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

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

On Wed, 2017-11-15 at 18:09 +0800, Ming Lei wrote:
> 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.

Fine, so lose the snide comment and set the cmnd to NULL *before*
calling free so we narrow the race window.

James