[lvc-project] [PATCH] lib: free pagelist on error in iov_iter_extract_pages()

[Dmitry Antipov]

Since 'iov_iter_extract_pages()' may allocate new pagelist if the passed
one isn't large enough, the worst-case scenario may be:

...
struct page *stack_pages[SMALL];
struct page **pages = stack_pages;
...
if (iov_iter_extract_pages(i..., &pages, ...) <= 0) {
        /* Even in case of error, new pagelist may be allocated */
        if (pages != stack_pages)
                kvfree(pages);                                  [1]
        /* The rest of error handling and return */
}
/* Regular flow */
...
if (pages != stack_pages)
        kvfree(pages);
...
return 0;

If you're unlucky so SMALL amount of pages wasn't enough and new
pagelist was allocated, missing [1] causes the memory leak similar
to one I've recently observed and fixed for 6.12 in [2]. So adjust
'iov_iter_extract_pages()' to make such a cleanup itself rather than
rely on caller's handling on error paths, thus making [1] not needed.

[2] https://lore.kernel.org/stable/20260505094529.406783-1-dmantipov@yandex.ru/T/#u

Suggested-by: Fedor Pchelkin <pchelkin at ispras.ru>
Signed-off-by: Dmitry Antipov <dmantipov at yandex.ru>
---
 lib/iov_iter.c | 54 ++++++++++++++++++++++++++++++--------------------
 1 file changed, 33 insertions(+), 21 deletions(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 243662af1af7..46dd11913df0 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1807,7 +1807,8 @@ static ssize_t iov_iter_extract_user_pages(struct iov_iter *i,
  *  (*) Use with ITER_DISCARD is not supported as that has no content.
  *
  * On success, the function sets *@pages to the new pagelist, if allocated, and
- * sets *offset0 to the offset into the first page.
+ * sets *offset0 to the offset into the first page. On error, new pagelist
+ * is freed if was allocated, and *@pages sets back to its original value.
  *
  * It may also return -ENOMEM and -EFAULT.
  */
@@ -1818,31 +1819,42 @@ ssize_t iov_iter_extract_pages(struct iov_iter *i,
 			       iov_iter_extraction_t extraction_flags,
 			       size_t *offset0)
 {
+	struct page **oldpages = *pages;
+	int ret;
+
 	maxsize = min_t(size_t, min_t(size_t, maxsize, i->count), MAX_RW_COUNT);
 	if (!maxsize)
 		return 0;
 
 	if (likely(user_backed_iter(i)))
-		return iov_iter_extract_user_pages(i, pages, maxsize,
-						   maxpages, extraction_flags,
-						   offset0);
-	if (iov_iter_is_kvec(i))
-		return iov_iter_extract_kvec_pages(i, pages, maxsize,
-						   maxpages, extraction_flags,
-						   offset0);
-	if (iov_iter_is_bvec(i))
-		return iov_iter_extract_bvec_pages(i, pages, maxsize,
-						   maxpages, extraction_flags,
-						   offset0);
-	if (iov_iter_is_folioq(i))
-		return iov_iter_extract_folioq_pages(i, pages, maxsize,
-						     maxpages, extraction_flags,
-						     offset0);
-	if (iov_iter_is_xarray(i))
-		return iov_iter_extract_xarray_pages(i, pages, maxsize,
-						     maxpages, extraction_flags,
-						     offset0);
-	return -EFAULT;
+		ret = iov_iter_extract_user_pages(i, pages, maxsize,
+						  maxpages, extraction_flags,
+						  offset0);
+	else if (iov_iter_is_kvec(i))
+		ret = iov_iter_extract_kvec_pages(i, pages, maxsize,
+						  maxpages, extraction_flags,
+						  offset0);
+	else if (iov_iter_is_bvec(i))
+		ret = iov_iter_extract_bvec_pages(i, pages, maxsize,
+						  maxpages, extraction_flags,
+						  offset0);
+	else if (iov_iter_is_folioq(i))
+		ret = iov_iter_extract_folioq_pages(i, pages, maxsize,
+						    maxpages, extraction_flags,
+						    offset0);
+	else if (iov_iter_is_xarray(i))
+		ret = iov_iter_extract_xarray_pages(i, pages, maxsize,
+						    maxpages, extraction_flags,
+						    offset0);
+	else
+		ret = -EFAULT;
+
+	if (unlikely(ret) && *pages && *pages != oldpages) {
+		kvfree(*pages);
+		*pages = oldpages;
+	}
+
+	return ret;
 }
 EXPORT_SYMBOL_GPL(iov_iter_extract_pages);
 
-- 
2.54.0