[PATCH v3 8/9] RFC: net: limit folio refcount when splicing pages into skbs

["David Hildenbrand (Arm)" <david@kernel.org>]

Various networking features use MSG_SPLICE_PAGES to add pages to skbs,
whereby skb_append_pagefrags() takes a page reference for each fragment it
adds. Pages spliced from pipes were already checked when entering the
pipe, but in-kernel users like sunrpc pass pagecache pages directly. This
allows for exceeding folio refcount limits.

This is not specific to large pagecache folios, but with them it is a
lot easier to reach the refcount limit, as page table mappings can cheaply
create many references.

Let's use try_get_page() in skb_append_pagefrags() and fail with -EFAULT.
Don't use -EMSGSIZE, as TCP would just retry with a new skb. The only
caller, skb_splice_from_iter(), handles that.

Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
Cc: stable@vger.kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 net/core/skbuff.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index f473c74956b8b..5e3f910eeea0b 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4702,8 +4702,13 @@ int skb_append_pagefrags(struct sk_buff *skb, struct page *page,
 	if (skb_can_coalesce(skb, i, page, offset)) {
 		skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], size);
 	} else if (i < max_frags) {
+		/*
+		 * Don't use -EMSGSIZE, as callers would just retry with a new
+		 * skb.
+		 */
+		if (!try_get_page(page))
+			return -EFAULT;
 		skb_zcopy_downgrade_managed(skb);
-		get_page(page);
 		skb_fill_page_desc_noacc(skb, i, page, offset, size);
 	} else {
 		return -EMSGSIZE;
@@ -7575,8 +7580,9 @@ static void skb_splice_csum_page(struct sk_buff *skb, struct page *page,
  * possible, copying them to fragments if not possible (such as if they're slab
  * pages).
  *
- * Returns the amount of data spliced/copied or -EMSGSIZE if there's
- * insufficient space in the buffer to transfer anything.
+ * Returns the amount of data spliced/copied, -EMSGSIZE if there's
+ * insufficient space in the buffer to transfer anything, or -EFAULT if we
+ * could not take a page reference.
  */
 ssize_t skb_splice_from_iter(struct sk_buff *skb, struct iov_iter *iter,
 			     ssize_t maxsize)

-- 
2.56.0