[PATCH bpf] net: return page_pool frags to the pool in bpf_xdp_shrink_data()

bpf_xdp_shrink_data() releases a fully-shrunk frag through
__xdp_return(netmem, mem_type, false, zc_frag), selecting the freer from
the rxq's mem type.  Under generic XDP the rxq's xdp_rxq_info often has no
mem model registered (e.g. loopback), so mem_type stays
MEM_TYPE_PAGE_SHARED == 0 even though the frag was allocated from a page
pool by skb_pp_cow_data().  __xdp_return() then frees the page through
page_frag_free() instead of page_pool_put_full_netmem(), so the page is
returned to the buddy allocator while its page_pool accounting is still
live:

  BUG: Bad page state in process A-146-S3_S1  pfn:4694b
  page dumped because: page_pool leak
   page_frag_free+0xf0/0x11c
   __xdp_return+0x160/0x640
   bpf_xdp_adjust_tail+0x5a4/0xbac
   bpf_prog_run_generic_xdp+0x4a8/0x1204

Before falling back to __xdp_return(), try the page_pool path: if the
frag really is a page_pool page, napi_pp_put_page() recycles it through
the pool and we are done.

Fixes: 4dce1a0d7cf3 ("bpf: Support pulling non-linear xdp data")
Assisted-by: LLM
Signed-off-by: Dudu Lu <redacted>
---
Tested base
-----------
Detected on Linux 7.3-rc4 (commit 93f51579e7df), arm64 QEMU guest.
Re-checked against mainline f47d223627f8 (v7.3-341): the affected code
is unchanged between the two, so the bug is still present there.
Checked against linux-next next-20261021 (commit 9a482184d910): the
affected code is unchanged there as well, so the bug is still present in
linux-next too.

Reproducer status
-----------------
A reproducer was written and executed on an arm64 QEMU guest (generic XDP on
loopback); the kernel reported "BUG: Bad page state ... page_pool leak".  The
reproducer source is not posted here per Documentation/process/security-bugs.rst;
it is available on request.

Triggering conditions
---------------------
The ability to load a generic-XDP program (CAP_BPF / CAP_NET_ADMIN) on a
device whose rxq has no mem model but whose skbs are page_pool-backed (loopback
via skb_cow_data_for_xdp()).

Notes
-----
Built and checkpatch-clean: this patch was applied on top of 7.3-rc4
(commit 93f51579e7df) and a full arm64 kernel (defconfig) was built with
it -- no new warnings or errors.  scripts/checkpatch.pl reports 0 errors,
0 warnings.

 net/core/filter.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index 70dc621672f2..94bc367cee12 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c

@@ -36,6 +36,7 @@
 #include <net/protocol.h>
 #include <net/netlink.h>
 #include <linux/skbuff.h>
+#include <linux/skbuff_ref.h>
 #include <linux/skmsg.h>
 #include <net/sock.h>
 #include <net/flow_dissector.h>

@@ -4380,6 +4381,18 @@ static bool bpf_xdp_shrink_data(struct xdp_buff *xdp, skb_frag_t *frag,
 	}
 
 	if (release) {
+		/* The frag may be page_pool-backed even when the rxq's
+		 * xdp_rxq_info carries no mem model (generic XDP on a
+		 * driver that never registers one, e.g. loopback).  In that
+		 * case mem_type is MEM_TYPE_PAGE_SHARED and __xdp_return()
+		 * would free the page through page_frag_free(), leaving the
+		 * page_pool's accounting dangling -- "page_pool leak".  Route
+		 * page_pool pages back through the pool instead.
+		 */
+		if (IS_ENABLED(CONFIG_PAGE_POOL) &&
+		    mem_type == MEM_TYPE_PAGE_SHARED && napi_pp_put_page(netmem))
+			return release;
+
 		__xdp_return(netmem, mem_type, false, zc_frag);
 	} else {
 		if (!tail)