Re: [PATCH V4] scsi_debugfs: fix crash in scsi_show_rq()

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2017-11-14 at 08:55 +0800, Ming Lei wrote:
> Hi James,
> 
> On Mon, Nov 13, 2017 at 10:55:52AM -0800, James Bottomley wrote:
> > 
> > On Sat, 2017-11-11 at 10:43 +0800, Ming Lei wrote:
> > > 
> > > So from CPU1's review, cmd->cmnd is in a remote NUMA node,
> > > __scsi_format_command() is executed much slower than
> > > mempool_free().
> > > So when mempool_free() returns, __scsi_format_command() may not
> > > fetched the buffer in L1 cache yet, then use-after-free
> > > is still triggered.
> > > 
> > > That is why I say this use-after-free is inevitable no matter
> > > 'setting SCpnt->cmnd to NULL before calling mempool_free()' or
> > > not.
> > 
> > The bottom line is that there are several creative ways around this
> > but the proposed code is currently broken and simply putting a
> > comment in saying so doesn't make it acceptable.
> 
> As I explained above, I didn't see one really workable way. Or please
> correct it if I am wrong.

I simply can't believe it's beyond the wit of man to solve a use after
free race.  About 40% of kernel techniques are devoted to this.  All I
really care about is not losing the PI information we previously had.
 I agree with Bart that NULL cmnd is a good indicator, so it seems
reasonable to use it.  If you have another mechanism, feel free to
propose it.

James