[PATCH v3 9/9] vfs, fdtable: prevent bounds-check bypass via speculative execution

[Dan Williams]

Expectedly, static analysis reports that '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.

Cc: Al Viro <v...@zeniv.linux.org.uk>
Co-developed-by: Elena Reshetova <elena.reshet...@intel.com>
Signed-off-by: Dan Williams <dan.j.willi...@intel.com>
---
 include/linux/fdtable.h |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index 1c65817673db..9731f1a255db 100644
--- 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>
@@ -81,9 +82,11 @@ struct dentry;
 static inline struct file *__fcheck_files(struct files_struct *files, unsigned 
int fd)
 {
        struct fdtable *fdt = rcu_dereference_raw(files->fdt);
+       struct file __rcu **fdp;
 
-       if (fd < fdt->max_fds)
-               return rcu_dereference_raw(fdt->fd[fd]);
+       fdp = array_ptr(fdt->fd, fd, fdt->max_fds);
+       if (fdp)
+               return rcu_dereference_raw(*fdp);
        return NULL;
 }