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From: Thomas Gleixner <tglx@linutronix.de>
To: Andy Lutomirski <luto@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	Linus Torvalds <torvalds@linuxfoundation.org>,
	Stephen Hemminger <stephen@networkplumber.org>,
	Willy Tarreau <w@1wt.eu>, Juergen Gross <jgross@suse.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [patch V2 06/16] x86/io: Speedup schedule out of I/O bitmap user
Date: Tue, 12 Nov 2019 18:08:27 +0100 (CET)
Message-ID: <alpine.DEB.2.21.1911121807260.1833@nanos.tec.linutronix.de> (raw)
In-Reply-To: <CALCETrUcY_DhZC8CH0NhoRp_r6mh4v1Z2dmhsdErV8wx6FsLaw@mail.gmail.com>

On Tue, 12 Nov 2019, Andy Lutomirski wrote:
> On Mon, Nov 11, 2019 at 2:35 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> > @@ -50,6 +48,11 @@ long ksys_ioperm(unsigned long from, uns
> >                  * limit correct.
> >                  */
> >                 preempt_disable();
> > +               t->io_bitmap_ptr = bitmap;
> > +               set_thread_flag(TIF_IO_BITMAP);
> > +               /* Make the bitmap base in the TSS valid */
> > +               tss = this_cpu_ptr(&cpu_tss_rw);
> > +               tss->x86_tss.io_bitmap_base = IO_BITMAP_OFFSET_VALID;
> >                 refresh_tss_limit();
> >                 preempt_enable();
> >         }
> 
> It's not shown in the diff, but the very next line of code turns
> preemption back off.  This means that we might schedule right here
> with TIF_IO_BITMAP set, the base set to VALID, but the wrong data in
> the bitmap.  I *think* this will actually end up being okay, but it
> certainly makes understanding the code harder.  Can you adjust the
> code so that preemption stays off?
> 
> More importantly, the code below this modifies the TSS copy in place
> instead of writing a whole new copy.  But now that you've added your
> optimization, the TSS copy might be *someone else's* IO bitmap.  So I
> think you might end up with more io ports allowed than you intended.
> For example:
> 
> Task A uses ioperm() to enable all ports.
> Switch to task B.  Now the TSS base is INVALID but all bitmap bits are still 0.
> Task B calls ioperm().
> 
> The code will set the base to VALID and will correctly set up the
> thread's copy of the bitmap, but I think the copy will only update the
> bits 0 through whatever ioperm() touched and not the bits above that
> in the TSS.

Yeah, you are right. Did not think about that. Will fix that up.
 
> I would believe that this is fixed later in your patch set.  If so,
> perhaps you should just memcpy() the whole thing without trying to
> optimize in this patch and then let the changes later re-optimize it
> as appropriate.  IOW change memcpy(tss->io_bitmap, t->io_bitmap_ptr,
> bytes_updated); to memcpy(..., BYTES_PER_LONG * IO_BITMAP_LONGS) or
> similar.

Right.

Thanks for spotting that!

       tglx

  reply index

Thread overview: 57+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-11 22:03 [patch V2 00/16] x86/iopl: Prevent user space from using CLI/STI with iopl(3) Thomas Gleixner
2019-11-11 22:03 ` [patch V2 01/16] x86/ptrace: Prevent truncation of bitmap size Thomas Gleixner
2019-11-12 15:34   ` Andy Lutomirski
2019-11-11 22:03 ` [patch V2 02/16] x86/process: Unify copy_thread_tls() Thomas Gleixner
2019-11-11 22:03 ` [patch V2 03/16] x86/cpu: Unify cpu_init() Thomas Gleixner
2019-11-11 22:03 ` [patch V2 04/16] x86/tss: Fix and move VMX BUILD_BUG_ON() Thomas Gleixner
2019-11-11 22:44   ` Paolo Bonzini
2019-11-12 15:37   ` Andy Lutomirski
2019-11-11 22:03 ` [patch V2 05/16] x86/iopl: Cleanup include maze Thomas Gleixner
2019-11-12 15:37   ` Andy Lutomirski
2019-11-11 22:03 ` [patch V2 06/16] x86/io: Speedup schedule out of I/O bitmap user Thomas Gleixner
2019-11-12 16:00   ` Andy Lutomirski
2019-11-12 17:08     ` Thomas Gleixner [this message]
2019-11-11 22:03 ` [patch V2 07/16] x86/ioperm: Move iobitmap data into a struct Thomas Gleixner
2019-11-12 16:02   ` Andy Lutomirski
2019-11-12 17:08     ` Thomas Gleixner
2019-11-11 22:03 ` [patch V2 08/16] x86/ioperm: Add bitmap sequence number Thomas Gleixner
2019-11-12  9:22   ` Peter Zijlstra
2019-11-12  9:55     ` [patch V2 08/16] x86/ioperm: Add bitmap sequence numberc Thomas Gleixner
2019-11-12 16:08   ` [patch V2 08/16] x86/ioperm: Add bitmap sequence number Andy Lutomirski
2019-11-12 17:10     ` Thomas Gleixner
2019-11-11 22:03 ` [patch V2 09/16] x86/ioperm: Move TSS bitmap update to exit to user work Thomas Gleixner
2019-11-12 16:16   ` Andy Lutomirski
2019-11-12 17:20     ` Thomas Gleixner
2019-11-12 17:41       ` Andy Lutomirski
2019-11-12 17:46         ` Linus Torvalds
2019-11-13  8:30           ` Peter Zijlstra
2019-11-11 22:03 ` [patch V2 10/16] x86/ioperm: Remove bitmap if all permissions dropped Thomas Gleixner
2019-11-12 17:43   ` Andy Lutomirski
2019-11-11 22:03 ` [patch V2 11/16] x86/ioperm: Share I/O bitmap if identical Thomas Gleixner
2019-11-12  7:14   ` Ingo Molnar
2019-11-12  7:17     ` Thomas Gleixner
2019-11-12  7:52       ` Ingo Molnar
2019-11-12  9:15   ` Peter Zijlstra
2019-11-12  9:51     ` Thomas Gleixner
2019-11-14 11:02     ` David Laight
2019-11-14 12:39       ` Thomas Gleixner
2019-11-14 13:09       ` Peter Zijlstra
2019-11-14 13:22         ` David Laight
2019-11-12 18:12   ` Andy Lutomirski
2019-11-11 22:03 ` [patch V2 12/16] selftests/x86/ioperm: Extend testing so the shared bitmap is exercised Thomas Gleixner
2019-11-11 22:03 ` [patch V2 13/16] x86/iopl: Fixup misleading comment Thomas Gleixner
2019-11-12 18:14   ` Andy Lutomirski
2019-11-11 22:03 ` [patch V2 14/16] x86/iopl: Restrict iopl() permission scope Thomas Gleixner
2019-11-11 23:03   ` Thomas Gleixner
2019-11-12  6:32     ` Ingo Molnar
2019-11-12  8:42   ` Ingo Molnar
2019-11-12 10:07     ` Thomas Gleixner
2019-11-12 18:35   ` Andy Lutomirski
2019-11-11 22:03 ` [patch V2 15/16] x86/iopl: Remove legacy IOPL option Thomas Gleixner
2019-11-12 18:37   ` Andy Lutomirski
2019-11-12 19:40     ` Thomas Gleixner
2019-11-11 22:03 ` [patch V2 16/16] selftests/x86/iopl: Extend test to cover IOPL emulation Thomas Gleixner
2019-11-12  7:40 ` [PATCH] x86/iopl: Factor out IO-bitmap related TSS fields into 'struct x86_io_bitmap' Ingo Molnar
2019-11-12  7:59   ` [PATCH] x86/iopl: Harmonize 'struct io_bitmap' and 'struct x86_io_bitmap' nomenclature Ingo Molnar
2019-11-12  8:11   ` [PATCH] x86/iopl: Clear up the role of the two bitmap copying fields Ingo Molnar
2019-11-12  8:15   ` [PATCH] x86/iopl: Rename <asm/iobitmap.h> to <asm/io_bitmap.h> Ingo Molnar

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