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, 101 insertions, 0 deletions

diff --git a/arch/arm/common/vlock.S b/arch/arm/common/vlock.S
new file mode 100644
index 000000000..c5eaed5a7
--- /dev/null
+++ b/arch/arm/common/vlock.S
@@ -0,0 +1,101 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * vlock.S - simple voting lock implementation for ARM
+ *
+ * Created by:	Dave Martin, 2012-08-16
+ * Copyright:	(C) 2012-2013  Linaro Limited
+ *
+ * This algorithm is described in more detail in
+ * Documentation/arch/arm/vlocks.rst.
+ */
+
+#include <linux/linkage.h>
+#include "vlock.h"
+
+.arch armv7-a
+
+/* Select different code if voting flags  can fit in a single word. */
+#if VLOCK_VOTING_SIZE > 4
+#define FEW(x...)
+#define MANY(x...) x
+#else
+#define FEW(x...) x
+#define MANY(x...)
+#endif
+
+@ voting lock for first-man coordination
+
+.macro voting_begin rbase:req, rcpu:req, rscratch:req
+	mov	\rscratch, #1
+	strb	\rscratch, [\rbase, \rcpu]
+	dmb
+.endm
+
+.macro voting_end rbase:req, rcpu:req, rscratch:req
+	dmb
+	mov	\rscratch, #0
+	strb	\rscratch, [\rbase, \rcpu]
+	dsb	st
+	sev
+.endm
+
+/*
+ * The vlock structure must reside in Strongly-Ordered or Device memory.
+ * This implementation deliberately eliminates most of the barriers which
+ * would be required for other memory types, and assumes that independent
+ * writes to neighbouring locations within a cacheline do not interfere
+ * with one another.
+ */
+
+@ r0: lock structure base
+@ r1: CPU ID (0-based index within cluster)
+ENTRY(vlock_trylock)
+	add	r1, r1, #VLOCK_VOTING_OFFSET
+
+	voting_begin	r0, r1, r2
+
+	ldrb	r2, [r0, #VLOCK_OWNER_OFFSET]	@ check whether lock is held
+	cmp	r2, #VLOCK_OWNER_NONE
+	bne	trylock_fail			@ fail if so
+
+	@ Control dependency implies strb not observable before previous ldrb.
+
+	strb	r1, [r0, #VLOCK_OWNER_OFFSET]	@ submit my vote
+
+	voting_end	r0, r1, r2		@ implies DMB
+
+	@ Wait for the current round of voting to finish:
+
+ MANY(	mov	r3, #VLOCK_VOTING_OFFSET			)
+0:
+ MANY(	ldr	r2, [r0, r3]					)
+ FEW(	ldr	r2, [r0, #VLOCK_VOTING_OFFSET]			)
+	cmp	r2, #0
+	wfene
+	bne	0b
+ MANY(	add	r3, r3, #4					)
+ MANY(	cmp	r3, #VLOCK_VOTING_OFFSET + VLOCK_VOTING_SIZE	)
+ MANY(	bne	0b						)
+
+	@ Check who won:
+
+	dmb
+	ldrb	r2, [r0, #VLOCK_OWNER_OFFSET]
+	eor	r0, r1, r2			@ zero if I won, else nonzero
+	bx	lr
+
+trylock_fail:
+	voting_end	r0, r1, r2
+	mov	r0, #1				@ nonzero indicates that I lost
+	bx	lr
+ENDPROC(vlock_trylock)
+
+@ r0: lock structure base
+ENTRY(vlock_unlock)
+	dmb
+	mov	r1, #VLOCK_OWNER_NONE
+	strb	r1, [r0, #VLOCK_OWNER_OFFSET]
+	dsb	st
+	sev
+	bx	lr
+ENDPROC(vlock_unlock)