On Wed, 7 Feb 2024 00:11:12 +0900 "Masami Hiramatsu (Google)" <mhiramat@xxxxxxxxxx> wrote: > From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> > > Improve push and data reserve operation on the shadow stack for > several sequencial interrupts. > > To push a ret_stack or data entry on the shadow stack, we need to > prepare an index (offset) entry before updating the stack pointer > (curr_ret_stack) so that unwinder from interrupts can find the > next return address from the shadow stack. Currently we do write index, > update the curr_ret_stack, and rewrite it again. But that is not enough > for the case if two interrupts happens and the first one breaks it. > For example, > > 1. write reserved index entry at ret_stack[new_index - 1] and ret addr. > 2. interrupt comes. > 2.1. push new index and ret addr on ret_stack. > 2.2. pop it. (corrupt entries on new_index - 1) > 3. return from interrupt. > 4. update curr_ret_stack = new_index > 5. interrupt comes again. > 5.1. unwind <------ may not work. I'm curious if you saw this happen? That is, did you trigger this or only noticed it by inspection? This code is already quite complex, I would like to simplify it more before we try to fix rare race conditions that only affect the unwinder. Let's hold off on this change. -- Steve > > To avoid this issue, this introduces a new rsrv_ret_stack stack > reservation pointer and a new push code (slow path) to commit > previous reserved code forcibly. > > 0. update rsrv_ret_stack = new_index. > 1. write reserved index entry at ret_stack[new_index - 1] and ret addr. > 2. interrupt comes. > 2.0. if rsrv_ret_stack != curr_ret_stack, add reserved index > entry on ret_stack[rsrv_ret_stack - 1] to point the previous > ret_stack pointed by ret_stack[curr_ret_stack - 1]. and > update curr_ret_stack = rsrv_ret_stack. > 2.1. push new index and ret addr on ret_stack. > 2.2. pop it. (corrupt entries on new_index - 1) > 3. return from interrupt. > 4. update curr_ret_stack = new_index > 5. interrupt comes again. > 5.1. unwind works, because curr_ret_stack points the previously > saved ret_stack. > 5.2. this can do push/pop operations too. > 6. return from interrupt. > 7. rewrite reserved index entry at ret_stack[new_index] again. > > This maybe a bit heavier but safer. > > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
