[PATCH v2] block: Fix start and length check added to iov_iter_extract_bvecs()

[David Howells <dhowells@redhat.com>]

Commit 14b007e17881 added an address check using iter_iov_addr() and a
length check using iter_iov_len() to iov_iter_extract_bvecs(), but these
cannot be used so and are unsafe in this circumstance as the functions have
hardwired assumptions about the iterator type.  They should only be used
with ITER_UBUF or ITER_IOVEC-type iterators and work with ITER_KVEC; they
should not be used with ITER_BVEC, ITER_FOLIOQ, ITER_XARRAY or ITER_DISCARD
iterators.

This change proved to be a problem for cachefiles as an iterator of type
ITER_FOLIOQ is passed and iter_iov_addr() and iter_iov_len() both
malfunction because iter->__iov in iter_iov() is not pointing to an iovec
array.

Fix this by using iov_iter_alignment() instead for anything other than
ITER_UBUF, ITER_IOVEC or ITER_KVEC.

Fixes: 14b007e17881 ("block: validate user space vectors during extraction")
Suggested-by: Keith Busch <kbusch@kernel.org>
cc: Keith Busch <kbusch@kernel.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: Hannes Reinecke <hare@kernel.org>
cc: Christoph Hellwig <hch@lst.de>
cc: Alexander Viro <viro@zeniv.linux.org.uk>
cc: Paulo Alcantara <pc@manguebit.org>
cc: netfs@lists.linux.dev
cc: linux-block@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
---
 lib/iov_iter.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 6665372ecf71..c489569a5815 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1921,15 +1921,22 @@ ssize_t iov_iter_extract_bvecs(struct iov_iter *iter, struct bio_vec *bv,
 		unsigned short max_vecs, unsigned mem_align_mask,
 		iov_iter_extraction_t extraction_flags)
 {
-	unsigned long start = (unsigned long)iter_iov_addr(iter);
 	unsigned short entries_left = max_vecs - *nr_vecs;
 	unsigned short nr_pages, i = 0;
 	size_t left, offset, len;
 	struct page **pages;
 	ssize_t size;
 
-	if ((start | iter_iov_len(iter)) & mem_align_mask)
+	if (likely(iter_is_ubuf(iter) ||
+		   iter_is_iovec(iter) ||
+		   iov_iter_is_kvec(iter))) {
+		unsigned long start = (unsigned long)iter_iov_addr(iter);
+
+		if ((start | iter_iov_len(iter)) & mem_align_mask)
+			return -EINVAL;
+	} else if (iov_iter_alignment(iter) & mem_align_mask) {
 		return -EINVAL;
+	}
 
 	/*
 	 * Move page array up in the allocated memory for the bio vecs as far as