linux-kernel - Re: [PATCH hotfix 6.12 3/8] mm: refactor map_deny_write

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Message-ID: <3de53e7fmsql2t7byzoqonyt7e22bztucjcypykvqiystbalw3@2vwnvh7jcfed>
Date: Wed, 23 Oct 2024 10:30:46 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH hotfix 6.12 3/8] mm: refactor map_deny_write_exec()

* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [241022 16:41]:
> 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@...gle.com>
> Fixes: deb0f6562884 ("mm/mmap: undo ->mmap() when arch_validate_flags() fails")
> Cc: stable <stable@...nel.org>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>

> ---
>  include/linux/mman.h | 21 ++++++++++++++++++---
>  mm/mmap.c            |  2 +-
>  mm/mprotect.c        |  2 +-
>  3 files changed, 20 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/mman.h b/include/linux/mman.h
> index bcb201ab7a41..d71f83b821ef 100644
> --- a/include/linux/mman.h
> +++ b/include/linux/mman.h
> @@ -188,16 +188,31 @@ static inline bool arch_memory_deny_write_exec_supported(void)
>   *
>   *	d)	mmap(PROT_READ | PROT_EXEC)
>   *		mmap(PROT_READ | PROT_EXEC | PROT_BTI)

nit: the arguments are not in the comments (which is a pre-existing
issue).

> + *
> + * This is only applicable if the user has set the Memory-Deny-Write-Execute
> + * (MDWE) protection mask for the current process.
> + *
> + * @old specifies the VMA flags the VMA originally possessed, and @new the ones
> + * we propose to set.
> + *
> + * Return: false if proposed change is OK, true if not ok and should be denied.
>   */
> -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)

Nit: somehow "new" is special in my editor.  Also, I'm too lazy to fix
my editor so I'll live with it.

>  {
> +	/* 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. */
> +	if (!(old & VM_EXEC))
>  		return true;
> 
>  	return false;
> diff --git a/mm/mmap.c b/mm/mmap.c
> index d55c58e99a54..66edf0ebba94 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1508,7 +1508,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  		vma_set_anonymous(vma);
>  	}
> 
> -	if (map_deny_write_exec(vma, vma->vm_flags)) {
> +	if (map_deny_write_exec(vma->vm_flags, vma->vm_flags)) {
>  		error = -EACCES;
>  		goto close_and_free_vma;
>  	}
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 0c5d6d06107d..6f450af3252e 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -810,7 +810,7 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
>  			break;
>  		}
> 
> -		if (map_deny_write_exec(vma, newflags)) {
> +		if (map_deny_write_exec(vma->vm_flags, newflags)) {
>  			error = -EACCES;
>  			break;
>  		}
> --
> 2.47.0