iov_iter: use kmemdup_array for dup_iter to harden against overflow

While auditing the Linux 7.0-rc2 kernel, I identified a potential security
vulnerability in the iov_iter framework's memory allocation logic.

The dup_iter() function, which is exported via EXPORT_SYMBOL, currently
uses kmemdup() with a raw multiplication to allocate the duplicate iovec array:

new->iov = kmemdup(from->iov, nr_segs * sizeof(struct iovec), gfp);

The hazard here is that dup_iter() relies on a primitive multiplication without
any integrated overflow check. Since nr_segs is often derived from user-space
input, this line is vulnerable to integer overflow (on 32-bit systems or
via type narrowing), potentially leading to a small allocation followed by a
large out-of-bounds memory copy. Furthermore, it allows for unbounded memory
allocations, as the function lacks intrinsic knowledge of safe limits.

On the 7.0-rc2 branch, several high-impact callchains still rely on this
exported function:

drivers/usb/gadget/function/f_fs.c:
The ffs_epfile_read_iter() path demonstrates why relying on dup_iter() is
dangerous: it performs allocation based on user input before verifying driver
state. This confirms that dup_iter() must be hardened internally as it cannot
assume pre-validated input.

drivers/usb/gadget/legacy/inode.c:
The ep_read_iter() path illustrates how dup_iter()’s lack of boundary awareness
compounds resource risks. When combined with other allocations, it creates
a multiplier effect for kernel memory pressure.

This patch replaces kmemdup() with kmemdup_array(), which utilizes
check_mul_overflow() to ensure the allocation size is calculated safely,
hardening dup_iter() against malicious or malformed inputs from its callers

Signed-off-by: Wang Haoran <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>

Author: Wang Haoran

lib/iov_iter.c

@@ -1224,13 +1224,13 @@ const void *dup_iter(struct iov_iter *new, struct iov_iter *old, gfp_t flags)
 {
 	*new = *old;
 	if (iov_iter_is_bvec(new))
-		return new->bvec = kmemdup(new->bvec,
-				    new->nr_segs * sizeof(struct bio_vec),
+		return new->bvec = kmemdup_array(new->bvec,
+				    new->nr_segs, sizeof(struct bio_vec),
 				    flags);
 	else if (iov_iter_is_kvec(new) || iter_is_iovec(new))
 		/* iovec and kvec have identical layout */
-		return new->__iov = kmemdup(new->__iov,
-				   new->nr_segs * sizeof(struct iovec),
+		return new->__iov = kmemdup_array(new->__iov,
+				   new->nr_segs, sizeof(struct iovec),
 				   flags);
 	return NULL;
 }