tree 6590f391e486be20e581ca2628f99476899ccefb
parent 23aba521568584e9cd41a697551aa6a4cfb4c6a6
author Pierre-Clément Tosi <ptosi@google.com> 1682091893 +0100
committer Pierre-Clément Tosi <ptosi@google.com> 1682583578 +0100

vmbase: Handle stack overflows

Allocate the first page of the writable_data region for a stack to be
used by the exception handlers so that faults caused by accesses to the
SP from the main thread can be handled. As fault handlers push the
register file to the stack on entry, we would previously enter an
exception loop on stack overflow.

This works by reserving SP_EL0 for the "main" thread and relying on
current_exception_sp0 (from exceptions.S) switching to SP_EL1 when
taking an exception. SP_EL1 is first initialized to the bottom of the
newly allocated page while SP_EL0 is initialized to the value previously
used (note that SP_EL0 was previously unused).

Use the linker script to ensure that there is always at least one full
page between the end of .bss and the top (i.e. smallest address) of the
stack, in order to guarantee a permission fault on stack overflow, if
the MMU has been set properly.

Rely on the fact that the page preceding the page containing the EH
stack isn't mapped R/W to catch EH stack overflows (which would still
end up in an exception loop).

Bug: 279209532
Test: atest MicrodroidTests
Change-Id: Ie5a8dc06348bfb7db2742b1affec4d162d8b538c