On Tue, 2018-08-28 at 20:46 -0700, Andy Lutomirski wrote: > On Tue, Aug 28, 2018 at 10:56 AM, Rik van Riel <riel@xxxxxxxxxxx> > wrote: > > On Mon, 27 Aug 2018 16:04:16 -0700 > > Andy Lutomirski <luto@xxxxxxxxxx> wrote: > > > > > The 0day bot is still chewing on this, but I've tested it a bit > > > locally > > > and it seems to do the right thing. > > > > Hi Andy, > > > > the version of the patch below should fix the bug we talked about > > in email yesterday. It should automatically cover kernel threads > > in lazy TLB mode, because current->mm will be NULL, while the > > cpu_tlbstate.loaded_mm should never be NULL. > > > > That's better than mine. I tweaked it a bit and added some > debugging, > and I got this: > > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/fixes&id=dd956eba16646fd0b15c3c0741269dfd84452dac > > I made the loaded_mm handling a little more conservative to make it > more obvious that switch_mm_irqs_off() is safe regardless of exactly > when it gets called relative to switching current. I am not convinced that the dance of writing cpu_tlbstate.loaded_mm twice, with a barrier on each end, is useful or necessary. At the time switch_mm_irqs_off returns, nmi_uaccess_ok() will still return false, because we have not switched "current" to the task that owns the next mm_struct yet. We just have to make sure to: 1) Change cpu_tlbstate.loaded_mm before we manipulate CR3, and 2) Change "current" only once enough of the mm stuff has been switched, __switch_to seems to get that right. Between the time switch_mm_irqs_off() sets cpu_tlbstate to the next mm, and __switch_to moves() over current, nmi_uaccess_ok() will return false. -- All Rights Reversed.
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