diff mbox series

[v2,4/5] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl

Message ID 20230517150321.2890206-5-revest@chromium.org
State New
Headers
Series MDWE without inheritance |

Commit Message

Florent Revest May 17, 2023, 3:03 p.m. UTC 
  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/sched/coredump.h   | 10 ++++++++++
 include/uapi/linux/prctl.h       |  1 +
 kernel/fork.c                    |  2 +-
 kernel/sys.c                     | 24 +++++++++++++++++++++---
 tools/include/uapi/linux/prctl.h |  1 +
 5 files changed, 34 insertions(+), 4 deletions(-)

Comments

David Hildenbrand May 22, 2023, 9:01 a.m. UTC | #1 
On 17.05.23 17:03, 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.
> 
> Signed-off-by: Florent Revest <revest@chromium.org>
> ---
>   include/linux/sched/coredump.h   | 10 ++++++++++
>   include/uapi/linux/prctl.h       |  1 +
>   kernel/fork.c                    |  2 +-
>   kernel/sys.c                     | 24 +++++++++++++++++++++---
>   tools/include/uapi/linux/prctl.h |  1 +
>   5 files changed, 34 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
> index 0ee96ea7a0e9..11f5e3dacb4e 100644
> --- a/include/linux/sched/coredump.h
> +++ b/include/linux/sched/coredump.h
> @@ -91,4 +91,14 @@ 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_HAS_MDWE_NO_INHERIT	30
> +
> +#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) |			\
> +				(1UL << MMF_HAS_MDWE_NO_INHERIT));	\
> +	new_flags & MMF_INIT_MASK;					\
> +})

Why the desire for macros here? :)

We have a single user of MMF_INIT_FLAGS, why not inline or use a proper 
inline function?
Florent Revest May 22, 2023, 4:11 p.m. UTC | #2 
On Mon, May 22, 2023 at 11:01 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 17.05.23 17:03, Florent Revest wrote:
> > +#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) |                  \
> > +                             (1UL << MMF_HAS_MDWE_NO_INHERIT));      \
> > +     new_flags & MMF_INIT_MASK;                                      \
> > +})
>
> Why the desire for macros here? :)

I just thought that's what the cool kids do nowadays ?! :)

Eh, I'm joking, I completely agree. Somehow this was suggested to me
in v1 as a macro and I didn't think of questioning that, but a static
inline function should be more readable indeed! I will fix this in v3.

> We have a single user of MMF_INIT_FLAGS, why not inline or use a proper
> inline function?

I have a slight preference for a separate function, so we don't spill
over too much of this logic in fork.c.

Thanks for the review David!
Catalin Marinas May 23, 2023, 4:36 p.m. UTC | #3 
On Wed, May 17, 2023 at 05:03:20PM +0200, Florent Revest wrote:
> diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
> index 0ee96ea7a0e9..11f5e3dacb4e 100644
> --- a/include/linux/sched/coredump.h
> +++ b/include/linux/sched/coredump.h
> @@ -91,4 +91,14 @@ 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_HAS_MDWE_NO_INHERIT	30
> +
> +#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) |			\
> +				(1UL << MMF_HAS_MDWE_NO_INHERIT));	\
> +	new_flags & MMF_INIT_MASK;					\
> +})

A function is better indeed, not sure who came up with this macro idea ;).

> diff --git a/kernel/sys.c b/kernel/sys.c
> index 339fee3eff6a..320eae3b12ab 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2368,9 +2368,25 @@ static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
>  	if (arg3 || arg4 || arg5)
>  		return -EINVAL;
>  
> -	if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN))
> +	if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN | PR_MDWE_NO_INHERIT))
>  		return -EINVAL;
>  
> +	/* NO_INHERIT only makes sense with REFUSE_EXEC_GAIN */
> +	if (bits & PR_MDWE_NO_INHERIT && !(bits & PR_MDWE_REFUSE_EXEC_GAIN))
> +		return -EINVAL;
> +
> +	/* Can't gain NO_INHERIT from !NO_INHERIT */
> +	if (bits & PR_MDWE_NO_INHERIT &&
> +	    test_bit(MMF_HAS_MDWE, &current->mm->flags) &&
> +	    !test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags))
> +		return -EPERM;
> +
> +	if (bits & PR_MDWE_NO_INHERIT)
> +		set_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags);
> +	else if (test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags)
> +		 && !(bits & PR_MDWE_REFUSE_EXEC_GAIN))
> +		return -EPERM; /* Cannot unset the flag */

Is this about not unsetting the MMF_HAS_MDWE bit? We already have a
check further down that covers this case.

Related to this, do we want to allow unsetting MMF_HAS_MDWE_NO_INHERIT?
It looks like it can't be unset but no error either. The above check,
IIUC, looks more like ensuring we don't clear MMF_HAS_MDWE.

Maybe we should tighten the logic here a bit and not allow any changes
after the initial flag setting:

current->mm->flags == 0, we allow:
	bits == 0 or
	bits == PR_MDWE_REFUSE_EXEC_GAIN or
	bits == PR_MDWE_REFUSE_EXEC_GAIN | PR_MDWE_NO_INHERIT

current->mm->flags != 0 (some bits were set), we only allow the exactly
the same bit combination or -EPERM.

So basically build the flags based on the PR_* input bits and compare
them with current->mm->flags when not 0, return -EPERM if different. I
think this preserves the ABI as we only have a single bit currently and
hopefully makes the logic here easier to parse.

> +
>  	if (bits & PR_MDWE_REFUSE_EXEC_GAIN)
>  		set_bit(MMF_HAS_MDWE, &current->mm->flags);
>  	else if (test_bit(MMF_HAS_MDWE, &current->mm->flags))
> @@ -2385,8 +2401,10 @@ static inline int prctl_get_mdwe(unsigned long arg2, unsigned long arg3,
>  	if (arg2 || arg3 || arg4 || arg5)
>  		return -EINVAL;
>  
> -	return test_bit(MMF_HAS_MDWE, &current->mm->flags) ?
> -		PR_MDWE_REFUSE_EXEC_GAIN : 0;
> +	return (test_bit(MMF_HAS_MDWE, &current->mm->flags) ?
> +		PR_MDWE_REFUSE_EXEC_GAIN : 0) |
> +		(test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags) ?
> +		PR_MDWE_NO_INHERIT : 0);
>  }

Just personal preference, use explicit 'if' blocks and add bits to a
local variable variable than multiple ternary operators.
Florent Revest May 26, 2023, 7:05 p.m. UTC | #4 
On Tue, May 23, 2023 at 6:36 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Wed, May 17, 2023 at 05:03:20PM +0200, Florent Revest wrote:
> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index 339fee3eff6a..320eae3b12ab 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -2368,9 +2368,25 @@ static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
> >       if (arg3 || arg4 || arg5)
> >               return -EINVAL;
> >
> > -     if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN))
> > +     if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN | PR_MDWE_NO_INHERIT))
> >               return -EINVAL;
> >
> > +     /* NO_INHERIT only makes sense with REFUSE_EXEC_GAIN */
> > +     if (bits & PR_MDWE_NO_INHERIT && !(bits & PR_MDWE_REFUSE_EXEC_GAIN))
> > +             return -EINVAL;
> > +
> > +     /* Can't gain NO_INHERIT from !NO_INHERIT */
> > +     if (bits & PR_MDWE_NO_INHERIT &&
> > +         test_bit(MMF_HAS_MDWE, &current->mm->flags) &&
> > +         !test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags))
> > +             return -EPERM;
> > +
> > +     if (bits & PR_MDWE_NO_INHERIT)
> > +             set_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags);
> > +     else if (test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags)
> > +              && !(bits & PR_MDWE_REFUSE_EXEC_GAIN))
> > +             return -EPERM; /* Cannot unset the flag */

Ugh, I had to proofread this 3 times to figure out what I was trying
to do, not great. :(

> Is this about not unsetting the MMF_HAS_MDWE bit? We already have a
> check further down that covers this case.

Actually, this was about not being able to unset _both_ NO_INHERIT and
HAS_MDWE (this would gain capabilities)... While still being able to
unset NO_INHERIT only (this reduces capabilities)

> Related to this, do we want to allow unsetting MMF_HAS_MDWE_NO_INHERIT?
> It looks like it can't be unset but no error either.

But - sigh - as you point out, that second part wasn't implemented. It
looks like I intended to add an:

else
  clear_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags);

after that block but forgot... The idea was that:
- setting no_inherit is always allowed
- unsetting it is allowed iff we don't also unset REFUSE_EXEC_GAIN

The "consecutive_prctl_flags" selftests in patch 5 were supposed to
make the assumptions here easier to read and verify. This logic was
meant to be covered by  "cant_disable_mdwe_no_inherit" and
"can_lower_privileges" but these tests only check that the "set" prctl
succeeds and not that the end result is the expected one so I missed
this. I'll improve the selftest in the next revision too so we can
catch this more easily next time.

> The above check,
> IIUC, looks more like ensuring we don't clear MMF_HAS_MDWE.

Indeed, without the else, it was useless.

> Maybe we should tighten the logic here a bit and not allow any changes
> after the initial flag setting:
>
> current->mm->flags == 0, we allow:
>         bits == 0 or
>         bits == PR_MDWE_REFUSE_EXEC_GAIN or
>         bits == PR_MDWE_REFUSE_EXEC_GAIN | PR_MDWE_NO_INHERIT
>
> current->mm->flags != 0 (some bits were set), we only allow the exactly
> the same bit combination or -EPERM.
>
> So basically build the flags based on the PR_* input bits and compare
> them with current->mm->flags when not 0, return -EPERM if different. I
> think this preserves the ABI as we only have a single bit currently and
> hopefully makes the logic here easier to parse.

On one hand, I thought it would be nice to have the ability to
transition from NO_INHERIT | REFUSE_EXEC_GAIN to REFUSE_EXEC_GAIN
because, conceptually, it seems to me that we shouldn't prevent a
process from dropping further capabilities.

On the other hand, I don't think I have an actual need for that
transition and I agree that if we don't try to handle it, the code
here would become a lot easier to reason about. I'd certainly sleep
better at night with the smaller likelihood of having screwed
something up... :)

Unless someone feels strongly otherwise, I'll do what you suggest in v3

> > @@ -2385,8 +2401,10 @@ static inline int prctl_get_mdwe(unsigned long arg2, unsigned long arg3,
> >       if (arg2 || arg3 || arg4 || arg5)
> >               return -EINVAL;
> >
> > -     return test_bit(MMF_HAS_MDWE, &current->mm->flags) ?
> > -             PR_MDWE_REFUSE_EXEC_GAIN : 0;
> > +     return (test_bit(MMF_HAS_MDWE, &current->mm->flags) ?
> > +             PR_MDWE_REFUSE_EXEC_GAIN : 0) |
> > +             (test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags) ?
> > +             PR_MDWE_NO_INHERIT : 0);
> >  }
>
> Just personal preference, use explicit 'if' blocks and add bits to a
> local variable variable than multiple ternary operators.

Will do!
diff mbox series

Patch

diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
index 0ee96ea7a0e9..11f5e3dacb4e 100644
--- a/include/linux/sched/coredump.h
+++ b/include/linux/sched/coredump.h
@@ -91,4 +91,14 @@  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_HAS_MDWE_NO_INHERIT	30
+
+#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) |			\
+				(1UL << MMF_HAS_MDWE_NO_INHERIT));	\
+	new_flags & MMF_INIT_MASK;					\
+})
+
 #endif /* _LINUX_SCHED_COREDUMP_H */
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 6e9af6cbc950..dacbe824e7c3 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -284,6 +284,7 @@  struct prctl_mm_map {
 /* Memory deny write / execute */
 #define PR_SET_MDWE			65
 # define PR_MDWE_REFUSE_EXEC_GAIN	(1UL << 0)
+# define PR_MDWE_NO_INHERIT		(1UL << 1)
 
 #define PR_GET_MDWE			66
 
diff --git a/kernel/fork.c b/kernel/fork.c
index d17995934eb4..62d52ad99937 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1284,7 +1284,7 @@  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;
diff --git a/kernel/sys.c b/kernel/sys.c
index 339fee3eff6a..320eae3b12ab 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2368,9 +2368,25 @@  static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
 	if (arg3 || arg4 || arg5)
 		return -EINVAL;
 
-	if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN))
+	if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN | PR_MDWE_NO_INHERIT))
 		return -EINVAL;
 
+	/* NO_INHERIT only makes sense with REFUSE_EXEC_GAIN */
+	if (bits & PR_MDWE_NO_INHERIT && !(bits & PR_MDWE_REFUSE_EXEC_GAIN))
+		return -EINVAL;
+
+	/* Can't gain NO_INHERIT from !NO_INHERIT */
+	if (bits & PR_MDWE_NO_INHERIT &&
+	    test_bit(MMF_HAS_MDWE, &current->mm->flags) &&
+	    !test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags))
+		return -EPERM;
+
+	if (bits & PR_MDWE_NO_INHERIT)
+		set_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags);
+	else if (test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags)
+		 && !(bits & PR_MDWE_REFUSE_EXEC_GAIN))
+		return -EPERM; /* Cannot unset the flag */
+
 	if (bits & PR_MDWE_REFUSE_EXEC_GAIN)
 		set_bit(MMF_HAS_MDWE, &current->mm->flags);
 	else if (test_bit(MMF_HAS_MDWE, &current->mm->flags))
@@ -2385,8 +2401,10 @@  static inline int prctl_get_mdwe(unsigned long arg2, unsigned long arg3,
 	if (arg2 || arg3 || arg4 || arg5)
 		return -EINVAL;
 
-	return test_bit(MMF_HAS_MDWE, &current->mm->flags) ?
-		PR_MDWE_REFUSE_EXEC_GAIN : 0;
+	return (test_bit(MMF_HAS_MDWE, &current->mm->flags) ?
+		PR_MDWE_REFUSE_EXEC_GAIN : 0) |
+		(test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags) ?
+		PR_MDWE_NO_INHERIT : 0);
 }
 
 static int prctl_get_auxv(void __user *addr, unsigned long len)
diff --git a/tools/include/uapi/linux/prctl.h b/tools/include/uapi/linux/prctl.h
index 6e6563e97fef..f7448d99520c 100644
--- a/tools/include/uapi/linux/prctl.h
+++ b/tools/include/uapi/linux/prctl.h
@@ -284,6 +284,7 @@  struct prctl_mm_map {
 /* Memory deny write / execute */
 #define PR_SET_MDWE			65
 # define PR_MDWE_REFUSE_EXEC_GAIN	(1UL << 0)
+# define PR_MDWE_NO_INHERIT		(1UL << 1)
 
 #define PR_GET_MDWE			66