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[PATCH v3 4/5] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl

Florent Revest posted 5 patches 2 years, 7 months ago
There is a newer version of this series
[PATCH v3 4/5] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl
Posted by Florent Revest 2 years, 7 months ago 
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                     | 32 ++++++++++++++++++++++++++------
 tools/include/uapi/linux/prctl.h |  1 +
 5 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
index 0ee96ea7a0e9..1b37fa8fc723 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
+
+static inline unsigned long mmf_init_flags(unsigned long flags)
+{
+	if (flags & (1UL << MMF_HAS_MDWE_NO_INHERIT))
+		flags &= ~((1UL << MMF_HAS_MDWE) |
+			   (1UL << MMF_HAS_MDWE_NO_INHERIT));
+	return 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..bc3c762d378f 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..1a2dc3da43ea 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2362,19 +2362,41 @@ static int prctl_set_vma(unsigned long opt, unsigned long start,
 }
 #endif /* CONFIG_ANON_VMA_NAME */
 
+static inline unsigned long get_current_mdwe(void)
+{
+	unsigned long ret = 0;
+
+	if (test_bit(MMF_HAS_MDWE, &current->mm->flags))
+		ret |= PR_MDWE_REFUSE_EXEC_GAIN;
+	if (test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags))
+		ret |= PR_MDWE_NO_INHERIT;
+
+	return ret;
+}
+
 static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
 				 unsigned long arg4, unsigned long arg5)
 {
+	unsigned long current_bits;
+
 	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;
 
+	current_bits = get_current_mdwe();
+	if (current_bits && current_bits != bits)
+		return -EPERM; /* Cannot unset the flags */
+
+	if (bits & PR_MDWE_NO_INHERIT)
+		set_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags);
 	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))
-		return -EPERM; /* Cannot unset the flag */
 
 	return 0;
 }
@@ -2384,9 +2406,7 @@ 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 (int)get_current_mdwe();
 }
 
 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 6e9af6cbc950..dacbe824e7c3 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
 
-- 
2.41.0.255.g8b1d071c50-goog
Re: [PATCH v3 4/5] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl
Posted by Catalin Marinas 2 years, 5 months ago 
On Tue, Jul 04, 2023 at 05:36:28PM +0200, Florent Revest wrote:
>  static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
>  				 unsigned long arg4, unsigned long arg5)
>  {
> +	unsigned long current_bits;
> +
>  	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;
>  
> +	current_bits = get_current_mdwe();
> +	if (current_bits && current_bits != bits)
> +		return -EPERM; /* Cannot unset the flags */
> +
> +	if (bits & PR_MDWE_NO_INHERIT)
> +		set_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags);
>  	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))
> -		return -EPERM; /* Cannot unset the flag */
>  
>  	return 0;
>  }

This works for me, it's easier to read. I don't expect a use-case where
the app starts with MDWE no-inherit, forks some new processes and wants
to turn inherit on for further forking.

> @@ -2384,9 +2406,7 @@ 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 (int)get_current_mdwe();

Nitpick: the type conversion should be handled by the compiler as
prctl_get_mdwe() returns an int already.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Re: [PATCH v3 4/5] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl
Posted by Florent Revest 2 years, 5 months ago 
On Sun, Aug 27, 2023 at 3:10 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Tue, Jul 04, 2023 at 05:36:28PM +0200, Florent Revest wrote:
> > @@ -2384,9 +2406,7 @@ 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 (int)get_current_mdwe();
>
> Nitpick: the type conversion should be handled by the compiler as
> prctl_get_mdwe() returns an int already.

Ah yes. Not sure why I added this one... :) thank you!
Re: [PATCH v3 4/5] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl
Posted by Kees Cook 2 years, 5 months ago 
On Tue, Jul 04, 2023 at 05:36:28PM +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.
> 
> 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                     | 32 ++++++++++++++++++++++++++------
>  tools/include/uapi/linux/prctl.h |  1 +
>  5 files changed, 39 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
> index 0ee96ea7a0e9..1b37fa8fc723 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
> +
> +static inline unsigned long mmf_init_flags(unsigned long flags)
> +{
> +	if (flags & (1UL << MMF_HAS_MDWE_NO_INHERIT))
> +		flags &= ~((1UL << MMF_HAS_MDWE) |
> +			   (1UL << MMF_HAS_MDWE_NO_INHERIT));
> +	return 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..bc3c762d378f 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..1a2dc3da43ea 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2362,19 +2362,41 @@ static int prctl_set_vma(unsigned long opt, unsigned long start,
>  }
>  #endif /* CONFIG_ANON_VMA_NAME */
>  
> +static inline unsigned long get_current_mdwe(void)
> +{
> +	unsigned long ret = 0;
> +
> +	if (test_bit(MMF_HAS_MDWE, &current->mm->flags))
> +		ret |= PR_MDWE_REFUSE_EXEC_GAIN;
> +	if (test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags))
> +		ret |= PR_MDWE_NO_INHERIT;
> +
> +	return ret;
> +}
> +
>  static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
>  				 unsigned long arg4, unsigned long arg5)
>  {
> +	unsigned long current_bits;
> +
>  	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;
>  
> +	current_bits = get_current_mdwe();
> +	if (current_bits && current_bits != bits)
> +		return -EPERM; /* Cannot unset the flags */

I was pondering why PR_MDWE_NO_INHERIT can't be unset, but I guess it
doesn't matter. Anything forked with have it off, and any process
wanting to launch stuff before locking down can just skip running the
prctl() until later.

> +
> +	if (bits & PR_MDWE_NO_INHERIT)
> +		set_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags);
>  	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))
> -		return -EPERM; /* Cannot unset the flag */
>  
>  	return 0;
>  }
> @@ -2384,9 +2406,7 @@ 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 (int)get_current_mdwe();
>  }
>  
>  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 6e9af6cbc950..dacbe824e7c3 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
>  
> -- 
> 2.41.0.255.g8b1d071c50-goog
> 

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook
Re: [PATCH v3 4/5] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl
Posted by Catalin Marinas 2 years, 5 months ago 
On Fri, Aug 25, 2023 at 03:38:36PM -0700, Kees Cook wrote:
> On Tue, Jul 04, 2023 at 05:36:28PM +0200, Florent Revest wrote:
> >  static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
> >  				 unsigned long arg4, unsigned long arg5)
> >  {
> > +	unsigned long current_bits;
> > +
> >  	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;
> >  
> > +	current_bits = get_current_mdwe();
> > +	if (current_bits && current_bits != bits)
> > +		return -EPERM; /* Cannot unset the flags */
> 
> I was pondering why PR_MDWE_NO_INHERIT can't be unset, but I guess it
> doesn't matter. Anything forked with have it off, and any process
> wanting to launch stuff before locking down can just skip running the
> prctl() until later.

I had a similar doubt initially but then realised that the no-inherit
mode won't be inherited and concluded it's ok.

-- 
Catalin
Re: [PATCH v3 4/5] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl
Posted by Florent Revest 2 years, 5 months ago 
On Sun, Aug 27, 2023 at 3:09 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Fri, Aug 25, 2023 at 03:38:36PM -0700, Kees Cook wrote:
> > On Tue, Jul 04, 2023 at 05:36:28PM +0200, Florent Revest wrote:
> > >  static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
> > >                              unsigned long arg4, unsigned long arg5)
> > >  {
> > > +   unsigned long current_bits;
> > > +
> > >     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;
> > >
> > > +   current_bits = get_current_mdwe();
> > > +   if (current_bits && current_bits != bits)
> > > +           return -EPERM; /* Cannot unset the flags */
> >
> > I was pondering why PR_MDWE_NO_INHERIT can't be unset, but I guess it
> > doesn't matter. Anything forked with have it off, and any process
> > wanting to launch stuff before locking down can just skip running the
> > prctl() until later.
>
> I had a similar doubt initially but then realised that the no-inherit
> mode won't be inherited and concluded it's ok.

Indeed. We previously discussed that in
https://lore.kernel.org/all/CABRcYmLt2KsCoD8WzyCTxuY=6ppuWEqyLSDRXSsmXSxPLHtEzQ@mail.gmail.com/
and I agreed this doesn't matter for our use case and this keeps the
code a lot more maintainable :)

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