Re: [PATCH] x86/alternative: fix race in try_get_desc()

[Nadav Amit <namit@vmware.com>]

On Sep 21, 2022, at 12:52 AM, Peter Zijlstra <peterz@infradead.org> wrote:

> ⚠ External Email
> 
> On Tue, Sep 20, 2022 at 10:47:43PM +0000, Nadav Amit wrote:
> 
>> Fix this issue with small backportable patch. Instead of trying to make
>> RCU-like behavior for bp_desc, just eliminate the unnecessary level of
>> indirection of bp_desc, and hold the whole descriptor on the stack.
>> Anyhow, there is only a single descriptor at any given moment.
> 
> Because of text_mutex; indeed. No idea why I put that thing on the
> stack.
> 
> I've done a few minor edits to your patch, but it otherwise looks good
> to me.
> 
> ---
> Subject: x86/alternative: Fix race in try_get_desc()
> From: Nadav Amit <namit@vmware.com>
> Date: Tue, 20 Sep 2022 22:47:43 +0000
> 
> From: Nadav Amit <namit@vmware.com>
> 
> The text poke mechanism claims to have an RCU-like behavior, but it does
> not appear that there is any quiescent state to ensure that nobody holds
> reference to desc. As a result, the following race appears to be
> possible, which can lead to memory corruption.
> 
>  CPU0                                  CPU1
>  ----                                  ----
>  text_poke_bp_batch()
>  -> smp_store_release(&bp_desc, &desc)
> 
>  [ notice that desc is on
>    the stack                   ]
> 
>                                        poke_int3_handler()
> 
>                                        [ int3 might be kprobe's
>                                          so sync events are do not
>                                          help ]
> 
>                                        -> try_get_desc(descp=&bp_desc)
>                                           desc = __READ_ONCE(bp_desc)
> 
>                                           if (!desc) [false, success]
>  WRITE_ONCE(bp_desc, NULL);
>  atomic_dec_and_test(&desc.refs)
> 
>  [ success, desc space on the stack
>    is being reused and might have
>    non-zero value. ]
>                                        arch_atomic_inc_not_zero(&desc->refs)
> 
>                                        [ might succeed since desc points to
>                                          stack memory that was freed and might
>                                          be reused. ]
> 
> I encountered some occasional crashes of poke_int3_handler() when
> kprobes are set, while accessing desc->vec. The analysis has been done
> offline and I did not corroborate the cause of the crashes. Yet, it
> seems that this race might be the root cause.
> 
> Fix this issue with small backportable patch. Instead of trying to make
> RCU-like behavior for bp_desc, just eliminate the unnecessary level of
> indirection of bp_desc, and hold the whole descriptor on the stack.

Looks good to me. Thanks for improving my patch.

I just made one small mistake in commit message. It should say “hold
the whole descriptor as a global” in the line above.

I will send v2 with your changes and the updated commit message.

Thanks again.