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Merge tag 'its-for-linus-20250509' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull x86 ITS mitigation from Dave Hansen:
"Mitigate Indirect Target Selection (ITS) issue.
I'd describe this one as a good old CPU bug where the behavior is
_obviously_ wrong, but since it just results in bad predictions it
wasn't wrong enough to notice. Well, the researchers noticed and also
realized that thus bug undermined a bunch of existing indirect branch
mitigations.
Thus the unusually wide impact on this one. Details:
ITS is a bug in some Intel CPUs that affects indirect branches
including RETs in the first half of a cacheline. Due to ITS such
branches may get wrongly predicted to a target of (direct or indirect)
branch that is located in the second half of a cacheline. Researchers
at VUSec found this behavior and reported to Intel.
Affected processors:
- Cascade Lake, Cooper Lake, Whiskey Lake V, Coffee Lake R, Comet
Lake, Ice Lake, Tiger Lake and Rocket Lake.
Scope of impact:
- Guest/host isolation:
When eIBRS is used for guest/host isolation, the indirect branches
in the VMM may still be predicted with targets corresponding to
direct branches in the guest.
- Intra-mode using cBPF:
cBPF can be used to poison the branch history to exploit ITS.
Realigning the indirect branches and RETs mitigates this attack
vector.
- User/kernel:
With eIBRS enabled user/kernel isolation is *not* impacted by ITS.
- Indirect Branch Prediction Barrier (IBPB):
Due to this bug indirect branches may be predicted with targets
corresponding to direct branches which were executed prior to IBPB.
This will be fixed in the microcode.
Mitigation:
As indirect branches in the first half of cacheline are affected, the
mitigation is to replace those indirect branches with a call to thunk that
is aligned to the second half of the cacheline.
RETs that take prediction from RSB are not affected, but they may be
affected by RSB-underflow condition. So, RETs in the first half of
cacheline are also patched to a return thunk that executes the RET aligned
to second half of cacheline"
* tag 'its-for-linus-20250509' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
selftest/x86/bugs: Add selftests for ITS
x86/its: FineIBT-paranoid vs ITS
x86/its: Use dynamic thunks for indirect branches
x86/ibt: Keep IBT disabled during alternative patching
mm/execmem: Unify early execmem_cache behaviour
x86/its: Align RETs in BHB clear sequence to avoid thunking
x86/its: Add support for RSB stuffing mitigation
x86/its: Add "vmexit" option to skip mitigation on some CPUs
x86/its: Enable Indirect Target Selection mitigation
x86/its: Add support for ITS-safe return thunk
x86/its: Add support for ITS-safe indirect thunk
x86/its: Enumerate Indirect Target Selection (ITS) bug
Documentation: x86/bugs/its: Add ITS documentation
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.