Description

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.

References 8

• git.kernel.org https://git.kernel.org/stable/c/1a6059f5ed57f48edfe7159404ff7d538d9d405b
• git.kernel.org https://git.kernel.org/stable/c/2100e374251a8fc00cce1916cfc50f3cb652cbe3
• git.kernel.org https://git.kernel.org/stable/c/54fa25ffab2b700df5abd58c136d64a912c53953
• git.kernel.org https://git.kernel.org/stable/c/8bf8d5dade4c5e1d8a2386f29253ed28b5d87735
• git.kernel.org https://git.kernel.org/stable/c/df1da53a7e98f0b2a0eb2241c154f148f2f2c1d8
• git.kernel.org https://git.kernel.org/stable/c/e36a82bebbf7da814530d5a179bef9df5934b717
• git.kernel.org https://git.kernel.org/stable/c/ec15405b80fc15ffc87a23d01378ae061c1aba07
• git.kernel.org https://git.kernel.org/stable/c/f55cb52ec98b22125f5bda36391edb8894f7e8cf

Remediation