sys/uio: add updateiov()

This function take a struct uio previously created by copyinuio and and
updates the lengths of the user-space iovec to match those in the uio.

To reduce the risks of pointer leakage and cross-ABI pointer confusion,
lengths are updated individually.

Reviewed by:	jamie, jhb
Effort:		CHERI upstreaming
Sponsored by:	DARPA, AFRL, Innovate UK
Differential Revision:	https://reviews.freebsd.org/D60025

sys/kern/subr_uio.c

@@ -468,6 +468,24 @@ copyinuio(const void *iovp, u_int iovcnt, struct uio **uiop)
 	return (0);
 }
 
+/*
+ * Update the lengths of a userspace iovec to match those in a struct uio's
+ * iovec (previously created by copyinuio).
+ */
+int
+updateiov(const struct uio *uiop, void *uiovp)
+{
+	int i, error;
+	struct iovec *iovp = uiovp;
+
+	for (i = 0; i < uiop->uio_iovcnt; i++) {
+		error = suword(&iovp[i].iov_len, uiop->uio_iov[i].iov_len);
+		if (error != 0)
+			return (EFAULT);
+	}
+	return (0);
+}
+
 struct uio *
 allocuio(u_int iovcnt)
 {

sys/sys/syscallsubr.h

@@ -96,6 +96,13 @@ typedef int (copyin_hdtr_t)(const void *hdtrp, struct sf_hdtr *hdtr);
 typedef int (copyinuio_t)(const void *iovp, unsigned int iovcnt,
     struct uio **iov);
 
+/*
+ * A updateiov_t takes a pointer to a struct uio previously created with
+ * copyinuio_t and a pointer to the corresponding iovec in userspace.
+ * It updates all lengths in userspace to match those in the uio.
+ */
+typedef int(updateiov_t)(const struct uio *uiop, void *iovp);
+
 uint64_t at2cnpflags(u_int at_flags, u_int mask);
 int	kern___getcwd(struct thread *td, char *buf, enum uio_seg bufseg,
 	    size_t buflen, size_t path_max);

sys/sys/uio.h

@@ -99,6 +99,7 @@ int	uiomove_fromphys(struct vm_page *ma[], vm_offset_t offset, int n,
 int	uiomove_nofault(void *cp, int n, struct uio *uio);
 int	uiomove_object(struct vm_object *obj, off_t obj_size, struct uio *uio);
 int	uiomove_step(void *cp, void *base, size_t cnt, struct uio *uio);
+int	updateiov(const struct uio *uiop, void *iovp);
 
 #else /* !_KERNEL */