[3/4] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl

  This extends the current PR_SET_MDWE prctl arg with a bit to indicate
that the process doesn't want MDWE protection to propagate to children.

To implement this no-inherit mode, the tag in current->mm->flags must be
absent from MMF_INIT_MASK. This means that the encoding for "MDWE but
without inherit" is different in the prctl than in the mm flags. This
leads to a bit of bit-mangling in the prctl implementation.

Signed-off-by: Florent Revest <revest@chromium.org>
---
 include/linux/mman.h             |  8 +++++++-
 include/linux/sched/coredump.h   |  1 +
 include/uapi/linux/prctl.h       |  1 +
 kernel/sys.c                     | 29 +++++++++++++++++++++++------
 tools/include/uapi/linux/prctl.h |  1 +
 5 files changed, 33 insertions(+), 7 deletions(-)
  

On Thu, May 04, 2023 at 07:09:41PM +0200, Florent Revest wrote:
> This extends the current PR_SET_MDWE prctl arg with a bit to indicate
> that the process doesn't want MDWE protection to propagate to children.
> 
> To implement this no-inherit mode, the tag in current->mm->flags must be
> absent from MMF_INIT_MASK. This means that the encoding for "MDWE but
> without inherit" is different in the prctl than in the mm flags. This
> leads to a bit of bit-mangling in the prctl implementation.

That bit mangling is not that bad but it complicates the code a bit,
especially if we'll add new bits in the future. We also need to check
both the original and the no-inherit bits for each feature.

Another question is whether we want to support more fine-grained
inheriting or just a big knob that disables inheriting for all the
(future) MDWE flags.

I think a somewhat simpler way would be to clear the flags on fork(),
either based on a big MMF_HAS_MDWE_NO_INHERIT knob or individual ones.
Something like below (completely untested):

diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
index 0ee96ea7a0e9..ca83a0c8d19c 100644
--- a/include/linux/sched/coredump.h
+++ b/include/linux/sched/coredump.h
@@ -91,4 +91,12 @@ static inline int get_dumpable(struct mm_struct *mm)
 				 MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK)

 #define MMF_VM_MERGE_ANY	29
+
+#define MMF_INIT_FLAGS(flags)	({				\
+	unsigned long new_flags = flags;			\
+	if (new_flags & (1UL << MMF_HAS_MDWE_NO_INHERIT))	\
+		new_flags &= ~(1UL << MMF_HAS_MDWE_MASK);	\
+	new_flags & MMF_INIT_MASK;				\
+})
+
 #endif /* _LINUX_SCHED_COREDUMP_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index ed4e01daccaa..53f0b68a5451 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1288,7 +1288,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 	hugetlb_count_init(mm);

 	if (current->mm) {
-		mm->flags = current->mm->flags & MMF_INIT_MASK;
+		mm->flags = MMF_INIT_FLAGS(current->mm->flags);
 		mm->def_flags = current->mm->def_flags & VM_INIT_DEF_MASK;
 	} else {
 		mm->flags = default_dump_filter;

The checks in MMF_INIT_FLAGS() can grow in time if we add more bits in
there but we still only keep a single flag that determines whether the
feature is enabled (maybe that's more like bikeshedding at this moment
when we have a single bit).


(fun remark: I see you cc'ed nd@arm.com'; that's not a real person, it's
what our IT folk asked us to add on cc so that the Exchange server
doesn't append the legal disclaimer; most lists are covered already
without such cc but I guess people feel safer to add it, just in case)
  

On Fri, May 5, 2023 at 8:34 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Thu, May 04, 2023 at 07:09:41PM +0200, Florent Revest wrote:
> > This extends the current PR_SET_MDWE prctl arg with a bit to indicate
> > that the process doesn't want MDWE protection to propagate to children.
> >
> > To implement this no-inherit mode, the tag in current->mm->flags must be
> > absent from MMF_INIT_MASK. This means that the encoding for "MDWE but
> > without inherit" is different in the prctl than in the mm flags. This
> > leads to a bit of bit-mangling in the prctl implementation.
>
> That bit mangling is not that bad but it complicates the code a bit,
> especially if we'll add new bits in the future. We also need to check
> both the original and the no-inherit bits for each feature.

I agree :)

> Another question is whether we want to support more fine-grained
> inheriting or just a big knob that disables inheriting for all the
> (future) MDWE flags.

Yep, I can't think of a more fine-grained inheritance model off the
top of my head but it would be good to have a sane base for when the
need arises.

> I think a somewhat simpler way would be to clear the flags on fork(),
> either based on a big MMF_HAS_MDWE_NO_INHERIT knob or individual ones.
> Something like below (completely untested):
>
> diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
> index 0ee96ea7a0e9..ca83a0c8d19c 100644
> --- a/include/linux/sched/coredump.h
> +++ b/include/linux/sched/coredump.h
> @@ -91,4 +91,12 @@ static inline int get_dumpable(struct mm_struct *mm)
>                                  MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK)
>
>  #define MMF_VM_MERGE_ANY       29
> +
> +#define MMF_INIT_FLAGS(flags)  ({                              \
> +       unsigned long new_flags = flags;                        \
> +       if (new_flags & (1UL << MMF_HAS_MDWE_NO_INHERIT))       \
> +               new_flags &= ~(1UL << MMF_HAS_MDWE_MASK);       \
> +       new_flags & MMF_INIT_MASK;                              \
> +})
> +
>  #endif /* _LINUX_SCHED_COREDUMP_H */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index ed4e01daccaa..53f0b68a5451 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1288,7 +1288,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>         hugetlb_count_init(mm);
>
>         if (current->mm) {
> -               mm->flags = current->mm->flags & MMF_INIT_MASK;
> +               mm->flags = MMF_INIT_FLAGS(current->mm->flags);
>                 mm->def_flags = current->mm->def_flags & VM_INIT_DEF_MASK;
>         } else {
>                 mm->flags = default_dump_filter;
>
> The checks in MMF_INIT_FLAGS() can grow in time if we add more bits in
> there but we still only keep a single flag that determines whether the
> feature is enabled (maybe that's more like bikeshedding at this moment
> when we have a single bit).

Sounds good!

I had considered something like this but I was afraid I'd spill too
much logic into fork.c... I didn't think of making it a neat macro in
coredump.h. That's a good point, I'll do this in v2.

> (fun remark: I see you cc'ed nd@arm.com'; that's not a real person, it's
> what our IT folk asked us to add on cc so that the Exchange server
> doesn't append the legal disclaimer; most lists are covered already
> without such cc but I guess people feel safer to add it, just in case)

Ahah! I mostly just copied the cc list from Joey's series. I remember
wondering why I couldn't find any patch sent by this mysterious ND but
I thought that if they got such a cool username, surely they must have
been at ARM since the early days and have some important role... :)
Then... mister nd won't get to see my v2! Thanks for the heads up.