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| author | Chang S. Bae <chang.seok.bae@intel.com> | 2019-05-08 13:02:28 +0300 |
|---|---|---|
| committer | Thomas Gleixner <tglx@linutronix.de> | 2019-06-22 12:38:55 +0300 |
| commit | 5bf0cab60ee2c730ec91ae0aabc3146bcfed138b (patch) | |
| tree | e3eba1af6c79aa6ce8e25fa0d921c3a7a76f3a09 /Documentation | |
| parent | 708078f65721b46d82d9934a3f0b36a2b8ad0656 (diff) | |
| download | linux-5bf0cab60ee2c730ec91ae0aabc3146bcfed138b.tar.xz | |
x86/entry/64: Document GSBASE handling in the paranoid path
On a FSGSBASE system, the way to handle GSBASE 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.
[ tglx: Massaged doc and changelog ]
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Link: https://lkml.kernel.org/r/1557309753-24073-14-git-send-email-chang.seok.bae@intel.com
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/x86/entry_64.rst | 9 |
1 files changed, 9 insertions, 0 deletions
diff --git a/Documentation/x86/entry_64.rst b/Documentation/x86/entry_64.rst index a48b3f6ebbe8..b87c1d816aea 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 a FSGSBASE system, 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 GSBASE value needs to be read +out, saved and the kernel GSBASE value written. On exit the saved GSBASE +value needs to be restored unconditionally. The non paranoid entry/exit +code still uses SWAPGS unconditionally as the state is known. |