From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Tue, 30 Oct 2018 16:58:41 +0100 From: Peter Zijlstra Subject: Re: [PATCH 16/17] prmem: pratomic-long Message-ID: <20181030155841.GF8177@hirez.programming.kicks-ass.net> References: <20181023213504.28905-1-igor.stoppa@huawei.com> <20181023213504.28905-17-igor.stoppa@huawei.com> <20181025001312.GA3159@worktop.c.hoisthospitality.com> <806bfff0-8d62-9918-480d-ec791b93841e@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <806bfff0-8d62-9918-480d-ec791b93841e@gmail.com> To: Igor Stoppa Cc: Mimi Zohar , Kees Cook , Matthew Wilcox , Dave Chinner , James Morris , Michal Hocko , kernel-hardening@lists.openwall.com, linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org, igor.stoppa@huawei.com, Dave Hansen , Jonathan Corbet , Laura Abbott , Will Deacon , Boqun Feng , Arnd Bergmann , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org List-ID: On Mon, Oct 29, 2018 at 11:17:14PM +0200, Igor Stoppa wrote: > > > On 25/10/2018 01:13, Peter Zijlstra wrote: > > On Wed, Oct 24, 2018 at 12:35:03AM +0300, Igor Stoppa wrote: > > > +static __always_inline > > > +bool __pratomic_long_op(bool inc, struct pratomic_long_t *l) > > > +{ > > > + struct page *page; > > > + uintptr_t base; > > > + uintptr_t offset; > > > + unsigned long flags; > > > + size_t size = sizeof(*l); > > > + bool is_virt = __is_wr_after_init(l, size); > > > + > > > + if (WARN(!(is_virt || likely(__is_wr_pool(l, size))), > > > + WR_ERR_RANGE_MSG)) > > > + return false; > > > + local_irq_save(flags); > > > + if (is_virt) > > > + page = virt_to_page(l); > > > + else > > > + vmalloc_to_page(l); > > > + offset = (~PAGE_MASK) & (uintptr_t)l; > > > + base = (uintptr_t)vmap(&page, 1, VM_MAP, PAGE_KERNEL); > > > + if (WARN(!base, WR_ERR_PAGE_MSG)) { > > > + local_irq_restore(flags); > > > + return false; > > > + } > > > + if (inc) > > > + atomic_long_inc((atomic_long_t *)(base + offset)); > > > + else > > > + atomic_long_dec((atomic_long_t *)(base + offset)); > > > + vunmap((void *)base); > > > + local_irq_restore(flags); > > > + return true; > > > + > > > +} > > > > That's just hideously nasty.. and horribly broken. > > > > We're not going to duplicate all these kernel interfaces wrapped in gunk > > like that. > > one possibility would be to have macros which use typeof() on the parameter > being passed, to decide what implementation to use: regular or write-rare > > This means that type punning would still be needed, to select the > implementation. > > Would this be enough? Is there some better way? Like mentioned elsewhere; if you do write_enable() + write_disable() thingies, it all becomes: write_enable(); atomic_foo(&bar); write_disable(); No magic gunk infested duplication at all. Of course, ideally you'd then teach objtool about this (or a GCC plugin I suppose) to ensure any enable reached a disable. The alternative is something like: #define ALLOW_WRITE(stmt) do { write_enable(); do { stmt; } while (0); write_disable(); } while (0) which then allows you to write: ALLOW_WRITE(atomic_foo(&bar)); No duplication. > > Also, you _cannot_ call vunmap() with IRQs disabled. Clearly > > you've never tested this with debug bits enabled. > > I thought I had them. And I _did_ have them enabled, at some point. > But I must have messed up with the configuration and I failed to notice > this. > > I can think of a way it might work, albeit it's not going to be very pretty: > > * for the vmap(): if I understand correctly, it might sleep while obtaining > memory for creating the mapping. This part could be executed before > disabling interrupts. The rest of the function, instead, would be executed > after interrupts are disabled. > > * for vunmap(): after the writing is done, change also the alternate mapping > to read only, then enable interrupts and destroy the alternate mapping. > Making also the secondary mapping read only makes it equally secure as the > primary, which means that it can be visible also with interrupts enabled. That doesn't work if you wanted to do this write while you already have IRQs disabled for example.