linux-kernel - [PATCH v6 10/13] vfs, fdtable: prevent bounds-check bypass via speculative execution

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Message-ID: <151727418500.33451.17392199002892248656.stgit@dwillia2-desk3.amr.corp.intel.com>
Date:   Mon, 29 Jan 2018 17:03:05 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     tglx@...utronix.de, mingo@...nel.org
Cc:     linux-arch@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        torvalds@...ux-foundation.org, alan@...ux.intel.com
Subject: [PATCH v6 10/13] vfs,
 fdtable: prevent bounds-check bypass via speculative execution

'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 <viro@...iv.linux.org.uk>
Co-developed-by: Elena Reshetova <elena.reshetova@...el.com>
Signed-off-by: Dan Williams <dan.j.williams@...el.com>
---
 include/linux/fdtable.h |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index 1c65817673db..41615f38bcff 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>
@@ -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;
 }