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Date: Mon, 21 Dec 2015 14:52:40 -0800 From: Andy Lutomirski <luto@...capital.net> To: Dave Hansen <dave.hansen@...ux.intel.com> Cc: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...nel.org>, Christoph Hellwig <hch@....de>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Brian Gerst <brgerst@...il.com>, "H. Peter Anvin" <hpa@...or.com>, Linus Torvalds <torvalds@...ux-foundation.org> Subject: Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit? On Dec 22, 2015 2:04 AM, "Dave Hansen" <dave.hansen@...ux.intel.com> wrote: > > On 12/18/2015 03:16 PM, Andy Lutomirski wrote: > > Hrm. We might also want an option to change pkru and/or baseline_pkru > > in all threads in the current mm. That's optional but it could be > > handy. Maybe it would be as simple as having the allocate-a-pkey call > > have an option to set an initial baseline value and an option to > > propagate that initial value to pre-existing threads. > > Do you mean actively going in and changing PKRU in other threads? I > fear that will be dangerous. > > IMNHO, whatever we do, I think we need to ensure that _raw_ PKRU calls > are allowed (somehow). Raw in this case would mean a thread calling > WRPKRU without a system call and without checking in with what any other > threads are doing. > > Let's say baseline_pkru=0x004 (we're access-disabling PKEY[1] and using > it for execute-only). Now, a thread is trying to do this: > > pkey2 = sys_pkey_alloc(); // now pkey2=2 > tmp = rdpkru(); // 0x004 > tmp |= 0x10; // set PKRU[2].AD=1 > wrpkru(tmp); > > While another thread does: > > pkey4 = pkey_alloc(); // pkey4=4 > sys_pkey_set(pkey4, ACCESS_DISABLE, SET_BASELINE_ALL_THREADS); > > Without some kind of locking, that's going to race. We could do all the > locking in the kernel, but that requires that the kernel do all the PKRU > writing, which I'd really like to avoid. > > I think the closest we can get reasonably is to have the kernel track > the baseline_pkru and then allow userspace to query it in case userspace > decides that thread needs to update its thread-local PKRU from the baseline. Yeah, fair point. Let's skip the modify-other-threads thing. Perhaps this is silly, but what if the default were changed to deny reads and writes for unallocated keys? Is there a use case that breaks? --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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