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, 64 insertions, 0 deletions

diff --git a/lib/kasprintf.c b/lib/kasprintf.c
new file mode 100644
index 000000000..cd2f5974e
--- /dev/null
+++ b/lib/kasprintf.c
@@ -0,0 +1,64 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  linux/lib/kasprintf.c
+ *
+ *  Copyright (C) 1991, 1992  Linus Torvalds
+ */
+
+#include <linux/stdarg.h>
+#include <linux/export.h>
+#include <linux/slab.h>
+#include <linux/types.h>
+#include <linux/string.h>
+
+/* Simplified asprintf. */
+char *kvasprintf(gfp_t gfp, const char *fmt, va_list ap)
+{
+	unsigned int first, second;
+	char *p;
+	va_list aq;
+
+	va_copy(aq, ap);
+	first = vsnprintf(NULL, 0, fmt, aq);
+	va_end(aq);
+
+	p = kmalloc_track_caller(first+1, gfp);
+	if (!p)
+		return NULL;
+
+	second = vsnprintf(p, first+1, fmt, ap);
+	WARN(first != second, "different return values (%u and %u) from vsnprintf(\"%s\", ...)",
+	     first, second, fmt);
+
+	return p;
+}
+EXPORT_SYMBOL(kvasprintf);
+
+/*
+ * If fmt contains no % (or is exactly %s), use kstrdup_const. If fmt
+ * (or the sole vararg) points to rodata, we will then save a memory
+ * allocation and string copy. In any case, the return value should be
+ * freed using kfree_const().
+ */
+const char *kvasprintf_const(gfp_t gfp, const char *fmt, va_list ap)
+{
+	if (!strchr(fmt, '%'))
+		return kstrdup_const(fmt, gfp);
+	if (!strcmp(fmt, "%s"))
+		return kstrdup_const(va_arg(ap, const char*), gfp);
+	return kvasprintf(gfp, fmt, ap);
+}
+EXPORT_SYMBOL(kvasprintf_const);
+
+char *kasprintf(gfp_t gfp, const char *fmt, ...)
+{
+	va_list ap;
+	char *p;
+
+	va_start(ap, fmt);
+	p = kvasprintf(gfp, fmt, ap);
+	va_end(ap);
+
+	return p;
+}
+EXPORT_SYMBOL(kasprintf);