[Bug 2060909] Re: Apply mitigations for the native BHI hardware vulnerabilty

This bug is awaiting verification that the linux-aws/6.8.0-1009.9 kernel
in -proposed solves the problem. Please test the kernel and update this
bug with the results. If the problem is solved, change the tag
'verification-needed-noble-linux-aws' to 'verification-done-noble-linux-
aws'. If the problem still exists, change the tag 'verification-needed-
noble-linux-aws' to 'verification-failed-noble-linux-aws'.


If verification is not done by 5 working days from today, this fix will
be dropped from the source code, and this bug will be closed.


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** Tags added: kernel-spammed-noble-linux-aws-v2 verification-needed-noble-linux-aws

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https://bugs.launchpad.net/bugs/2060909

Title:
Apply mitigations for the native BHI hardware vulnerabilty

Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Noble:
Fix Released

Bug description:
[Impact]

Branch History Injection (BHI) attacks may allow a malicious
application to influence indirect branch prediction in kernel by
poisoning the branch history. eIBRS isolates indirect branch targets
in ring0.

The BHB can still influence the choice of indirect branch predictor
entry, and although branch predictor entries are isolated between
modes when eIBRS is enabled, the BHB itself is not isolated between
modes.

Previously the only known real-world BHB attack vector was via
unprivileged eBPF. Further research has found attacks that don't
require unprivileged eBPF.

See also:
https://www.phoronix.com/news/Linux-BHI-Branch-History-Inject

[Test case]

https://www.vusec.net/projects/native-bhi/

[Fix]

Backport from upstream the merge that introduces spectre_bhi= boot
option to control BHI mitigation:

 2bb69f5fc721 ("Merge tag 'nativebhi' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip")
 ed2e8d49b54d ("KVM: x86: Add BHI_NO")
 95a6ccbdc719 ("x86/bhi: Mitigate KVM by default")
 ec9404e40e8f ("x86/bhi: Add BHI mitigation knob")
 be482ff95009 ("x86/bhi: Enumerate Branch History Injection (BHI) bug")
 0f4a837615ff ("x86/bhi: Define SPEC_CTRL_BHI_DIS_S")
 7390db8aea0d ("x86/bhi: Add support for clearing branch history at syscall entry")
 1e3ad78334a6 ("x86/syscall: Don't force use of indirect calls for system calls")
 0cd01ac5dcb1 ("x86/bugs: Change commas to semicolons in 'spectre_v2' sysfs file")

Also set spectre_bhi=auto by default, that will rely on the BHI_DIS_S
hardware control if it's available on the system CPUs, otherwise a
proper software sequence will be executed at VMexit.

NOTE: we may get these changes via stable update in 6.8, when that
happens we can drop this backport and apply the patch set like any
other regular stable update.

[Regression potential]

We may experience performance regressions with this new mitigation
enabled, especially in VMs and CPUs that don't have the BHI hardware
support capability (due to the extra software sequence executed at
VMexit).

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