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Date:   Sat,  9 May 2020 13:36:45 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, tglx@...utronix.de, 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>
Subject: [PATCH v11 08/18] x86/entry/64: Document GSBASE handling in the paranoid path

From: "Chang S. Bae" <chang.seok.bae@...el.com>

On FSGSBASE systems, the way to handle GS base in the paranoid path is
different from the existing SWAPGS-based entry/exit path handling. Document
the reason and what has to be done for FSGSBASE enabled systems.

Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
Reviewed-by: Tony Luck <tony.luck@...el.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: Tony Luck <tony.luck@...el.com>
Cc: Andi Kleen <ak@...ux.intel.com>
---
 Documentation/x86/entry_64.rst | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/Documentation/x86/entry_64.rst b/Documentation/x86/entry_64.rst
index a48b3f6ebbe87..0499a40723af3 100644
--- a/Documentation/x86/entry_64.rst
+++ b/Documentation/x86/entry_64.rst
@@ -108,3 +108,12 @@ We try to only use IST entries and the paranoid entry code for vectors
 that absolutely need the more expensive check for the GS base - and we
 generate all 'normal' entry points with the regular (faster) paranoid=0
 variant.
+
+On FSGSBASE systems, however, user space can set GS without kernel
+interaction. It means the value of GS base itself does not imply anything,
+whether a kernel value or a user space value. So, there is no longer a safe
+way to check whether the exception is entering from user mode or kernel
+mode in the paranoid entry code path. So the GS base value needs to be read
+out, saved and the kernel GS base value written. On exit, the saved GS base
+value needs to be restored unconditionally. The non-paranoid entry/exit
+code still uses SWAPGS unconditionally as the state is known.
-- 
2.20.1

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