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

[Kees Cook]

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@xxxxxxxxxxxx>
> ---
>  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@xxxxxxxxxxxx>

-- 
Kees Cook