Re: [PATCH] fork,memcg: fix crash in free_thread_stack on memcg charge fail

[Roman Gushchin]

On Fri, Dec 14, 2018 at 11:17:26PM -0500, Rik van Riel wrote:
> Changeset 9b6f7e163cd0 ("mm: rework memcg kernel stack accounting")
> will result in fork failing if allocating a kernel stack for a task
> in dup_task_struct exceeds the kernel memory allowance for that cgroup.
> 
> Unfortunately, it also results in a crash.
> 
> This is due to the code jumping to free_stack and calling free_thread_stack
> when the memcg kernel stack charge fails, but without tsk->stack pointing
> at the freshly allocated stack.
> 
> This in turn results in the vfree_atomic in free_thread_stack oopsing
> with a backtrace like this:
> 
> #5 [ffffc900244efc88] die at ffffffff8101f0ab
>  #6 [ffffc900244efcb8] do_general_protection at ffffffff8101cb86
>  #7 [ffffc900244efce0] general_protection at ffffffff818ff082
>     [exception RIP: llist_add_batch+7]
>     RIP: ffffffff8150d487  RSP: ffffc900244efd98  RFLAGS: 00010282
>     RAX: 0000000000000000  RBX: ffff88085ef55980  RCX: 0000000000000000
>     RDX: ffff88085ef55980  RSI: 343834343531203a  RDI: 343834343531203a
>     RBP: ffffc900244efd98   R8: 0000000000000001   R9: ffff8808578c3600
>     R10: 0000000000000000  R11: 0000000000000001  R12: ffff88029f6c21c0
>     R13: 0000000000000286  R14: ffff880147759b00  R15: 0000000000000000
>     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
>  #8 [ffffc900244efda0] vfree_atomic at ffffffff811df2c7
>  #9 [ffffc900244efdb8] copy_process at ffffffff81086e37
> #10 [ffffc900244efe98] _do_fork at ffffffff810884e0
> #11 [ffffc900244eff10] sys_vfork at ffffffff810887ff
> #12 [ffffc900244eff20] do_syscall_64 at ffffffff81002a43
>     RIP: 000000000049b948  RSP: 00007ffcdb307830  RFLAGS: 00000246
>     RAX: ffffffffffffffda  RBX: 0000000000896030  RCX: 000000000049b948
>     RDX: 0000000000000000  RSI: 00007ffcdb307790  RDI: 00000000005d7421
>     RBP: 000000000067370f   R8: 00007ffcdb3077b0   R9: 000000000001ed00
>     R10: 0000000000000008  R11: 0000000000000246  R12: 0000000000000040
>     R13: 000000000000000f  R14: 0000000000000000  R15: 000000000088d018
>     ORIG_RAX: 000000000000003a  CS: 0033  SS: 002b
> 
> The simplest fix is to assign tsk->stack right where it is allocated.

Good catch!

Acked-by: Roman Gushchin <guro@xxxxxx>

Thanks!