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
