linux-kernel - RE: [PATCH 13/15] x86/fsgsbase/64: With FSGSBASE, compare GS bases on paranoid

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Date:   Tue, 20 Mar 2018 10:16:36 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Chang S. Bae'" <chang.seok.bae@...el.com>,
        "x86@...nel.org" <x86@...nel.org>
CC:     "luto@...nel.org" <luto@...nel.org>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "markus.t.metzger@...el.com" <markus.t.metzger@...el.com>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "ravi.v.shankar@...el.com" <ravi.v.shankar@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: RE: [PATCH 13/15] x86/fsgsbase/64: With FSGSBASE, compare GS bases on
 paranoid_entry

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.

	David