Messages in this thread Patch in this message | | | From | Sasha Levin <> | Subject | [PATCH v12 08/18] x86/entry/64: Document GSBASE handling in the paranoid path | Date | Mon, 11 May 2020 00:53:01 -0400 |
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From: "Chang S. Bae" <chang.seok.bae@intel.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@intel.com> Signed-off-by: Sasha Levin <sashal@kernel.org> Reviewed-by: Tony Luck <tony.luck@intel.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Borislav Petkov <bp@alien8.de> Cc: Andy Lutomirski <luto@kernel.org> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Tony Luck <tony.luck@intel.com> Cc: Andi Kleen <ak@linux.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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