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, 26 insertions, 0 deletions

diff --git a/drivers/video/nomodeset.c b/drivers/video/nomodeset.c
new file mode 100644
index 000000000..13cc8b719
--- /dev/null
+++ b/drivers/video/nomodeset.c
@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/module.h>
+#include <linux/types.h>
+
+#include <video/nomodeset.h>
+
+static bool video_nomodeset;
+
+bool video_firmware_drivers_only(void)
+{
+	return video_nomodeset;
+}
+EXPORT_SYMBOL(video_firmware_drivers_only);
+
+static int __init disable_modeset(char *str)
+{
+	video_nomodeset = true;
+
+	pr_warn("Booted with the nomodeset parameter. Only the system framebuffer will be available\n");
+
+	return 1;
+}
+
+/* Disable kernel modesetting */
+__setup("nomodeset", disable_modeset);