Description

In the Linux kernel, the following vulnerability has been resolved:

x86/fred: Clear WFE in missing-ENDBRANCH #CPs

An indirect branch instruction sets the CPU indirect branch tracker
(IBT) into WAIT_FOR_ENDBRANCH (WFE) state and WFE stays asserted
across the instruction boundary. When the decoder finds an
inappropriate instruction while WFE is set ENDBR, the CPU raises a #CP
fault.

For the "kernel IBT no ENDBR" selftest where #CPs are deliberately
triggered, the WFE state of the interrupted context needs to be
cleared to let execution continue. Otherwise when the CPU resumes
from the instruction that just caused the previous #CP, another
missing-ENDBRANCH #CP is raised and the CPU enters a dead loop.

This is not a problem with IDT because it doesn't preserve WFE and
IRET doesn't set WFE. But FRED provides space on the entry stack
(in an expanded CS area) to save and restore the WFE state, thus the
WFE state is no longer clobbered, so software must clear it.

Clear WFE to avoid dead looping in ibt_clear_fred_wfe() and the
!ibt_fatal code path when execution is allowed to continue.

Clobbering WFE in any other circumstance is a security-relevant bug.

[ dhansen: changelog rewording ]

Classification

CVE ID: CVE-2024-56761

Affected Products

Vendor: Linux

Product: Linux

Exploit Prediction Scoring System (EPSS)

EPSS Score: 0.04% (probability of being exploited)

EPSS Percentile: 5.08% (scored less or equal to compared to others)

EPSS Date: 2025-02-04 (when was this score calculated)

References

https://git.kernel.org/stable/c/b939f108e86b76119428a6fa4e92491e09ac7867
https://git.kernel.org/stable/c/dc81e556f2a017d681251ace21bf06c126d5a192

Timeline