vfs, fdtable: Prevent bounds-check bypass via speculative execution

commit 56c30ba7b348b90484969054d561f711ba196507

'fd' is a user controlled value that is used as a data dependency to
read from the 'fdt->fd' array.  In order to avoid potential leaks of
kernel memory values, block speculative execution of the instruction
stream that could issue reads based on an invalid 'file *' returned from
__fcheck_files.

Co-developed-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: kernel-hardening@lists.openwall.com
Cc: gregkh@linuxfoundation.org
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: torvalds@linux-foundation.org
Cc: alan@linux.intel.com
Link: https://lkml.kernel.org/r/151727418500.33451.17392199002892248656.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index 1c65817673db17f62f668c3760e115d6728b6d21..41615f38bcff3ec5740fa7368bde90933ac746da 100644 (file)
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -10,6 +10,7 @@
 #include <linux/compiler.h>
 #include <linux/spinlock.h>
 #include <linux/rcupdate.h>
+#include <linux/nospec.h>
 #include <linux/types.h>
 #include <linux/init.h>
 #include <linux/fs.h>
@@ -82,8 +83,10 @@ static inline struct file *__fcheck_files(struct files_struct *files, unsigned i
 {
        struct fdtable *fdt = rcu_dereference_raw(files->fdt);
 
-       if (fd < fdt->max_fds)
+       if (fd < fdt->max_fds) {
+               fd = array_index_nospec(fd, fdt->max_fds);
                return rcu_dereference_raw(fdt->fd[fd]);
+       }
        return NULL;
 }