Re: Possible race in copy of fpu->state in copy_process against the exeve'ing parent?

[Ingo Molnar <mingo@kernel.org>]

* Jianyu Zhan <nasa4836@gmail.com> wrote:

> Hi,
> 
> I encountered a panic on a Linux-3.2 kernel on a x86_64 machine, and
> suspect it is a race condition.  And I checked the current mainline
> and found it was fixed unintendedly.
> 
> So I hope x86/fpu maintainer help verify this.  Thanks verfy much.
> 
> 
> The panic stack trace :
> 
>  #0 [ffff88529d33f990] try_crashdump at ffffffff8105b8ca
>  #1 [ffff88529d33f9a0] dump_on_panic at ffffffff8105b965
>  #2 [ffff88529d33fa60] notifier_call_chain at ffffffff8139f784
>  #3 [ffff88529d33fac0] atomic_notifier_call_chain at ffffffff8139f81d
>  #4 [ffff88529d33fad0] panic at ffffffff8139971c
>  #5 [ffff88529d33fb50] oops_end at ffffffff8139d34a
>  #6 [ffff88529d33fb80] no_context at ffffffff81021569
>  #7 [ffff88529d33fbd0] __bad_area_nosemaphore at ffffffff81021730
>  #8 [ffff88529d33fc20] bad_area at ffffffff810217ac
>  #9 [ffff88529d33fc50] do_page_fault at ffffffff8139f509
> #10 [ffff88529d33fd70] page_fault at ffffffff8139caef
>     [exception RIP: prepare_to_copy+35]
> <------------------ PANIC !!!
>     RIP: ffffffff810013f4  RSP: ffff88529d33fe20  RFLAGS: 00010286
>     RAX: 00000000ffffffff  RBX: 0000000001200011  RCX: ffff884fe73f6320
>     RDX: 00000000ffffffff  RSI: 00007fff07d36bd0  RDI: 0000000000000000
>     RBP: ffff88529d33fe20   R8: 00007f5c4a209770   R9: 0000000000000000
>     R10: 00007f5c4a209770  R11: 0000000000000202  R12: 0000000000000000
>     R13: 0000000000000000  R14: ffff884fe73f6320  R15: 0000000000000001
>     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
> #11 [ffff88529d33fe28] copy_process at ffffffff81038211
> #12 [ffff88529d33fea8] do_fork at ffffffff810393ec
> #13 [ffff88529d33ff38] sys_clone at ffffffff81009118
> #14 [ffff88529d33ff48] stub_clone at ffffffff813a31d3
> 
> 
> crash7> dis -r prepare_to_copy+35
> 0xffffffff810013d1 <prepare_to_copy>:   push   %rbp
> 0xffffffff810013d2 <prepare_to_copy+1>: mov    %rsp,%rbp
> 0xffffffff810013d5 <prepare_to_copy+4>: nopl   0x0(%rax,%rax,1)
> 0xffffffff810013da <prepare_to_copy+9>: mov    %rdi,%rcx
> 0xffffffff810013dd <prepare_to_copy+12>:        cmpl   $0x0,0x4d8(%rdi)
> 0xffffffff810013e4 <prepare_to_copy+19>:        je
> 0xffffffff8100142e <prepare_to_copy+93>
> 0xffffffff810013e6 <prepare_to_copy+21>:        mov    0x4e0(%rdi),%rdi
> 0xffffffff810013ed <prepare_to_copy+28>:        xchg   %ax,%ax
> 0xffffffff810013ef <prepare_to_copy+30>:        or     $0xffffffff,%eax
> 0xffffffff810013f2 <prepare_to_copy+33>:        mov    %eax,%edx
> 0xffffffff810013f4 <prepare_to_copy+35>:        xsaveopt64 (%rdi)
> <---- PANIC HERE
> 
> when panic the %rdi is 0x0000000000000000, which is fpu->state.
> 
> 
> 
> So  I suspect there is a possible race:
> 
> 
>    Parent:
> 
>        sys_execve
>          do_execve
>            do_execve_common
>              search_binary_handler
>                 load_elf_binary
>                   start_thread
>                     start_thread_common
>                        free_thread_xstate(current)
>                          fpu_free
>                             fpu->state = NULL
> 
> 
>     Child:
> 
>         sys_clone
>           do_fork
>              copy_process
>                 dup_task_struct
>                    prepare_to_copy
>                       unlazy_fpu
>                          __save_init_fpu
>                            fpu_save_init
>                              fpu_xsave(fpu)   <---- fpu->sate is NULL,
> so cause a
>                                                                 NULL
> dereference.
> 
> Scenario:  Parent is still exeve'ing,  and just set fpu->state to NULL,
> and the a concurrent clone() forks a Child and in which  fpu_xsave()
> tries to fpu_xsave, when fpu->state is NULL.
> 
> The race window seems quite small, and I have checked the Parent's
> 'sum_exec_runtime' is 536920255(~0.53s).
> 
> I checked the mainline, and found commit 304bceda6a18(" x86, fpu: use
> non-lazy fpu restore for processors supporting xsave") seems
> unintendedly fix this?

So I'm not sure I understand the suggested race. Separate tasks have separate 
fpu->state states, so a parallel execve() and clone() has no effect on each other. 
There's no FPU state sharing.

Thanks,

	Ingo