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

[Ming Lei <ming.lei@redhat.com>]

On Tue, Nov 14, 2017 at 10:14:52AM -0800, James Bottomley wrote:
> 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.

Hi James,

This patch is my proposal, no others thought of yet.

We can fix the use-after-free easily via lock, rcu and ..., but some cost
has to pay. In this case, we can't wait too long in show_rq(), otherwise
we may lose important debug info, so I do not have better way.

IMO this use-after-free is actually no harm, I don't think we have to fix
it, but it should be better to not let utility warn on this case.


Thanks,
Ming