Date: Thu, 31 Aug 2017 10:47:27 +0100 From: Mark Rutland <mark.rutland@....com> To: Juerg Haefliger <juerg.haefliger@...onical.com> Cc: Tycho Andersen <tycho@...ker.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, kernel-hardening@...ts.openwall.com, Marco Benatto <marco.antonio.780@...il.com> Subject: Re: [PATCH v5 04/10] arm64: Add __flush_tlb_one() On Thu, Aug 31, 2017 at 11:43:53AM +0200, Juerg Haefliger wrote: > On 08/30/2017 06:47 PM, Tycho Andersen wrote: > > On Wed, Aug 30, 2017 at 07:31:25AM +0200, Juerg Haefliger wrote: > >> > >> > >> On 08/23/2017 07:04 PM, Mark Rutland wrote: > >>> On Wed, Aug 23, 2017 at 10:58:42AM -0600, Tycho Andersen wrote: > >>>> Hi Mark, > >>>> > >>>> On Mon, Aug 14, 2017 at 05:50:47PM +0100, Mark Rutland wrote: > >>>>> That said, is there any reason not to use flush_tlb_kernel_range() > >>>>> directly? > >>>> > >>>> So it turns out that there is a difference between __flush_tlb_one() and > >>>> flush_tlb_kernel_range() on x86: flush_tlb_kernel_range() flushes all the TLBs > >>>> via on_each_cpu(), where as __flush_tlb_one() only flushes the local TLB (which > >>>> I think is enough here). > >>> > >>> That sounds suspicious; I don't think that __flush_tlb_one() is > >>> sufficient. > >>> > >>> If you only do local TLB maintenance, then the page is left accessible > >>> to other CPUs via the (stale) kernel mappings. i.e. the page isn't > >>> exclusively mapped by userspace. > >> > >> We flush all CPUs to get rid of stale entries when a new page is > >> allocated to userspace that was previously allocated to the kernel. > >> Is that the scenario you were thinking of? > > > > I think there are two cases, the one you describe above, where the > > pages are first allocated, and a second one, where e.g. the pages are > > mapped into the kernel because of DMA or whatever. In the case you > > describe above, I think we're doing the right thing (which is why my > > test worked correctly, because it tested this case). > > > > In the second case, when the pages are unmapped (i.e. the kernel is > > done doing DMA), do we need to flush the other CPUs TLBs? I think the > > current code is not quite correct, because if multiple tasks (CPUs) > > map the pages, only the TLB of the last one is flushed when the > > mapping is cleared, because the tlb is only flushed when ->mapcount > > drops to zero, leaving stale entries in the other TLBs. It's not clear > > to me what to do about this case. > > For this to happen, multiple CPUs need to have the same userspace page > mapped at the same time. Is this a valid scenario? I believe so. I think you could trigger that with a multi-threaded application running across several CPUs. All those threads would share the same page tables. Thanks, Mark.
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