From: Dave Hansen <dave.hansen@intel.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
Dan Williams <dan.j.williams@intel.com>
Cc: "Luck, Tony" <tony.luck@intel.com>,
Andy Lutomirski <luto@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Borislav Petkov <bp@alien8.de>, stable <stable@vger.kernel.org>,
the arch/x86 maintainers <x86@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>,
Paul Mackerras <paulus@samba.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Erwin Tsaur <erwin.tsaur@intel.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
linux-nvdimm <linux-nvdimm@lists.01.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe()
Date: Fri, 1 May 2020 13:17:32 -0700 [thread overview]
Message-ID: <b1dbb13d-f812-684f-2892-5004674db0f7@intel.com> (raw)
In-Reply-To: <CAHk-=wiPkwF2+y6wZd=VD9BooKxHRWhSVW8dr+WSeeSPkJk7kQ@mail.gmail.com>
On 5/1/20 11:28 AM, Linus Torvalds wrote:
> Plus on x86 you can't reasonably even have different code sequences
> for that case, because CLAC/STAC don't have a "enable users read
> accesses" vs "write accesses" case. It's an all-or-nothing "enable
> user faults".
>
> We _used_ to have a difference on x86, back when we did the whole "fs
> segment points to user space".
Protection keys might give us _some_ of this back. If we're doing a
copy_from_user(), we could (logically) do:
stac()
save_pkru()
pkru |= ~0x55555555
... do userspace read
restore_pkru()
clac()
That *should* generate a fault if we try to write to userspace in there
because PKRU affects all user *addresses* (PTEs with _PAGE_USER set) not
user-mode *accesses*.
Properly stashing the value off and context switching it correctly would
be fun, but probably not impossible to pull off. You actually wouldn't
even technically need to restore PKRU in this path. It would just need
to be restored before the thread runs userspace or hits a copy_to_user()
equivalent.
I can't imagine this would all be worth the trouble, but there are
crazier people out there than me.
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next prev parent reply other threads:[~2020-05-01 20:17 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-30 8:24 [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe() Dan Williams
2020-04-30 8:25 ` [PATCH v2 1/2] copy_safe: Rename memcpy_mcsafe() to copy_safe() Dan Williams
2020-04-30 8:25 ` [PATCH v2 2/2] x86/copy_safe: Introduce copy_safe_fast() Dan Williams
2020-04-30 14:02 ` [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe() Linus Torvalds
2020-04-30 16:51 ` Andy Lutomirski
2020-04-30 17:17 ` Linus Torvalds
2020-04-30 18:42 ` Andy Lutomirski
2020-04-30 19:22 ` Luck, Tony
2020-04-30 19:50 ` Linus Torvalds
2020-04-30 20:25 ` Luck, Tony
2020-04-30 23:52 ` Dan Williams
2020-05-01 0:10 ` Linus Torvalds
2020-05-01 0:23 ` Andy Lutomirski
2020-05-01 0:39 ` Linus Torvalds
2020-05-01 1:10 ` Andy Lutomirski
2020-05-01 14:09 ` Luck, Tony
2020-05-03 0:29 ` Andy Lutomirski
2020-05-04 20:05 ` Luck, Tony
2020-05-04 20:26 ` Andy Lutomirski
2020-05-04 21:30 ` Dan Williams
2020-05-01 0:24 ` Linus Torvalds
2020-05-01 1:20 ` Andy Lutomirski
2020-05-01 1:21 ` Dan Williams
2020-05-01 18:28 ` Linus Torvalds
2020-05-01 20:17 ` Dave Hansen [this message]
2020-05-03 12:57 ` David Laight
2020-05-04 18:33 ` Dan Williams
2020-05-11 15:24 ` Vivek Goyal
2020-04-30 19:51 ` Dan Williams
2020-04-30 20:07 ` Andy Lutomirski
2020-05-01 7:46 ` David Laight
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