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.
> Cc: Christoph Hellwig <h...@infradead.org>
> Cc: James Bottomley <james.bottom...@hansenpartnership.com>
> Cc: Andrea Arcangeli <aarca...@redhat.com>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: Ingo Molnar <mi...@redhat.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Darren Hart <dvh...@infradead.org>
> Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
> Signed-off-by: Jason Wang <jasow...@redhat.com>
> ---
> 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.
> @@ -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?
> 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.
> 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
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