TLDRVuln - Cybersecurity Vulnerabilities Made Simple

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

KVM: arm64: Unregister redistributor for failed vCPU creation

Alex reports that syzkaller has managed to trigger a use-after-free when

tearing down a VM:

BUG: KASAN: slab-use-after-free in kvm_put_kvm+0x300/0xe68 virt/kvm/kvm_main.c:5769

Read of size 8 at addr ffffff801c6890d0 by task syz.3.2219/10758

CPU: 3 UID: 0 PID: 10758 Comm: syz.3.2219 Not tainted 6.11.0-rc6-dirty #64

Hardware name: linux,dummy-virt (DT)

Call trace:

dump_backtrace+0x17c/0x1a8 arch/arm64/kernel/stacktrace.c:317

show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:324

__dump_stack lib/dump_stack.c:93 [inline]

dump_stack_lvl+0x94/0xc0 lib/dump_stack.c:119

print_report+0x144/0x7a4 mm/kasan/report.c:377

kasan_report+0xcc/0x128 mm/kasan/report.c:601

__asan_report_load8_noabort+0x20/0x2c mm/kasan/report_generic.c:381

kvm_put_kvm+0x300/0xe68 virt/kvm/kvm_main.c:5769

kvm_vm_release+0x4c/0x60 virt/kvm/kvm_main.c:1409

__fput+0x198/0x71c fs/file_table.c:422

____fput+0x20/0x30 fs/file_table.c:450

task_work_run+0x1cc/0x23c kernel/task_work.c:228

do_notify_resume+0x144/0x1a0 include/linux/resume_user_mode.h:50

el0_svc+0x64/0x68 arch/arm64/kernel/entry-common.c:169

el0t_64_sync_handler+0x90/0xfc arch/arm64/kernel/entry-common.c:730

el0t_64_sync+0x190/0x194 arch/arm64/kernel/entry.S:598

Upon closer inspection, it appears that we do not properly tear down the

MMIO registration for a vCPU that fails creation late in the game, e.g.

a vCPU w/ the same ID already exists in the VM.

It is important to consider the context of commit that introduced this bug

by moving the unregistration out of __kvm_vgic_vcpu_destroy(). That

change correctly sought to avoid an srcu v. config_lock inversion by

breaking up the vCPU teardown into two parts, one guarded by the

config_lock.

Fix the use-after-free while avoiding lock inversion by adding a

special-cased unregistration to __kvm_vgic_vcpu_destroy(). This is safe

because failed vCPUs are torn down outside of the config_lock.