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

Catalin Marinas
2023-05-23T16:36:22Z

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