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From: Andy Lutomirski <luto@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>,
	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 09/16] x86/ioperm: Move TSS bitmap update to exit to user work
Date: Tue, 12 Nov 2019 09:41:58 -0800
Message-ID: <CALCETrWX7POruLpr27mVoZ-CtVjz35tJBaZz0FNy9_eXfZo_fg@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1911121811150.1833@nanos.tec.linutronix.de>

On Tue, Nov 12, 2019 at 9:20 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Tue, 12 Nov 2019, Andy Lutomirski wrote:
> > On Mon, Nov 11, 2019 at 2:35 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> > >
> > > There is no point to update the TSS bitmap for tasks which use I/O bitmaps
> > > on every context switch. It's enough to update it right before exiting to
> > > user space.
> > +
> > > +static inline void switch_to_bitmap(unsigned long tifp)
> > > +{
> > > +       /*
> > > +        * Invalidate I/O bitmap if the previous task used it. If the next
> > > +        * task has an I/O bitmap it will handle it on exit to user mode.
> > > +        */
> > > +       if (tifp & _TIF_IO_BITMAP)
> > > +               tss_invalidate_io_bitmap(this_cpu_ptr(&cpu_tss_rw));
> > > +}
> >
> > Shouldn't you be invalidating the io bitmap if the *next* task doesn't
> > use?  Or is the rule that, when a non-io-bitmap-using task is running,
> > even in kernel mode, the io bitmap is always invalid.
>
> Well it does not make much of a difference whether we do the above or
> !(tifn & _TIF_IO_BITMAP). We always end up in that code when one of the
> involved tasks has TIF_IO_BITMAP set. I decided to use the sched out check
> because that makes it clear that this is the end of the valid I/O
> bitmap. If the next task has TIF_IO_BITMAP set as well, then it will anyway
> end up in the exit to user mode update code. Clearing it here ensures that
> even if the exit to user mode malfunctions the bitmap cannot be leaked.
>
> > As it stands, you need exit_thread() to invalidate the bitmap.  I
> > assume it does, but I can't easily see it in the middle of the series
> > like this.
>
> It does.
>
> > IOW your code might be fine, but it could at least use some comments
> > in appropriate places (exit_to_usermode_loop()?) that we guarantee
> > that, if the bit is *clear*, then the TSS has the io bitmap marked
> > invalid.  And add an assertion under CONFIG_DEBUG_ENTRY.
> >
> > Also, do you need to update EXIT_TO_USERMODE_LOOP_FLAGS?
>
> No, the TIF_IO_BITMAP check is done once after the loop has run and it
> would not make any sense in the loop as TIF_IO_BITMAP cannot be cleared
> there and we'd loop forever. The other usermode loop flags are transient.
>

Right.  But your diff tool *said* the diff was in
exit_to_usermode_loop().  Can you look at your .gitconfig and see if
you have something weird going on?

--Andy

  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
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 [this message]
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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