From: Andy Lutomirski <luto@kernel.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: David Laight <David.Laight@aculab.com>,
"Chang S. Bae" <chang.seok.bae@intel.com>,
"x86@kernel.org" <x86@kernel.org>,
"luto@kernel.org" <luto@kernel.org>,
"ak@linux.intel.com" <ak@linux.intel.com>,
"markus.t.metzger@intel.com" <markus.t.metzger@intel.com>,
"tony.luck@intel.com" <tony.luck@intel.com>,
"ravi.v.shankar@intel.com" <ravi.v.shankar@intel.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Dave Hansen <dave.hansen@linux.intel.com>
Subject: Re: [PATCH 13/15] x86/fsgsbase/64: With FSGSBASE, compare GS bases on paranoid_entry
Date: Wed, 21 Mar 2018 00:36:10 +0000 [thread overview]
Message-ID: <CALCETrX0ew6xcbxiKDkpGw8CJsF0TWNZc=GHuVEPbDpfZ-X3ww@mail.gmail.com> (raw)
In-Reply-To: <191d8212-6740-3131-1653-d057f522843c@zytor.com>
On Tue, Mar 20, 2018 at 8:07 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 03/20/18 03:16, David Laight wrote:
>> From: Chang S. Bae
>>> Sent: 19 March 2018 17:49
>> ...
>>> When FSGSBASE is enabled, SWAPGS needs if and only if (current)
>>> GS base is not the kernel's.
>>>
>>> FSGSBASE instructions allow user to write any value on GS base;
>>> even negative. Sign check on the current GS base is not
>>> sufficient. Fortunately, reading GS base is fast. Kernel GS
>>> base is also known from the offset table with the CPU number.
>> ...
>>
>> Use code might want to put a negative value into GSBASE.
>> While it is normal to put a valid address into GSBASE there
>> is no reason why the code can't put an offset into GSBASE,
>> in which case it might be negative.
>>
>> Yes, I know you can't put arbitrary 64bit values into GSBASE.
>> But the difference between 2 user pointers will always be valid.
>>
>
> You don't have a choice: you can't control what userspace puts in there.
> Anything that depends on a specific value is inherently unsafe.
>
> But we also don't need swapgs when we have rdgsbase/wrgsbase available.
> We can indeed just unconditionally save it (via rdgsbase) into the stack
> frame and wrgsbase the correct percpu value. In that case it might be
> necessary in order to avoid insane complexity to also save/restore the
> gs selector.
This is exactly what the old code did. I liked the old code better.
>
> Is it going to be faster? *Probably* not as swapgs is designed to be
> fast; it does, however, eliminate the need to RDMSR/WRMSR inside the
> kernel task switch as the user space gsbase will simply live on the
> stack. (This is assuming we do this unconditionally on every method of
> kernel entry, including non-paranoid. I'm not sure if we ever care
> about the userspace GS/GSBASE inside a paranoid handler, but if we do it
> would be rather messy to find if we do this conditionally.
>
> Now...
>
> + ALTERNATIVE "jmp .Lparanoid_entry_no_fsgsbase", \
> + "RDGSBASE %rdx", X86_FEATURE_FSGSBASE
> + READ_KERNEL_GSBASE %rax
>
> READ_KERNEL_GSBASE here seems like a Really Bad Name[TM] for this macro,
> since it seems to imply reading MSR_KERNEL_GS_BASE, rather than finding
> the current percpu offset. I would prefer calling it something like
> FIND_PERCPU_BASE or something like that.
I think we should revert to what the old patches did here.
next prev parent reply other threads:[~2018-03-21 0:36 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-19 17:49 [PATCH 00/15] x86: Enable FSGSBASE instructions Chang S. Bae
2018-03-19 17:49 ` [PATCH 01/15] x86/fsgsbase/64: Introduce FS/GS base helper functions Chang S. Bae
2018-03-19 17:49 ` [PATCH 02/15] x86/fsgsbase/64: Make ptrace read FS/GS base accurately Chang S. Bae
2018-03-19 17:49 ` [PATCH 03/15] x86/fsgsbase/64: Use FS/GS base helpers in core dump Chang S. Bae
2018-03-19 17:49 ` [PATCH 04/15] x86/fsgsbase/64: Factor out load FS/GS segments from __switch_to Chang S. Bae
2018-03-19 17:49 ` [PATCH 05/15] x86/ptrace: A new macro to get an offset of user_regs_struct Chang S. Bae
2018-03-19 17:49 ` [PATCH 06/15] x86/fsgsbase/64: Add putregs() to handle multiple elements' setting Chang S. Bae
2018-03-19 17:49 ` [PATCH 07/15] x86/fsgsbase/64: putregs() in a reverse order Chang S. Bae
2018-03-19 17:49 ` [PATCH 08/15] x86/fsgsbase/64: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE Chang S. Bae
2018-03-19 17:49 ` [PATCH 09/15] x86/fsgsbase/64: Add intrinsics/macros for FSGSBASE instructions Chang S. Bae
2018-03-19 17:49 ` [PATCH 10/15] x86/fsgsbase/64: Enable FSGSBASE instructions in helper functions Chang S. Bae
2018-03-19 17:49 ` [PATCH 11/15] x86/fsgsbase/64: Preserve FS/GS state in __switch_to if FSGSBASE is on Chang S. Bae
2018-03-19 17:49 ` [PATCH 12/15] x86/fsgsbase/64: When copying a thread, use FSGSBASE if enabled Chang S. Bae
2018-03-19 17:49 ` [PATCH 13/15] x86/fsgsbase/64: With FSGSBASE, compare GS bases on paranoid_entry Chang S. Bae
2018-03-19 20:05 ` Dave Hansen
2018-03-19 21:12 ` Bae, Chang Seok
2018-03-20 10:16 ` David Laight
2018-03-20 20:07 ` H. Peter Anvin
2018-03-20 20:36 ` Bae, Chang Seok
2018-03-21 0:36 ` Andy Lutomirski [this message]
2018-03-21 21:56 ` H. Peter Anvin
2018-03-20 14:58 ` Andy Lutomirski
2018-03-20 16:33 ` Bae, Chang Seok
2018-03-20 17:03 ` Bae, Chang Seok
2018-03-21 22:03 ` H. Peter Anvin
2018-03-22 1:37 ` Andy Lutomirski
2018-03-19 17:49 ` [PATCH 14/15] x86/fsgsbase/64: Support legacy behavior when FS/GS updated by ptracer Chang S. Bae
2018-03-20 15:04 ` Andy Lutomirski
2018-03-20 16:33 ` Bae, Chang Seok
2018-03-21 0:47 ` Andy Lutomirski
2018-03-21 7:01 ` Metzger, Markus T
2018-03-22 1:39 ` Andy Lutomirski
2018-03-21 15:11 ` Bae, Chang Seok
2018-03-22 1:40 ` Andy Lutomirski
2018-03-22 15:45 ` Bae, Chang Seok
2018-03-22 16:07 ` Andy Lutomirski
2018-03-22 16:46 ` Bae, Chang Seok
2018-03-22 16:53 ` Andy Lutomirski
2018-03-22 21:17 ` Bae, Chang Seok
2018-03-22 21:28 ` Andy Lutomirski
2018-03-19 17:49 ` [PATCH 15/15] x86/fsgsbase/64: Enable FSGSBASE by default and add a chicken bit Chang S. Bae
2018-03-20 15:05 ` [PATCH 00/15] x86: Enable FSGSBASE instructions Andy Lutomirski
2018-03-20 15:06 ` Andy Lutomirski
2018-03-20 16:33 ` Bae, Chang Seok
2018-03-21 0:40 ` Andy Lutomirski
2018-03-21 15:15 ` Bae, Chang Seok
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