On Wed, Nov 15, 2023 at 12:45:45AM +0000, Edgecombe, Rick P wrote:

On Tue, 2023-11-14 at 20:05 +0000, Mark Brown wrote:

quoted

+               if (size < 8)
+                       return (unsigned long)ERR_PTR(-EINVAL);

What is the intention here? The check in map_shadow_stack is to leave
space for the token, but here there is no token.

It was to ensure that there is sufficient space for at least one entry
on the stack.

I think for CLONE_VM we should not require a non-zero size. Speaking of
CLONE_VM we should probably be clear on what the expected behavior is
for situations when a new shadow stack is not usually allocated.
!CLONE_VM || CLONE_VFORK will use the existing shadow stack. Should we
require shadow_stack_size be zero in this case, or just ignore it? I'd
lean towards requiring it to be zero so userspace doesn't pass garbage
in that we have to accommodate later. What we could possibly need to do
around that though, I'm not sure. What do you think?

Yes, requiring it to be zero in that case makes sense I think.

quoted

+++ b/include/linux/sched/task.h

@@ -41,6 +41,8 @@ struct kernel_clone_args {

        void *fn_arg;
        struct cgroup *cgrp;
        struct css_set *cset;
+       unsigned long shadow_stack;

Was this ^ left in accidentally? Elsewhere in this patch it is getting
checked too.

Yes, it's just bitrot from removing the pointer.