FineIBT Looks Like It Could Be Ready For Linux 6.2
The FineIBT support has been added to TIP's x86/core branch where the core x86 changes normally reside until being submitted as part of the next kernel cycle's merge window. Given the timing and barring any issues from being raised with FineIBT, now that it's reached TIP x86/core it's likely to then be submitted for the Linux 6.2 merge window in December.
Intel engineers worked on this "Fine Indirect Branch Tracking" code. As explained in the patch:
Implement an alternative CFI scheme that merges both the fine-grained nature of kCFI but also takes full advantage of the coarse grained hardware CFI as provided by IBT.
To contrast:
kCFI is a pure software CFI scheme and relies on being able to read text -- specifically the instruction *before* the target symbol, and does the hash validation *before* doing the call (otherwise control flow is compromised already).
FineIBT is a software and hardware hybrid scheme; by ensuring every branch target starts with a hash validation it is possible to place the hash validation after the branch. This has several advantages:
o the (hash) load is avoided; no memop; no RX requirement.
o IBT WAIT-FOR-ENDBR state is a speculation stop; by placing the hash validation in the immediate instruction after the branch target there is a minimal speculation window and the whole is a viable defence against SpectreBHB.
o Kees feels obliged to mention it is slightly more vulnerable when the attacker can write code.
Obviously this patch relies on kCFI, but additionally it also relies on the padding from the call-depth-tracking patches. It uses this padding to place the hash-validation while the call-sites are re-written to modify the indirect target to be 16 bytes in front of the original target, thus hitting this new preamble.
Notably, there is no hardware that needs call-depth-tracking (Skylake) and supports IBT (Tigerlake and onwards).
The config option for this kernel feature is under "FINEIBT".
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