[PATCH net] vhost: don't use kmap() to log dirty pages

[Jason Wang]

Vhost log dirty pages directly to a userspace bitmap through GUP and
kmap_atomic() since kernel doesn't have a set_bit_to_user()
helper. This will cause issues for the arch that has virtually tagged
caches. The way to fix is to keep using userspace virtual
address. Fortunately, futex has arch_futex_atomic_op_inuser() which
could be used for setting a bit to user.

Note there're several cases that futex helper can fail e.g a page
fault or the arch that doesn't have the support. For those cases, a
simplified get_user()/put_user() pair protected by a global mutex is
provided as a fallback. The fallback may lead false positive that
userspace may see more dirty pages.

Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Darren Hart <dvhart@xxxxxxxxxxxxx>
Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
---
Changes from RFC V2:
- drop GUP and provide get_user()/put_user() fallbacks
- round down log_base
Changes from RFC V1:
- switch to use arch_futex_atomic_op_inuser()
---
 drivers/vhost/vhost.c | 54 ++++++++++++++++++++++++++++-----------------------
 1 file changed, 30 insertions(+), 24 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 351af88..7fa05ba 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -31,6 +31,7 @@
 #include <linux/sched/signal.h>
 #include <linux/interval_tree_generic.h>
 #include <linux/nospec.h>
+#include <asm/futex.h>
 
 #include "vhost.h"
 
@@ -43,6 +44,8 @@
 MODULE_PARM_DESC(max_iotlb_entries,
 	"Maximum number of iotlb entries. (default: 2048)");
 
+static DEFINE_MUTEX(vhost_log_lock);
+
 enum {
 	VHOST_MEMORY_F_LOG = 0x1,
 };
@@ -1692,28 +1695,31 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
 }
 EXPORT_SYMBOL_GPL(vhost_dev_ioctl);
 
-/* TODO: This is really inefficient.  We need something like get_user()
- * (instruction directly accesses the data, with an exception table entry
- * returning -EFAULT). See Documentation/x86/exception-tables.txt.
- */
-static int set_bit_to_user(int nr, void __user *addr)
+static int set_bit_to_user(int nr, u32 __user *addr)
 {
-	unsigned long log = (unsigned long)addr;
-	struct page *page;
-	void *base;
-	int bit = nr + (log % PAGE_SIZE) * 8;
+	u32 old;
 	int r;
 
-	r = get_user_pages_fast(log, 1, 1, &page);
-	if (r < 0)
-		return r;
-	BUG_ON(r != 1);
-	base = kmap_atomic(page);
-	set_bit(bit, base);
-	kunmap_atomic(base);
-	set_page_dirty_lock(page);
-	put_page(page);
+	r = arch_futex_atomic_op_inuser(FUTEX_OP_OR, 1 << nr, &old, addr);
+	if (r) {
+		/* Fallback through get_user()/put_user(), this may
+		 * lead false positive that userspace may see more
+		 * dirty pages. A mutex is used to synchronize log
+		 * access between vhost threads.
+		 */
+		mutex_lock(&vhost_log_lock);
+		r = get_user(old, addr);
+		if (r)
+			goto err;
+		r = put_user(old | 1 << nr, addr);
+		if (r)
+			goto err;
+		mutex_unlock(&vhost_log_lock);
+	}
 	return 0;
+err:
+	mutex_unlock(&vhost_log_lock);
+	return r;
 }
 
 static int log_write(void __user *log_base,
@@ -1725,13 +1731,13 @@ static int log_write(void __user *log_base,
 	if (!write_length)
 		return 0;
 	write_length += write_address % VHOST_PAGE_SIZE;
+	log_base = (void __user *)((u64)log_base & ~0x3ULL);
+	write_page += ((u64)log_base & 0x3ULL) * 8;
 	for (;;) {
-		u64 base = (u64)(unsigned long)log_base;
-		u64 log = base + write_page / 8;
-		int bit = write_page % 8;
-		if ((u64)(unsigned long)log != log)
-			return -EFAULT;
-		r = set_bit_to_user(bit, (void __user *)(unsigned long)log);
+		u32 __user *log = (u32 __user *)log_base + write_page / 32;
+		int bit = write_page % 32;
+
+		r = set_bit_to_user(bit, log);
 		if (r < 0)
 			return r;
 		if (write_length <= VHOST_PAGE_SIZE)
-- 
1.8.3.1