CVE-2026-74646: misc: fastrpc: take fl->lock when moving mmaps on interrupted invoke

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

misc: fastrpc: take fl->lock when moving mmaps on interrupted invoke

When an invoke is interrupted by a signal, waitforcompletioninterruptible() returns -ERESTARTSYS and fastrpcinternalinvoke() moves every buffer from fl->mmaps onto cctx->invokeinterruptedmmaps. This listdel()/listaddtail() walk runs without holding fl->lock, the lock that serialises fl->mmaps in fastrpcreqmmap() and fastrpcreqmunmap() everywhere else.

Take fl->lock around the move, matching every other fl->mmaps accessor.