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

[Catalin Marinas <catalin.marinas@arm.com>]

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.

-- 
Catalin