[RFC] potential UAF in kvm_spapr_tce_attach_iommu_group() (was Re: [PATCH 11/19] switch simple users of fdget() to CLASS(fd, ...))

[Michael Ellerman]

Linus Torvalds <torvalds at linux-foundation.org> writes:
> On Sun, 9 Jun 2024 at 19:45, Al Viro <viro at zeniv.linux.org.uk> wrote:
>>
>> Unless I'm misreading that code (entirely possible), this fdput() shouldn't
>> be done until we are done with stt.
>
> Ack. That looks right to me.
>
> If I follow it right, the lifetime of stt is tied to the lifetime of
> the file (plus RCU), so doing fdput early and then dropping the RCU
> lock means that stt may not be valid any more later.

Yep. I added a sleep after the fdput and was able to get KASAN to catch
it (below).

I'll send a fix patch tomorrow, just using fdput(), and then the CLASS
conversion can go on top later.

cheers


==================================================================
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:
[c00020008c2a7860] [c0000000027d4d50] dump_stack_lvl+0xb4/0x108 (unreliable)
[c00020008c2a78a0] [c00000000072dfa8] print_report+0x2b4/0x6ec
[c00020008c2a7990] [c00000000072d898] kasan_report+0x118/0x2b0
[c00020008c2a7aa0] [c00000000072ff38] __asan_load4+0xb8/0xd0
[c00020008c2a7ac0] [c00800001b343140] kvm_spapr_tce_attach_iommu_group+0x298/0x720 [kvm]
[c00020008c2a7b90] [c00800001b31d61c] kvm_vfio_set_attr+0x524/0xac0 [kvm]
[c00020008c2a7c60] [c00800001b3083ec] kvm_device_ioctl+0x144/0x240 [kvm]
[c00020008c2a7cd0] [c0000000007e052c] sys_ioctl+0x62c/0x1810
[c00020008c2a7df0] [c000000000038d90] system_call_exception+0x190/0x440
[c00020008c2a7e50] [c00000000000d15c] system_call_vectored_common+0x15c/0x2ec
--- interrupt: 3000 at 0x7fff8af5bedc
NIP:  00007fff8af5bedc LR: 00007fff8af5bedc CTR: 0000000000000000
REGS: c00020008c2a7e80 TRAP: 3000   Not tainted  (6.10.0-rc3-next-20240612-dirty)
MSR:  900000000280f033 <SF,HV,VEC,VSX,EE,PR,FP,ME,IR,DR,RI,LE>  CR: 44002482  XER: 00000000
IRQMASK: 0 
GPR00: 0000000000000036 00007fffda53b1f0 00007fff8b066d00 0000000000000006 
GPR04: 000000008018aee1 00007fffda53b270 0000000000000008 00007fff8ac0e9e0 
GPR08: 0000000000000006 0000000000000000 0000000000000000 0000000000000000 
GPR12: 0000000000000000 00007fff8b2ca540 0000000000000000 0000000000000000 
GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 
GPR20: 0000000000000000 0000000000000000 0000000000000000 00000000100101c0 
GPR24: 00007fff8b2bf840 00007fff8b2c0000 00007fffda53b728 0000000000000001 
GPR28: 00007fffda53b838 0000000000000006 0000000000000001 0000000000000005 
NIP [00007fff8af5bedc] 0x7fff8af5bedc
LR [00007fff8af5bedc] 0x7fff8af5bedc
--- interrupt: 3000

Allocated by task 2505:
 kasan_save_stack+0x48/0x80
 kasan_save_track+0x2c/0x50
 kasan_save_alloc_info+0x44/0x60
 __kasan_kmalloc+0xd0/0x120
 __kmalloc_noprof+0x214/0x670
 kvm_vm_ioctl_create_spapr_tce+0x10c/0x420 [kvm]
 kvm_arch_vm_ioctl+0x5fc/0x890 [kvm]
 kvm_vm_ioctl+0xa54/0x13d0 [kvm]
 sys_ioctl+0x62c/0x1810
 system_call_exception+0x190/0x440
 system_call_vectored_common+0x15c/0x2ec

Freed by task 0:
 kasan_save_stack+0x48/0x80
 kasan_save_track+0x2c/0x50
 kasan_save_free_info+0xac/0xd0
 __kasan_slab_free+0x120/0x210
 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

Last potentially related work creation:
 kasan_save_stack+0x48/0x80
 __kasan_record_aux_stack+0xcc/0x130
 __call_rcu_common.constprop.0+0x8c/0x8e0
 kvm_spapr_tce_release+0x29c/0xbc10 [kvm]
 __fput+0x22c/0x630
 sys_close+0x70/0xe0
 system_call_exception+0x190/0x440
 system_call_vectored_common+0x15c/0x2ec

The buggy address belongs to the object at c000200027552c00
 which belongs to the cache kmalloc-256 of size 256
The buggy address is located 48 bytes inside of
 freed 256-byte region [c000200027552c00, c000200027552d00)

The buggy address belongs to the physical page:
page: refcount:1 mapcount:0 mapping:0000000000000000 index:0xc000200027551800 pfn:0x20002755
flags: 0x83ffff800000000(node=8|zone=0|lastcpupid=0x7ffff)
page_type: 0xfdffffff(slab)
raw: 083ffff800000000 c000000007010d80 5deadbeef0000122 0000000000000000
raw: c000200027551800 0000000080800078 00000001fdffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 c000200027552b00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 c000200027552b80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>c000200027552c00: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                     ^
 c000200027552c80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 c000200027552d00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================
Disabling lock debugging due to kernel taint