On 2019/5/9 下午9:18, Michael S. Tsirkin wrote:
On Thu, May 09, 2019 at 08:58:00AM -0400, Jason Wang wrote:
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 archs (few non popular ones) that don't implement futex
helper, we can't log dirty pages. We can fix them e.g for non
virtually tagged archs implement a kmap fallback on top or simply
disable LOG_ALL features of vhost.
- The helper also requires userspace pointer is located at 4-byte
boundary, need to check during dirty log setting
Why check? Round it down.
Will do this.
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 V1:
- switch to use arch_futex_atomic_op_inuser()
---
drivers/vhost/vhost.c | 35 +++++++++++++++++------------------
1 file changed, 17 insertions(+), 18 deletions(-)
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 351af88..4e5a004 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"
@@ -1652,6 +1653,10 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
r = -EFAULT;
break;
}
+ if (p & 0x3) {
+ r = -EINVAL;
+ break;
+ }
for (i = 0; i < d->nvqs; ++i) {
struct vhost_virtqueue *vq;
void __user *base = (void __user *)(unsigned long)p;
That's an ABI change and might break some userspace. I don't think
it's necessary: you are changing individual bits anyway.
Right.
@@ -1692,31 +1697,27 @@ 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);
OK so the trick is that page is pinned so you don't expect
arch_futex_atomic_op_inuser below to fail. get_user_pages_fast
guarantees page is not going away but does it guarantee PTE won't be
invaidated or write protected?
Good point, then I think we probably need to do manual fixup through
fixup_user_fault() if arch_futex_atomic_op_in_user() fail.
if (r < 0)
return r;
BUG_ON(r != 1);
- base = kmap_atomic(page);
- set_bit(bit, base);
- kunmap_atomic(base);
+
+ r = arch_futex_atomic_op_inuser(FUTEX_OP_ADD, 1 << nr, &old, addr);
+ /* TODO: fallback to kmap() when -ENOSYS? */
+
Add a comment why this won't fail? Maybe warn on EFAULT?
Also down the road a variant that does not need tricks like this is
still nice to have.
Ok. Let me post a V3.
Thanks
set_page_dirty_lock(page);
put_page(page);
- return 0;
+ return r;
}
-static int log_write(void __user *log_base,
+static int log_write(u32 __user *log_base,
u64 write_address, u64 write_length)
{
u64 write_page = write_address / VHOST_PAGE_SIZE;
@@ -1726,12 +1727,10 @@ static int log_write(void __user *log_base,
return 0;
write_length += write_address % VHOST_PAGE_SIZE;
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 = 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
