[PATCH 15/18] 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.

Based on an original patch by Elena Reshetova.

Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 include/linux/fdtable.h |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index 1c65817673db..4a147c5c2533 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -81,9 +81,10 @@ 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]);
+	if ((fdp = nospec_array_ptr(fdt->fd, fd, fdt->max_fds)))
+		return rcu_dereference_raw(*fdp);
 	return NULL;
 }