Re: [PATCH v4 2/5] userfaultfd: add /dev/userfaultfd for fine grained access control

On Jul 19, 2022, at 12:56 PM, Axel Rasmussen axelrasmussen@google.com wrote:

+static int new_userfaultfd(bool is_syscall, int flags) { struct userfaultfd_ctx *ctx; int fd;

• if (!sysctl_unprivileged_userfaultfd &&

•    (flags & UFFD_USER_MODE_ONLY) == 0 &&
  

•    !capable(CAP_SYS_PTRACE)) {
  

• printk_once(KERN_WARNING "uffd: Set unprivileged_userfaultfd "
  

• 	"sysctl knob to 1 if kernel faults must be handled "
  

• 	"without obtaining CAP_SYS_PTRACE capability\n");
  

• if (is_syscall && !userfaultfd_syscall_allowed(flags)) return -EPERM;

• }

  BUG_ON(!current->mm);

@@ -2098,8 +2105,42 @@ SYSCALL_DEFINE1(userfaultfd, int, flags) return fd; }

+SYSCALL_DEFINE1(userfaultfd, int, flags) +{

• return new_userfaultfd(true, flags);

+}

Not critical, but why not to put the userfaultfd_syscall_allowed() check here? You would be able to lose the “is_syscall”.

I also had a small comment for patch 5.

But these are minor issues, so for the series:

Acked-by: Nadav Amit namit@vmware.com

[ Sorry again for misunderstanding the scheme you were using is similar to KVM and therefore reasonable. ]