preempted dup_mm misses TLB invalidate

[Nickolas Fortino]

I’ve noticed an issue in simulation where the Linux kernel is executing 
a user process when the page tables and TLBs have gotten out of sync. 
The page tables have a page marked as user read only, but the TLB has 
the page marked as user read/write.

I’ve traced the issue back to the handling of copy on write pages 
generated from the ‘do_fork’, ‘copy_process’, ‘dup_mm’, ‘dup_mmap’ call 
stack. If run without interruption, ‘dup_mmap’ calls 
‘flush_tlb_mm(oldmm)’ on completion, avoiding any issues. In this case, 
however, about 4 million instructions after ‘dup_mm’ is called, 
‘copy_pte_range’ yields to another thread via __cond_resched. About 20 
million instructions later, a user process with the ASID of the source 
mm is scheduled. This process performs a store to a page modified from 
read/write to read only in the copy on write logic of ‘copy_one_pte’. 
Because the TLB was not invalidated, the store hits on a TLB entry with 
read/write permissions and succeeds without a fault.

What invariant in the Linux kernel is supposed to prevent this from 
happening? Note I have not observed user visible corruption, but it 
seems very unlikely a successful store to a page marked as read only in 
the kernel is safe.

For reference, this issue was found with the Linux 3.7 kernel coming 
from the Linaro 12.12 release available at 
http://releases.linaro.org/12.12/android/vexpress