[PATCH v8 12/17] x86/entry/64: Document GSBASE handling in the paranoid path

[Chang S. Bae]

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@xxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---

Changes from v7:
* Massaged doc and changelog by Thomas
* Used 'GS base' consistently, instead of 'GSBASE'
---
 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 a48b3f6..0499a40 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.
-- 
2.7.4