On Wed, Nov 15, 2017 at 08:04:49PM +0800, Ming Lei wrote: > 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? Hi James, Are you fine with V4? As I explained, the use-after-free can't be avoided, we have to make scsi_show_rq() to survive that, so we don't need to touch code in free path. Also we won't lose debug info too early in this way, not like 'set the cmnd to NULL *before* calling free'. Thanks, Ming
