linux-kernel - [PATCH] lkdtm: Test KUAP directional user access unlocks on powerpc

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Message-Id: <20200131053157.22463-1-ruscur@russell.cc>
Date:   Fri, 31 Jan 2020 16:31:57 +1100
From:   Russell Currey <ruscur@...sell.cc>
To:     keescook@...omium.org
Cc:     linux-kernel@...r.kernel.org, dja@...ens.net, mpe@...erman.id.au,
        kernel-hardening@...ts.openwall.com, linuxppc-dev@...ts.ozlabs.org,
        christophe.leroy@....fr, Russell Currey <ruscur@...sell.cc>
Subject: [PATCH] lkdtm: Test KUAP directional user access unlocks on powerpc

Kernel Userspace Access Prevention (KUAP) on powerpc supports
allowing only one access direction (Read or Write) when allowing access
to or from user memory.

A bug was recently found that showed that these one-way unlocks never
worked, and allowing Read *or* Write would actually unlock Read *and*
Write.  We should have a test case for this so we can make sure this
doesn't happen again.

Like ACCESS_USERSPACE, the correct result is for the test to fault.

At the time of writing this, the upstream kernel still has this bug
present, so the test will allow both accesses whereas ACCESS_USERSPACE
will correctly fault.

Signed-off-by: Russell Currey <ruscur@...sell.cc>
---
 drivers/misc/lkdtm/core.c  |  3 +++
 drivers/misc/lkdtm/lkdtm.h |  3 +++
 drivers/misc/lkdtm/perms.c | 43 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 49 insertions(+)

diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
index ee0d6e721441..baef3c6f48d6 100644
--- a/drivers/misc/lkdtm/core.c
+++ b/drivers/misc/lkdtm/core.c
@@ -137,6 +137,9 @@ static const struct crashtype crashtypes[] = {
 	CRASHTYPE(EXEC_USERSPACE),
 	CRASHTYPE(EXEC_NULL),
 	CRASHTYPE(ACCESS_USERSPACE),
+#ifdef CONFIG_PPC_KUAP
+	CRASHTYPE(ACCESS_USERSPACE_KUAP),
+#endif
 	CRASHTYPE(ACCESS_NULL),
 	CRASHTYPE(WRITE_RO),
 	CRASHTYPE(WRITE_RO_AFTER_INIT),
diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
index c56d23e37643..406a3fb32e6f 100644
--- a/drivers/misc/lkdtm/lkdtm.h
+++ b/drivers/misc/lkdtm/lkdtm.h
@@ -57,6 +57,9 @@ void lkdtm_EXEC_RODATA(void);
 void lkdtm_EXEC_USERSPACE(void);
 void lkdtm_EXEC_NULL(void);
 void lkdtm_ACCESS_USERSPACE(void);
+#ifdef CONFIG_PPC_KUAP
+void lkdtm_ACCESS_USERSPACE_KUAP(void);
+#endif
 void lkdtm_ACCESS_NULL(void);
 
 /* lkdtm_refcount.c */
diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 62f76d506f04..2c9aa0114333 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -10,6 +10,9 @@
 #include <linux/mman.h>
 #include <linux/uaccess.h>
 #include <asm/cacheflush.h>
+#ifdef CONFIG_PPC_KUAP
+#include <asm/uaccess.h>
+#endif
 
 /* Whether or not to fill the target memory area with do_nothing(). */
 #define CODE_WRITE	true
@@ -200,6 +203,46 @@ void lkdtm_ACCESS_USERSPACE(void)
 	vm_munmap(user_addr, PAGE_SIZE);
 }
 
+/* Test that KUAP's directional user access unlocks work as intended */
+#ifdef CONFIG_PPC_KUAP
+void lkdtm_ACCESS_USERSPACE_KUAP(void)
+{
+	unsigned long user_addr, tmp = 0;
+	unsigned long *ptr;
+
+	user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
+			    PROT_READ | PROT_WRITE | PROT_EXEC,
+			    MAP_ANONYMOUS | MAP_PRIVATE, 0);
+	if (user_addr >= TASK_SIZE) {
+		pr_warn("Failed to allocate user memory\n");
+		return;
+	}
+
+	if (copy_to_user((void __user *)user_addr, &tmp, sizeof(tmp))) {
+		pr_warn("copy_to_user failed\n");
+		vm_munmap(user_addr, PAGE_SIZE);
+		return;
+	}
+
+	ptr = (unsigned long *)user_addr;
+
+	/* Allowing "write to" should not allow "read from" */
+	allow_write_to_user(ptr, sizeof(unsigned long));
+	pr_info("attempting bad read at %px with write allowed\n", ptr);
+	tmp = *ptr;
+	tmp += 0xc0dec0de;
+	prevent_write_to_user(ptr, sizeof(unsigned long));
+
+	/* Allowing "read from" should not allow "write to" */
+	allow_read_from_user(ptr, sizeof(unsigned long));
+	pr_info("attempting bad write at %px with read allowed\n", ptr);
+	*ptr = tmp;
+	prevent_read_from_user(ptr, sizeof(unsigned long));
+
+	vm_munmap(user_addr, PAGE_SIZE);
+}
+#endif
+
 void lkdtm_ACCESS_NULL(void)
 {
 	unsigned long tmp;
-- 
2.25.0