From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ming Lei Subject: Re: [PATCH V4] scsi_debugfs: fix crash in scsi_show_rq() Date: Wed, 15 Nov 2017 20:04:50 +0800 Message-ID: <20171115120449.GD908@ming.t460p> References: <20171110090144.11791-1-ming.lei@redhat.com> <1510332718.3095.2.camel@HansenPartnership.com> <20171111024337.GB20382@ming.t460p> <1510599352.3336.20.camel@HansenPartnership.com> <20171114005544.GA20889@ming.t460p> <1510683292.3077.3.camel@HansenPartnership.com> <20171115100949.GC908@ming.t460p> <1510741680.3088.1.camel@HansenPartnership.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Return-path: Content-Disposition: inline In-Reply-To: <1510741680.3088.1.camel@HansenPartnership.com> Sender: linux-block-owner@vger.kernel.org To: James Bottomley Cc: Jens Axboe , "Martin K . Petersen" , Christoph Hellwig , linux-scsi@vger.kernel.org, linux-block@vger.kernel.org, John Garry , Bart Van Assche , Omar Sandoval , Hannes Reinecke List-Id: linux-scsi@vger.kernel.org On Wed, Nov 15, 2017 at 07:28:00PM +0900, James Bottomley wrote: > 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. Hi James, Given we can't avoid the use-after-free, how about not do that way so we won't lose the precious debug info too early? Thanks, Ming