 @@ __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;






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