CVE-2021-47350

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 isexecfault() 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 EXECUSERSPACE 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 setaccessflagsfilter().

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.

Until commit cbd7e6ca0210 ("powerpc/fault: Avoid heavy searchexceptiontables() verification"), an exec fault from kernel at a userspace address was indirectly caught by the lack of entry for that address in the exception tables. But after that commit the kernel mainly relies on KUAP or on core mm handling to catch wrong user accesses. Here the access is not wrong, so mm handles it. It is a minor fault because PAGEEXEC is not set, setaccessflagsfilter() should set PAGEEXEC and voila. But as isexecfault() returns false as explained in the beginning, setaccessflagsfilter() bails out without setting PAGE_EXEC flag, which leads to a forever minor exec fault.

As the kernel is not prepared to handle such exec faults, the thing to do is to fire in badkernelfault() for any exec fault taken by the kernel, as it was prior to commit d3ca587404b3.