Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes: > On Wed, Jun 16, 2021 at 1:00 PM Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: >> >> And even for debugging, I think it would be both easier and cheaper to >> just add a magic word to the entry stack instead. > > IOW, just add a > > unsigned long magic; > > to "struct switch_stack", and then make the stack switch code push that value. > > That would be cheap enough to be just unconditional, but you could > make it depend on a debug config option too, of course. > > It helps if 'xyz' is some constant that is easyish to generate. It > might not be a constant - maybe it could be the address of that > 'magic' field itself, so you'd just generate it with > > stq $r,($r) > > and it would be equally easy to just validate at lookup for that WARN_ON_ONCE(): > > WARN_ON_ONCE(switch_stack->magic != (unsigned long)&switch_stack->magic); > > or whatever. > > It's for debugging, not security. So it doesn't have to be some kind > of super-great magic number, just something easy to generate and check > (that isn't a common value like "0" that trivially exist on the stack > anyway). Fair enough. I was thinking for a moment that do_sigreturn might have a problem with that but restore_sigcontext makes it clear that struct switch_stack is not exposed to userspace. Do you know if struct switch_stack or pt_regs is ever exposeed to usespace? They are both defined in arch/alpha/include/uapi/asm/ptrace.h which makes me think userspace must see those definitions somewhere. Eric
