Hello,
I am trying to make pmd level of the translation table as read only so that any writes in the pte entries should cause a permission fault.
Current steps which i am doing are:
1) inside kernel space, allocating 2 pointers (say p, q) and allocating memory using vmalloc (4k size to occupy entire page)
2) assigning int data to both (say p=10, q= 20)
3) setting APTable bit for readonly for both the pointers (By reading manual, bit 61 and 62 denotes APTable)
4) swapping pte value for both the pointer in translation table
5) printing value for both the pointer
At step 4, access permission fault is not occurring.
I tried flushing the TLB, still at point 4 fault does not occur, but fault is occurring at random places (sometimes after point 5 or in between any points)
Can anyone tell me possible reasons why this is happening or some solution to fix it?
thanks
42Bastian Schick said:You may want to set the page table read-only. Not the PTE.
Wait, i think we are not on the same page.
Setting last level of the page table read only is working, but making upper level of the page table read only is not working.
Let's assume translation granule of 4kb with page size as 4kb. It will have 4 levels of page table.
4th level points to page. Here making ready only with AP[7:6] bits works.
3rd level pointing to 4th level, we need to set APTable[62:61]. Here its not working as it is supposed to.
I am confused that are there any system registers which disables hierarchical permission checks for page table.
42Bastian Schick said: I do not understand the benefit of it all. Esp. if your code runs with the same privilege as the kernel.
No, the point is to run our code in the secure world(EL3) which has more privileges than kernel(EL1). With this we can reduce our trusted computing base code size. Also we can add policies which can say when kernel is allowed to make changes.