CVE-2023-54232 — m68k: Only force 030 bus error if PC not in exception table

Affected: Linux

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

m68k: Only force 030 bus error if PC not in exception table

__get_kernel_nofault() does copy data in supervisor mode when forcing a task backtrace log through /proc/sysrq_trigger. This is expected cause a bus error exception on e.g. NULL pointer dereferencing when logging a kernel task has no workqueue associated. This bus error ought to be ignored.

Our 030 bus error handler is ill equipped to deal with this:

Whenever ssw indicates a kernel mode access on a data fault, we don't even attempt to handle the fault and instead always send a SEGV signal (or panic). As a result, the check for exception handling at the fault PC (buried in send_sig_fault() which gets called from do_page_fault() eventually) is never used.

In contrast, both 040 and 060 access error handlers do not care whether a fault happened on supervisor mode access, and will call do_page_fault() on those, ultimately honoring the exception table.

Add a check in bus_error030 to call do_page_fault() in case we do have an entry for the fault PC in our exception table.

I had attempted a fix for this earlier in 2019 that did rely on testing pagefault_disabled() (see link below) to achieve the same thing, but this patch should be more generic.

Tested on 030 Atari Falcon.

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CVE-2023-54232 — m68k: Only force 030 bus error if PC not in exception table