[PATCH v2 06/14] iov_iter: Fix a memory leak in iov_iter_extract_user_pages()

[David Howells <dhowells@redhat.com>]

There's a potential memory leak in callers of iov_iter_extract_user_pages()
whereby if a pages array is allocated in function, it isn't freed before
returning of an error or 0.

Now, it's not a leak per se in iov_iter_extract_user_pages() as, if an
array is allocated, it's returned through *pages, so it's incumbent on the
caller to free it.  However, not all callers do.

Fix this by freeing the table and clearing *pages before returning an error
or 0.  Note that iov_iter_extract_pages() and its subfunctions are allowed
to return 0 without returning an array (for instance if the iterator count
is 0).

Fixes: 7d58fe731028 ("iov_iter: Add a function to extract a page list from an iterator")
Closes: https://sashiko.dev/#/patchset/20260616100821.2062304-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: Christoph Hellwig <hch@infradead.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
 lib/iov_iter.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 1661c237b643..5a6434709716 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1756,6 +1756,7 @@ static ssize_t iov_iter_extract_user_pages(struct iov_iter *i,
 	unsigned long addr;
 	unsigned int gup_flags = 0;
 	size_t offset;
+	bool will_alloc = !*pages;
 	int res;
 
 	if (i->data_source == ITER_DEST)
@@ -1772,8 +1773,14 @@ static ssize_t iov_iter_extract_user_pages(struct iov_iter *i,
 	if (!maxpages)
 		return -ENOMEM;
 	res = pin_user_pages_fast(addr, maxpages, gup_flags, *pages);
-	if (unlikely(res <= 0))
+	if (unlikely(res <= 0)) {
+		if (will_alloc) {
+			kvfree(*pages);
+			*pages = NULL;
+		}
 		return res;
+	}
+
 	maxsize = min_t(size_t, maxsize, res * PAGE_SIZE - offset);
 	iov_iter_advance(i, maxsize);
 	return maxsize;