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, 43 insertions, 0 deletions

diff --git a/arch/powerpc/lib/string.S b/arch/powerpc/lib/string.S
new file mode 100644
index 000000000..c3f5e7d31
--- /dev/null
+++ b/arch/powerpc/lib/string.S
@@ -0,0 +1,43 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * String handling functions for PowerPC.
+ *
+ * Copyright (C) 1996 Paul Mackerras.
+ */
+#include <linux/export.h>
+#include <asm/ppc_asm.h>
+#include <asm/cache.h>
+
+	.text
+	
+_GLOBAL(strncmp)
+	PPC_LCMPI 0,r5,0
+	beq-	2f
+	mtctr	r5
+	addi	r5,r3,-1
+	addi	r4,r4,-1
+	.balign IFETCH_ALIGN_BYTES
+1:	lbzu	r3,1(r5)
+	cmpwi	1,r3,0
+	lbzu	r0,1(r4)
+	subf.	r3,r0,r3
+	beqlr	1
+	bdnzt	eq,1b
+	blr
+2:	li	r3,0
+	blr
+EXPORT_SYMBOL(strncmp)
+
+_GLOBAL(memchr)
+	PPC_LCMPI 0,r5,0
+	beq-	2f
+	mtctr	r5
+	addi	r3,r3,-1
+	.balign IFETCH_ALIGN_BYTES
+1:	lbzu	r0,1(r3)
+	cmpw	0,r0,r4
+	bdnzf	2,1b
+	beqlr
+2:	li	r3,0
+	blr
+EXPORT_SYMBOL(memchr)