openrisc: fix arbitrary kernel memory access via or1k_atomic syscallgrafted

sys_or1k_atomic() (syscall 244 in the "or1k" ABI) takes two user
pointers, v1 and v2, and swaps the words they point to in hand-written
assembly.

    l.lwz   r29,0(r4)
    l.lwz   r27,0(r5)
    l.sw    0(r4),r27
    l.sw    0(r5),r29

The pointers are not checked with access_ok(). The four memory
accesses also have no exception table entries.

A caller passes a kernel address as either pointer, and the syscall
reads from and writes to it directly.

This gives an unprivileged process a kernel read/write primitive. It
overwrites kernel data such as the sys_call_table, gaining code
execution in kernel context.

Check both pointers before entering the critical section. Add fixups
for the four memory accesses so faults on valid but unmapped user
addresses return -EFAULT.

[shorne@gmail.com: fix comment style]
Fixes: 9d02a4283e9c ("OpenRISC: Boot code")
Cc: stable@vger.kernel.org
Signed-off-by: Ali Ahmet Memis <ali@iusegentoo.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>

1 files changed, 70 insertions, 0 deletions

diff --git a/fs/nullfs.c b/fs/nullfs.c
new file mode 100644
index 000000000..fdbd3e5d3
--- /dev/null
+++ b/fs/nullfs.c
@@ -0,0 +1,70 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2026 Christian Brauner <brauner@kernel.org> */
+#include <linux/fs/super_types.h>
+#include <linux/fs_context.h>
+#include <linux/magic.h>
+
+static const struct super_operations nullfs_super_operations = {
+	.statfs	= simple_statfs,
+};
+
+static int nullfs_fs_fill_super(struct super_block *s, struct fs_context *fc)
+{
+	struct inode *inode;
+
+	s->s_maxbytes		= MAX_LFS_FILESIZE;
+	s->s_blocksize		= PAGE_SIZE;
+	s->s_blocksize_bits	= PAGE_SHIFT;
+	s->s_magic		= NULL_FS_MAGIC;
+	s->s_op			= &nullfs_super_operations;
+	s->s_export_op		= NULL;
+	s->s_xattr		= NULL;
+	s->s_time_gran		= 1;
+	s->s_d_flags		= 0;
+
+	inode = new_inode(s);
+	if (!inode)
+		return -ENOMEM;
+
+	/* nullfs is permanently empty... */
+	make_empty_dir_inode(inode);
+	simple_inode_init_ts(inode);
+	inode->i_ino	= 1;
+	/* ... and immutable. */
+	inode->i_flags |= S_IMMUTABLE;
+
+	s->s_root = d_make_root(inode);
+	if (!s->s_root)
+		return -ENOMEM;
+
+	return 0;
+}
+
+/*
+ * For now this is a single global instance. If needed we can make it
+ * mountable by userspace at which point we will need to make it
+ * multi-instance.
+ */
+static int nullfs_fs_get_tree(struct fs_context *fc)
+{
+	return get_tree_single(fc, nullfs_fs_fill_super);
+}
+
+static const struct fs_context_operations nullfs_fs_context_ops = {
+	.get_tree	= nullfs_fs_get_tree,
+};
+
+static int nullfs_init_fs_context(struct fs_context *fc)
+{
+	fc->ops		= &nullfs_fs_context_ops;
+	fc->global	= true;
+	fc->sb_flags	= SB_NOUSER;
+	fc->s_iflags	= SB_I_NOEXEC | SB_I_NODEV;
+	return 0;
+}
+
+struct file_system_type nullfs_fs_type = {
+	.name			= "nullfs",
+	.init_fs_context	= nullfs_init_fs_context,
+	.kill_sb		= kill_anon_super,
+};