CVE-2025-39966: iommufd: Fix race during abort for file descriptors

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

iommufd: Fix race during abort for file descriptors

fput() doesn't actually call fileoperations release() synchronously, it puts the file on a work queue and it will be released eventually.

This is normally fine, except for iommufd the file and the iommufdobject are tied to gether. The file has the object as it's privatedata and holds a users refcount, while the object is expected to remain alive as long as the file is.

When the allocation of a new object aborts before installing the file it will fput() the file and then go on to immediately kfree() the obj. This causes a UAF once the workqueue completes the fput() and tries to decrement the users refcount.

Fix this by putting the core code in charge of the file lifetime, and call fputsync() during abort to ensure that release() is called before kfree. fputsync() is a bit too tricky to open code in all the object implementations. Instead the objects tell the core code where the file pointer is and the core will take care of the life cycle.

If the object is successfully allocated then the file will hold a users refcount and the iommufdobject cannot be destroyed.

It is worth noting that close(); ioctl(IOMMUDESTROY); doesn't have an issue because close() is already using a synchronous version of fput().

The UAF looks like this:

BUG: KASAN: slab-use-after-free in iommufdeventqfopsrelease+0x45/0xc0 drivers/iommu/iommufd/eventq.c:376 Write of size 4 at addr ffff888059c97804 by task syz.0.46/6164

CPU: 0 UID: 0 PID: 6164 Comm: syz.0.46 Not tainted syzkaller #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025 Call Trace: <TASK> dumpstack lib/dumpstack.c:94 [inline] dumpstacklvl+0x116/0x1f0 lib/dumpstack.c:120 printaddressdescription mm/kasan/report.c:378 [inline] printreport+0xcd/0x630 mm/kasan/report.c:482 kasanreport+0xe0/0x110 mm/kasan/report.c:595 checkregioninline mm/kasan/generic.c:183 [inline] kasancheckrange+0x100/0x1b0 mm/kasan/generic.c:189 instrumentatomicreadwrite include/linux/instrumented.h:96 [inline] atomicfetchsubrelease include/linux/atomic/atomic-instrumented.h:400 [inline] refcountdec include/linux/refcount.h:455 [inline] refcountdec include/linux/refcount.h:476 [inline] iommufdeventqfopsrelease+0x45/0xc0 drivers/iommu/iommufd/eventq.c:376 fput+0x402/0xb70 fs/filetable.c:468 taskworkrun+0x14d/0x240 kernel/taskwork.c:227 resumeusermodework include/linux/resumeusermode.h:50 [inline] exittousermodeloop+0xeb/0x110 kernel/entry/common.c:43 exittousermodeprepare include/linux/irq-entry-common.h:225 [inline] syscallexittousermodework include/linux/entry-common.h:175 [inline] syscallexittousermode include/linux/entry-common.h:210 [inline] dosyscall64+0x41c/0x4c0 arch/x86/entry/syscall64.c:100 entrySYSCALL64afterhwframe+0x77/0x7f