Re: [kernel-hardening] [PATCH v5 04/10] arm64: Add __flush_tlb_one()

[Tycho Andersen <tycho@docker.com>]

On Wed, Aug 23, 2017 at 06:04:43PM +0100, Mark Rutland wrote:
> On Wed, Aug 23, 2017 at 10:58:42AM -0600, Tycho Andersen wrote:
> > Hi Mark,
> > 
> > On Mon, Aug 14, 2017 at 05:50:47PM +0100, Mark Rutland wrote:
> > > That said, is there any reason not to use flush_tlb_kernel_range()
> > > directly?
> > 
> > So it turns out that there is a difference between __flush_tlb_one() and
> > flush_tlb_kernel_range() on x86: flush_tlb_kernel_range() flushes all the TLBs
> > via on_each_cpu(), where as __flush_tlb_one() only flushes the local TLB (which
> > I think is enough here).
> 
> That sounds suspicious; I don't think that __flush_tlb_one() is
> sufficient.
> 
> If you only do local TLB maintenance, then the page is left accessible
> to other CPUs via the (stale) kernel mappings. i.e. the page isn't
> exclusively mapped by userspace.

I thought so too, so I tried to test it with something like the patch
below. But it correctly failed for me when using __flush_tlb_one(). I
suppose I'm doing something wrong in the test, but I'm not sure what.

Tycho


>From 1d1b0a18d56cf1634072096231bfbaa96cb2aa16 Mon Sep 17 00:00:00 2001
From: Tycho Andersen <tycho@docker.com>
Date: Tue, 22 Aug 2017 18:07:12 -0600
Subject: [PATCH] add XPFO_SMP test

Signed-off-by: Tycho Andersen <tycho@docker.com>
---
 drivers/misc/lkdtm.h      |   1 +
 drivers/misc/lkdtm_core.c |   1 +
 drivers/misc/lkdtm_xpfo.c | 139 ++++++++++++++++++++++++++++++++++++++++++----
 3 files changed, 130 insertions(+), 11 deletions(-)

diff --git a/drivers/misc/lkdtm.h b/drivers/misc/lkdtm.h
index fc53546113c1..34a6ee37f216 100644
--- a/drivers/misc/lkdtm.h
+++ b/drivers/misc/lkdtm.h
@@ -67,5 +67,6 @@ void lkdtm_USERCOPY_KERNEL(void);
 /* lkdtm_xpfo.c */
 void lkdtm_XPFO_READ_USER(void);
 void lkdtm_XPFO_READ_USER_HUGE(void);
+void lkdtm_XPFO_SMP(void);
 
 #endif
diff --git a/drivers/misc/lkdtm_core.c b/drivers/misc/lkdtm_core.c
index 164bc404f416..9544e329de4b 100644
--- a/drivers/misc/lkdtm_core.c
+++ b/drivers/misc/lkdtm_core.c
@@ -237,6 +237,7 @@ struct crashtype crashtypes[] = {
 	CRASHTYPE(USERCOPY_KERNEL),
 	CRASHTYPE(XPFO_READ_USER),
 	CRASHTYPE(XPFO_READ_USER_HUGE),
+	CRASHTYPE(XPFO_SMP),
 };
 
 
diff --git a/drivers/misc/lkdtm_xpfo.c b/drivers/misc/lkdtm_xpfo.c
index c72509128eb3..7600fdcae22f 100644
--- a/drivers/misc/lkdtm_xpfo.c
+++ b/drivers/misc/lkdtm_xpfo.c
@@ -4,22 +4,27 @@
 
 #include "lkdtm.h"
 
+#include <linux/cpumask.h>
 #include <linux/mman.h>
 #include <linux/uaccess.h>
 #include <linux/xpfo.h>
+#include <linux/kthread.h>
 
-void read_user_with_flags(unsigned long flags)
+#include <linux/delay.h>
+#include <linux/sched/task.h>
+
+#define XPFO_DATA 0xdeadbeef
+
+static unsigned long do_map(unsigned long flags)
 {
-	unsigned long user_addr, user_data = 0xdeadbeef;
-	phys_addr_t phys_addr;
-	void *virt_addr;
+	unsigned long user_addr, user_data = XPFO_DATA;
 
 	user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
 			    PROT_READ | PROT_WRITE | PROT_EXEC,
 			    flags, 0);
 	if (user_addr >= TASK_SIZE) {
 		pr_warn("Failed to allocate user memory\n");
-		return;
+		return 0;
 	}
 
 	if (copy_to_user((void __user *)user_addr, &user_data,
@@ -28,25 +33,61 @@ void read_user_with_flags(unsigned long flags)
 		goto free_user;
 	}
 
+	return user_addr;
+
+free_user:
+	vm_munmap(user_addr, PAGE_SIZE);
+	return 0;
+}
+
+static unsigned long *user_to_kernel(unsigned long user_addr)
+{
+	phys_addr_t phys_addr;
+	void *virt_addr;
+
 	phys_addr = user_virt_to_phys(user_addr);
 	if (!phys_addr) {
 		pr_warn("Failed to get physical address of user memory\n");
-		goto free_user;
+		return 0;
 	}
 
 	virt_addr = phys_to_virt(phys_addr);
 	if (phys_addr != virt_to_phys(virt_addr)) {
 		pr_warn("Physical address of user memory seems incorrect\n");
-		goto free_user;
+		return 0;
 	}
 
+	return virt_addr;
+}
+
+static void read_map(unsigned long *virt_addr)
+{
 	pr_info("Attempting bad read from kernel address %p\n", virt_addr);
-	if (*(unsigned long *)virt_addr == user_data)
-		pr_info("Huh? Bad read succeeded?!\n");
+	if (*(unsigned long *)virt_addr == XPFO_DATA)
+		pr_err("FAIL: Bad read succeeded?!\n");
 	else
-		pr_info("Huh? Bad read didn't fail but data is incorrect?!\n");
+		pr_err("FAIL: Bad read didn't fail but data is incorrect?!\n");
+}
+
+static void read_user_with_flags(unsigned long flags)
+{
+	unsigned long user_addr, *kernel;
+
+	user_addr = do_map(flags);
+	if (!user_addr) {
+		pr_err("FAIL: map failed\n");
+		return;
+	}
+
+	kernel = user_to_kernel(user_addr);
+	if (!kernel) {
+		pr_err("FAIL: user to kernel conversion failed\n");
+		goto free_user;
+	}
+
+	read_map(kernel);
 
- free_user:
+free_user:
 	vm_munmap(user_addr, PAGE_SIZE);
 }
 
@@ -60,3 +101,79 @@ void lkdtm_XPFO_READ_USER_HUGE(void)
 {
 	read_user_with_flags(MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB);
 }
+
+struct smp_arg {
+	struct completion map_done;
+	unsigned long *virt_addr;
+	unsigned int cpu;
+};
+
+static int smp_reader(void *parg)
+{
+	struct smp_arg *arg = parg;
+
+	if (arg->cpu != smp_processor_id()) {
+		pr_err("FAIL: scheduled on wrong CPU?\n");
+		return 0;
+	}
+
+	wait_for_completion(&arg->map_done);
+
+	if (arg->virt_addr)
+		read_map(arg->virt_addr);
+
+	return 0;
+}
+
+/* The idea here is to read from the kernel's map on a different thread than
+ * did the mapping (and thus the TLB flushing), to make sure that the page
+ * faults on other cores too.
+ */
+void lkdtm_XPFO_SMP(void)
+{
+	unsigned long user_addr;
+	struct task_struct *thread;
+	int ret;
+	struct smp_arg arg;
+
+	init_completion(&arg.map_done);
+
+	if (num_online_cpus() < 2) {
+		pr_err("not enough to do a multi cpu test\n");
+		return;
+	}
+
+	arg.cpu = (smp_processor_id() + 1) % num_online_cpus();
+	thread = kthread_create(smp_reader, &arg, "lkdtm_xpfo_test");
+	if (IS_ERR(thread)) {
+		pr_err("couldn't create kthread? %ld\n", PTR_ERR(thread));
+		return;
+	}
+
+	kthread_bind(thread, arg.cpu);
+	get_task_struct(thread);
+	wake_up_process(thread);
+
+	user_addr = do_map(MAP_PRIVATE | MAP_ANONYMOUS);
+	if (user_addr) {
+		arg.virt_addr = user_to_kernel(user_addr);
+		/* child thread checks for failure */
+	}
+
+	complete(&arg.map_done);
+
+	/* there must be a better way to do this. */
+	while (1) {
+		if (thread->exit_state)
+			break;
+		msleep_interruptible(100);
+	}
+
+	ret = kthread_stop(thread);
+	if (ret != SIGKILL)
+		pr_err("FAIL: thread wasn't killed: %d\n", ret);
+	put_task_struct(thread);
+
+	if (user_addr)
+		vm_munmap(user_addr, PAGE_SIZE);
+}
-- 
2.11.0