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, 46 insertions, 0 deletions

diff --git a/arch/hexagon/lib/modsi3.S b/arch/hexagon/lib/modsi3.S
new file mode 100644
index 000000000..9ea1c86ef
--- /dev/null
+++ b/arch/hexagon/lib/modsi3.S
@@ -0,0 +1,46 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (c) 2021, The Linux Foundation. All rights reserved.
+ */
+
+#include <linux/linkage.h>
+
+SYM_FUNC_START(__hexagon_modsi3)
+        {
+                p2 = cmp.ge(r0,#0)
+                r2 = abs(r0)
+                r1 = abs(r1)
+        }
+        {
+                r3 = cl0(r2)
+                r4 = cl0(r1)
+                p0 = cmp.gtu(r1,r2)
+        }
+        {
+                r3 = sub(r4,r3)
+                if (p0) jumpr r31
+        }
+        {
+                p1 = cmp.eq(r3,#0)
+                loop0(1f,r3)
+                r0 = r2
+                r2 = lsl(r1,r3)
+        }
+        .falign
+1:
+        {
+                p0 = cmp.gtu(r2,r0)
+                if (!p0.new) r0 = sub(r0,r2)
+                r2 = lsr(r2,#1)
+                if (p1) r1 = #0
+        }:endloop0
+        {
+                p0 = cmp.gtu(r2,r0)
+                if (!p0.new) r0 = sub(r0,r1)
+                if (p2) jumpr r31
+        }
+        {
+                r0 = neg(r0)
+                jumpr r31
+        }
+SYM_FUNC_END(__hexagon_modsi3)