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, 61 insertions, 0 deletions

diff --git a/arch/mips/mm/sc-debugfs.c b/arch/mips/mm/sc-debugfs.c
new file mode 100644
index 000000000..80ff39471
--- /dev/null
+++ b/arch/mips/mm/sc-debugfs.c
@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2015 Imagination Technologies
+ * Author: Paul Burton <paul.burton@mips.com>
+ */
+
+#include <asm/bcache.h>
+#include <asm/debug.h>
+#include <linux/uaccess.h>
+#include <linux/debugfs.h>
+#include <linux/init.h>
+
+static ssize_t sc_prefetch_read(struct file *file, char __user *user_buf,
+				size_t count, loff_t *ppos)
+{
+	bool enabled = bc_prefetch_is_enabled();
+	char buf[3];
+
+	buf[0] = enabled ? 'Y' : 'N';
+	buf[1] = '\n';
+	buf[2] = 0;
+
+	return simple_read_from_buffer(user_buf, count, ppos, buf, 2);
+}
+
+static ssize_t sc_prefetch_write(struct file *file,
+				 const char __user *user_buf,
+				 size_t count, loff_t *ppos)
+{
+	bool enabled;
+	int err;
+
+	err = kstrtobool_from_user(user_buf, count, &enabled);
+	if (err)
+		return err;
+
+	if (enabled)
+		bc_prefetch_enable();
+	else
+		bc_prefetch_disable();
+
+	return count;
+}
+
+static const struct file_operations sc_prefetch_fops = {
+	.open = simple_open,
+	.llseek = default_llseek,
+	.read = sc_prefetch_read,
+	.write = sc_prefetch_write,
+};
+
+static int __init sc_debugfs_init(void)
+{
+	struct dentry *dir;
+
+	dir = debugfs_create_dir("l2cache", mips_debugfs_dir);
+	debugfs_create_file("prefetch", S_IRUGO | S_IWUSR, dir, NULL,
+			    &sc_prefetch_fops);
+	return 0;
+}
+late_initcall(sc_debugfs_init);