CVE-2026-64061: netfs: Fix early put of sink folio in netfs_read_gaps()

In the Linux kernel, the following vulnerability has been resolved:

netfs: Fix early put of sink folio in netfsreadgaps()

Fix netfsreadgaps() to release the sink page it uses after waiting for the request to complete. The way the sink page is used is that an ITERBVEC-class iterator is created that has the gaps from the target folio at either end, but has the sink page tiled over the middle so that a single read op can fill in both gaps.

The bug was found by KASAN detecting a UAF on the generic/075 xfstest in the cifsd kernel thread that handles reception of data from the TCP socket:

BUG: KASAN: use-after-free in copytoiter+0x48a/0xa20 Write of size 885 at addr ffff888107f92000 by task cifsd/1285 CPU: 2 UID: 0 PID: 1285 Comm: cifsd Not tainted 7.0.0 #6 PREEMPT(lazy) Call Trace: dumpstacklvl+0x5d/0x80 printreport+0x17f/0x4f1 kasanreport+0x100/0x1e0 kasancheckrange+0x10f/0x1e0 asanmemcpy+0x3c/0x60 copytoiter+0x48a/0xa20 skbdatagramiter+0x2c9/0x430 skbcopydatagramiter+0x6e/0x160 tcprecvmsglocked+0xce0/0x1130 tcprecvmsg+0xeb/0x300 inetrecvmsg+0xcf/0x3a0 sockrecvmsg+0xea/0x100 cifsreadvfromsocket+0x3a6/0x4d0 [cifs] cifsreaditerfromsocket+0xdd/0x130 [cifs] cifsreadvreceive+0xaad/0xb10 [cifs] cifsdemultiplexthread+0x1148/0x1740 [cifs] kthread+0x1cf/0x210