CVE-2021-47350

/// Technical description

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

powerpc/mm: Fix lockup on kernel exec fault

The powerpc kernel is not prepared to handle exec faults from kernel.

Especially, the function is_exec_fault() will return 'false' when an

exec fault is taken by kernel, because the check is based on reading

current->thread.regs->trap which contains the trap from user.

For instance, when provoking a LKDTM EXEC_USERSPACE test,

current->thread.regs->trap is set to SYSCALL trap (0xc00), and

the fault taken by the kernel is not seen as an exec fault by

set_access_flags_filter().

Commit d7df2443cd5f ("powerpc/mm: Fix spurious segfaults on radix

with autonuma") made it clear and handled it properly. But later on

commit d3ca587404b3 ("powerpc/mm: Fix reporting of kernel execute

faults") removed that handling, introducing test based on error_code.

And here is the problem, because on the 603 all upper bits of SRR1

get cleared when the TLB instruction miss handler bails out to ISI.

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References