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