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| -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. |