Re: [PATCH hotfix 6.12 3/8] mm: refactor map_deny_write_exec()

[Jann Horn]

On Tue, Oct 22, 2024 at 10:41 PM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
> Refactor the map_deny_write_exec() to not unnecessarily require a VMA
> parameter but rather to accept VMA flags parameters, which allows us to use
> this function early in mmap_region() in a subsequent commit.
>
> While we're here, we refactor the function to be more readable and add some
> additional documentation.
>
> Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> Fixes: deb0f6562884 ("mm/mmap: undo ->mmap() when arch_validate_flags() fails")
> Cc: stable <stable@xxxxxxxxxx>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>

[..]
> -static inline bool map_deny_write_exec(struct vm_area_struct *vma,  unsigned long vm_flags)
> +static inline bool map_deny_write_exec(unsigned long old, unsigned long new)
>  {
> +       /* If MDWE is disabled, we have nothing to deny. */
>         if (!test_bit(MMF_HAS_MDWE, &current->mm->flags))
>                 return false;
>
> -       if ((vm_flags & VM_EXEC) && (vm_flags & VM_WRITE))
> +       /* If the new VMA is not executable, we have nothing to deny. */
> +       if (!(new & VM_EXEC))
> +               return false;
> +
> +       /* Under MDWE we absolutely do not accept writably executable... */
> +       if (new & VM_WRITE)
>                 return true;
>
> -       if (!(vma->vm_flags & VM_EXEC) && (vm_flags & VM_EXEC))
> +       /* ...nor newly executable VMAs. */

nit: maybe clarify this as "nor existing VMAs newly becoming
executable" or something like that


> +       if (!(old & VM_EXEC))
>                 return true;
>
>         return false;