From: "Chang S. Bae" <[email protected]> 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 <[email protected]> Signed-off-by: Sasha Levin <[email protected]> Reviewed-by: Tony Luck <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Borislav Petkov <[email protected]> Cc: Andy Lutomirski <[email protected]> Cc: H. Peter Anvin <[email protected]> Cc: Dave Hansen <[email protected]> Cc: Tony Luck <[email protected]> Cc: Andi Kleen <[email protected]> --- 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
