CVE-2024-41070

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

KVM: PPC: Book3S HV: Prevent UAF in kvm_spapr_tce_attach_iommu_group()

Al reported a possible use-after-free (UAF) in kvm_spapr_tce_attach_iommu_group().

It looks up `stt` from tablefd, but then continues to use it after doing

fdput() on the returned fd. After the fdput() the tablefd is free to be

closed by another thread. The close calls kvm_spapr_tce_release() and

then release_spapr_tce_table() (via call_rcu()) which frees `stt`.

Although there are calls to rcu_read_lock() in

kvm_spapr_tce_attach_iommu_group() they are not sufficient to prevent

the UAF, because `stt` is used outside the locked regions.

With an artifcial delay after the fdput() and a userspace program which

triggers the race, KASAN detects the UAF:

BUG: KASAN: slab-use-after-free in kvm_spapr_tce_attach_iommu_group+0x298/0x720 [kvm]

Read of size 4 at addr c000200027552c30 by task kvm-vfio/2505

CPU: 54 PID: 2505 Comm: kvm-vfio Not tainted 6.10.0-rc3-next-20240612-dirty #1

Hardware name: 8335-GTH POWER9 0x4e1202 opal:skiboot-v6.5.3-35-g1851b2a06 PowerNV

Call Trace:

dump_stack_lvl+0xb4/0x108 (unreliable)

print_report+0x2b4/0x6ec

kasan_report+0x118/0x2b0

__asan_load4+0xb8/0xd0

kvm_spapr_tce_attach_iommu_group+0x298/0x720 [kvm]

kvm_vfio_set_attr+0x524/0xac0 [kvm]

kvm_device_ioctl+0x144/0x240 [kvm]

sys_ioctl+0x62c/0x1810

system_call_exception+0x190/0x440

system_call_vectored_common+0x15c/0x2ec

...

Freed by task 0:

...

kfree+0xec/0x3e0

release_spapr_tce_table+0xd4/0x11c [kvm]

rcu_core+0x568/0x16a0

handle_softirqs+0x23c/0x920

do_softirq_own_stack+0x6c/0x90

do_softirq_own_stack+0x58/0x90

__irq_exit_rcu+0x218/0x2d0

irq_exit+0x30/0x80

arch_local_irq_restore+0x128/0x230

arch_local_irq_enable+0x1c/0x30

cpuidle_enter_state+0x134/0x5cc

cpuidle_enter+0x6c/0xb0

call_cpuidle+0x7c/0x100

do_idle+0x394/0x410

cpu_startup_entry+0x60/0x70

start_secondary+0x3fc/0x410

start_secondary_prolog+0x10/0x14

Fix it by delaying the fdput() until `stt` is no longer in use, which

is effectively the entire function. To keep the patch minimal add a call

to fdput() at each of the existing return paths. Future work can convert

the function to goto or __cleanup style cleanup.

With the fix in place the test case no longer triggers the UAF.