xen: Make struct privcmd_irqfd's layout architecture independent

Using indirect pointers in an ioctl command argument means that the
layout is architecture specific, in particular we can't use the same one
from 32-bit compat tasks. The general recommendation is to have __u64
members and use u64_to_user_ptr() to access it from the kernel if we are
unable to avoid the pointers altogether.

Fixes: f8941e6c ("xen: privcmd: Add support for irqfd")
Reported-by: Arnd Bergmann <arnd@kernel.org>
Closes: https://lore.kernel.org/all/268a2031-63b8-4c7d-b1e5-8ab83ca80b4a@app.fastmail.com/Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/a4ef0d4a68fc858b34a81fd3f9877d9b6898eb77.1697439990.git.viresh.kumar@linaro.orgSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/privcmd.c

@@ -935,7 +935,7 @@ static int privcmd_irqfd_assign(struct privcmd_irqfd *irqfd)
 		return -ENOMEM;
 	dm_op = kirqfd + 1;
 
-	if (copy_from_user(dm_op, irqfd->dm_op, irqfd->size)) {
+	if (copy_from_user(dm_op, u64_to_user_ptr(irqfd->dm_op), irqfd->size)) {
 		ret = -EFAULT;
 		goto error_kfree;
 	}

include/uapi/xen/privcmd.h

@@ -102,7 +102,7 @@ struct privcmd_mmap_resource {
 #define PRIVCMD_IRQFD_FLAG_DEASSIGN (1 << 0)
 
 struct privcmd_irqfd {
-	void __user *dm_op;
+	__u64 dm_op;
 	__u32 size; /* Size of structure pointed by dm_op */
 	__u32 fd;
 	__u32 flags;