[PATCH v6 10/24] netfs: Fix overrun check in netfs_extract_user_iter()

[David Howells <dhowells@redhat.com>]

Fix netfs_extract_user_iter() so that if iov_iter_extract_pages() overfills
pages[], then those pages don't get included in the iterator constructed at
the end of the function.  If there was an overfill, memory corruption has
already happened.

Fixes: 85dd2c8ff368 ("netfs: Add a function to extract a UBUF or IOVEC into a BVEC iterator")
Closes: https://sashiko.dev/#/patchset/20260427154639.180684-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
 fs/netfs/iterator.c | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/fs/netfs/iterator.c b/fs/netfs/iterator.c
index 429e4396e1b0..b375567e0520 100644
--- a/fs/netfs/iterator.c
+++ b/fs/netfs/iterator.c
@@ -72,21 +72,24 @@ ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,
 			break;
 		}
 
-		if (ret > count) {
-			pr_err("get_pages rc=%zd more than %zu\n", ret, count);
+		if (WARN(ret > count,
+			 "%s: extract_pages overrun %zd > %zu bytes\n",
+			 __func__, ret, count)) {
+			ret = -EIO;
 			break;
 		}
 
-		count -= ret;
-		ret += offset;
-		cur_npages = DIV_ROUND_UP(ret, PAGE_SIZE);
-
-		if (npages + cur_npages > max_pages) {
-			pr_err("Out of bvec array capacity (%u vs %u)\n",
-			       npages + cur_npages, max_pages);
+		cur_npages = DIV_ROUND_UP(offset + ret, PAGE_SIZE);
+		if (WARN(cur_npages > max_pages - npages,
+			 "%s: extract_pages overrun %u > %u pages\n",
+			 __func__, npages + cur_npages, max_pages)) {
+			ret = -EIO;
 			break;
 		}
 
+		count -= ret;
+		ret += offset;
+
 		for (i = 0; i < cur_npages; i++) {
 			len = ret > PAGE_SIZE ? PAGE_SIZE : ret;
 			bvec_set_page(bv + npages + i, *pages++, len - offset, offset);
@@ -97,6 +100,11 @@ ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,
 		npages += cur_npages;
 	}
 
+	/* Note: Don't try to clean up after EIO.  Either we got no pages, so
+	 * nothing to clean up, or we got a buffer overrun, memory corruption
+	 * and can't trust the stuff in the buffer (a WARN was emitted).
+	 */
+
 	if (ret < 0 && (ret == -ENOMEM || npages == 0)) {
 		for (i = 0; i < npages; i++)
 			unpin_user_page(bv[i].bv_page);