linux-kernel - Re: [PATCH v12 00/18] Enable FSGSBASE instructions

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Date:   Mon, 18 May 2020 16:53:35 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        bp@...en8.de, luto@...nel.org
Cc:     hpa@...or.com, dave.hansen@...el.com, tony.luck@...el.com,
        ak@...ux.intel.com, ravi.v.shankar@...el.com,
        chang.seok.bae@...el.com, Sasha Levin <sashal@...nel.org>,
        x86@...nel.org
Subject: Re: [PATCH v12 00/18] Enable FSGSBASE instructions


Cc: +x86@...nel.org ....

Sasha Levin <sashal@...nel.org> writes:

> Benefits:
> Currently a user process that wishes to read or write the FS/GS base must
> make a system call. But recent X86 processors have added new instructions
> for use in 64-bit mode that allow direct access to the FS and GS segment
> base addresses.  The operating system controls whether applications can
> use these instructions with a %cr4 control bit.
>
> In addition to benefits to applications, performance improvements to the
> OS context switch code are possible by making use of these instructions. A
> third party reported out promising performance numbers out of their
> initial benchmarking of the previous version of this patch series [9].
>
> Enablement check:
> The kernel provides information about the enabled state of FSGSBASE to
> applications using the ELF_AUX vector. If the HWCAP2_FSGSBASE bit is set in
> the AUX vector, the kernel has FSGSBASE instructions enabled and
> applications can use them.
>
> Kernel changes:
> Major changes made in the kernel are in context switch, paranoid path, and
> ptrace. In a context switch, a task's FS/GS base will be secured regardless
> of its selector. In the paranoid path, GS base is unconditionally
> overwritten to the kernel GS base on entry and the original GS base is
> restored on exit. Ptrace includes divergence of FS/GS index and base
> values.
>
> Security:
> For mitigating the Spectre v1 SWAPGS issue, LFENCE instructions were added
> on most kernel entries. Those patches are dependent on previous behaviors
> that users couldn't load a kernel address into the GS base. These patches
> change that assumption since the user can load any address into GS base.
> The changes to the kernel entry path in this patch series take account of
> the SWAPGS issue.