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From: Andy Lutomirski <luto@kernel.org>
To: "Bae, Chang Seok" <chang.seok.bae@intel.com>
Cc: Andrew Lutomirski <luto@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>, Andi Kleen <ak@linux.intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"Metzger, Markus T" <markus.t.metzger@intel.com>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH V2 00/15] x86: Enable FSGSBASE instructions
Date: Thu, 31 May 2018 13:37:58 -0700	[thread overview]
Message-ID: <CALCETrVAW5wuABK63h8frMuW_-tT29ZWPH2fPMiffOWQn8Lp2g@mail.gmail.com> (raw)
In-Reply-To: <1527789525-8857-1-git-send-email-chang.seok.bae@intel.com>

On Thu, May 31, 2018 at 10:58 AM Chang S. Bae <chang.seok.bae@intel.com> wrote:
>
> FSGSBASE is 64-bit instruction set to allow read/write
> FS/GS base from any privilege. As introduced from
> Ivybridge, enabling effort has been revolving quite long
> [2,3,4] for various reasons. After extended discussions [1],
> this patchset is proposed to introduce new ABIs of
> customizing FS/GS base (separate from its selector).


Thanks!

I have two general comments:

1. Can you try and generate a new version of patches 1-5 quickly?  I
think it would be nice to get them merged this cycle.

2. I spoke to hpa, and he said that, after further investigation of
how gdb works, a command like 'p $gs = 0x7' results in
PTRACE_POKEUSER.  He further suggested that it would therefore be
reasonable to have POKEUSER on gs refresh gsindex (assuming the poked
value is nonzero, sigh) and to make PTRACE_SETREGS iterate over the
registers in reverse order so that it behaves sanely.  Is this indeed
the case?

3. The ptrace behavior is sufficiently subtle that I think it needs a
test case.  Can you add a new selftest (or extend the existing
fsgsbase selftest) to do something like this:

 - Create an LDT entry in slot zero with base == 1.
 - Read out the hwcap bit indicating whether we have the new instructions on.
 - MOV 0x7 to %gs and use ptrace to read gsbase.  Confirm that the result is 1.
 - MOV 0x7 to %gs, do wrgsbase to change the base to 2 (if supported),
and use ptrace to read gsbase.  Confirm that the result is 2.
 - Same as previous test, but with 0x0 instead of 0x7.
 - Allocate a TLS segment with base == 3.  Load it into %gs. Use
ptrace to read gsbase.  Confirm that the result is 3.
 - Use ptrace to toggle %gs (using POKEUSER) back and forth between
0x0, 0x7, and the TLS segment.  In each case, immediately use ptrace
to read the base and confirm that you get the expected result.  Then
resume the tracee and read the base directly, confirming that you get
the expected result.
 - Use PTRACE_SETREGS to load gs = 0, gsbase = 4.  Confirm that
GETREGS returns those values back and confirm that they are in fact
loaded into the tracee.
 - Use PTRACE_SETREGS to load gs = 0x7, gsbase = 4.  Confirm that
GETREGS returns those values back and confirm that they have the
expected values (which will depend on the hwcap bit).  Also confirm
that the expected values are loaded into the tracee.

Does this seem reasonable?  The mov_ss_trap testcase has a nice bit of
code you can borrow to invoke ptrace operations on yourself.

  parent reply	other threads:[~2018-05-31 20:38 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-31 17:58 Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 01/15] x86/fsgsbase/64: Introduce FS/GS base helper functions Chang S. Bae
2018-05-31 20:14   ` Andy Lutomirski
2018-05-31 21:03     ` Bae, Chang Seok
2018-05-31 17:58 ` [PATCH V2 02/15] x86/fsgsbase/64: Make ptrace read FS/GS base accurately Chang S. Bae
2018-05-31 20:14   ` Andy Lutomirski
2018-05-31 20:31     ` hpa
2018-05-31 20:38       ` Andy Lutomirski
2018-05-31 17:58 ` [PATCH V2 03/15] x86/fsgsbase/64: Use FS/GS base helpers in core dump Chang S. Bae
2018-05-31 20:15   ` Andy Lutomirski
2018-05-31 21:03     ` Bae, Chang Seok
2018-05-31 17:58 ` [PATCH V2 04/15] x86/fsgsbase/64: Factor out load FS/GS segments from __switch_to Chang S. Bae
2018-05-31 20:16   ` Andy Lutomirski
2018-05-31 17:58 ` [PATCH V2 05/15] x86/vdso: Move out the CPU number store Chang S. Bae
2018-05-31 20:25   ` Andy Lutomirski
2018-05-31 21:06     ` Bae, Chang Seok
2018-06-05  7:02   ` [lkp-robot] [x86/vdso] f52001961d: BUG:kernel_hang_in_early-boot_stage,last_printk:Probing_EDD(edd=off_to_disable)...ok kernel test robot
2018-05-31 17:58 ` [PATCH V2 06/15] taint: Add taint for insecure Chang S. Bae
2018-05-31 20:25   ` Andy Lutomirski
2018-05-31 20:50     ` hpa
2018-05-31 17:58 ` [PATCH V2 07/15] x86/fsgsbase/64: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 08/15] x86/fsgsbase/64: Add intrinsics/macros for FSGSBASE instructions Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 09/15] x86/fsgsbase/64: Enable FSGSBASE instructions in helper functions Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 10/15] x86/fsgsbase/64: Preserve FS/GS state in __switch_to if FSGSBASE is on Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 11/15] x86/fsgsbase/64: When copying a thread, use FSGSBASE if enabled Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 12/15] x86/fsgsbase/64: Use per-CPU base as GS base on paranoid_entry Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 13/15] x86/fsgsbase/64: Enable FSGSBASE by default and add a chicken bit Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 14/15] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2 Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 15/15] x86/fsgsbase/64: Add documentation for FSGSBASE Chang S. Bae
2018-05-31 20:37 ` Andy Lutomirski [this message]
2018-05-31 21:11   ` [PATCH V2 00/15] x86: Enable FSGSBASE instructions Bae, Chang Seok

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