Catalin Marinas <catalin.marinas <at> arm.com> writes:

The Undef abort handler in the kernel reads the undefined instruction
from user space. If the page table was modified from another CPU, the
user access could fail and do_page_fault() will be executed with
interrupts disabled. This can potentially deadlock on ARM11MPCore or on
Cortex-A15 with erratum 798181 workaround enabled (both implying IPI for
TLB maintenance with page table lock held).

This patch enables the IRQs in __und_usr before attempting to read the
instruction from user space.

This patch moves enable_irq call from do_fpe directly to __und_usr handler,
but __und_svc handler also calls do_fpe (via call_fpe), so now this codepath
runs with disabled irqs. This behavior change doesn't look good for me.