Bug 2404107 (CVE-2025-39966)

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

iommufd: Fix race during abort for file descriptors

fput() doesn't actually call file_operations 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 iommufd_object
are tied to gether. The file has the object as it's private_data 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
__fput_sync() during abort to ensure that release() is called before
kfree. __fput_sync() 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 iommufd_object cannot be destroyed.

It is worth noting that close(); ioctl(IOMMU_DESTROY); 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 iommufd_eventq_fops_release+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>
     __dump_stack lib/dump_stack.c:94 [inline]
     dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
     print_address_description mm/kasan/report.c:378 [inline]
     print_report+0xcd/0x630 mm/kasan/report.c:482
     kasan_report+0xe0/0x110 mm/kasan/report.c:595
     check_region_inline mm/kasan/generic.c:183 [inline]
     kasan_check_range+0x100/0x1b0 mm/kasan/generic.c:189
     instrument_atomic_read_write include/linux/instrumented.h:96 [inline]
     atomic_fetch_sub_release include/linux/atomic/atomic-instrumented.h:400 [inline]
     __refcount_dec include/linux/refcount.h:455 [inline]
     refcount_dec include/linux/refcount.h:476 [inline]
     iommufd_eventq_fops_release+0x45/0xc0 drivers/iommu/iommufd/eventq.c:376
     __fput+0x402/0xb70 fs/file_table.c:468
     task_work_run+0x14d/0x240 kernel/task_work.c:227
     resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
     exit_to_user_mode_loop+0xeb/0x110 kernel/entry/common.c:43
     exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
     syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
     syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
     do_syscall_64+0x41c/0x4c0 arch/x86/entry/syscall_64.c:100
     entry_SYSCALL_64_after_hwframe+0x77/0x7f