linux-kernel - Re: [PATCH v2 5/6] mseal: Replace can_modify_mm

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Message-ID: <skp4dvg6h4otefmknyrg3jabqpwh4ixdi6zq645ij4wc3phmep@nzi6piu4gkk7>
Date: Fri, 9 Aug 2024 12:27:03 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Pedro Falcato <pedro.falcato@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, oliver.sang@...el.com,
        torvalds@...ux-foundation.org, jeffxu@...gle.com,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH v2 5/6] mseal: Replace can_modify_mm_madv with a vma
 variant

* Pedro Falcato <pedro.falcato@...il.com> [240807 17:13]:
> Replace can_modify_mm_madv() with a single vma variant, and associated
> checks in madvise.
> 
> While we're at it, also invert the order of checks in:
>  if (unlikely(is_ro_anon(vma) && !can_modify_vma(vma))
> 
> Checking if we can modify the vma itself (through vm_flags) is
> certainly cheaper than is_ro_anon() due to arch_vma_access_permitted()
> looking at e.g pkeys registers (with extra branches) in some
> architectures.

Doesn't this also allow for partial madvise success?  If you pass a
range across vmas, then it will fail once it encounters an mseal'ed vma.
This change should probably be reflected in the change log.


> 
> Signed-off-by: Pedro Falcato <pedro.falcato@...il.com>
> ---
>  mm/internal.h |  6 ++----
>  mm/madvise.c  | 13 +++----------
>  mm/mseal.c    | 17 ++++-------------
>  3 files changed, 9 insertions(+), 27 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 90f50f3c4cf..3f9a5c17626 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1373,8 +1373,7 @@ static inline bool can_modify_vma(struct vm_area_struct *vma)
>  
>  bool can_modify_mm(struct mm_struct *mm, unsigned long start,
>  		unsigned long end);
> -bool can_modify_mm_madv(struct mm_struct *mm, unsigned long start,
> -		unsigned long end, int behavior);
> +bool can_modify_vma_madv(struct vm_area_struct *vma, int behavior);
>  #else
>  static inline int can_do_mseal(unsigned long flags)
>  {
> @@ -1387,8 +1386,7 @@ static inline bool can_modify_mm(struct mm_struct *mm, unsigned long start,
>  	return true;
>  }
>  
> -static inline bool can_modify_mm_madv(struct mm_struct *mm, unsigned long start,
> -		unsigned long end, int behavior)
> +static inline bool can_modify_vma_madv(struct vm_area_struct *vma, int behavior)
>  {
>  	return true;
>  }
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 89089d84f8d..4e64770be16 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1031,6 +1031,9 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
>  	struct anon_vma_name *anon_name;
>  	unsigned long new_flags = vma->vm_flags;
>  
> +	if (unlikely(!can_modify_vma_madv(vma, behavior)))
> +		return -EPERM;
> +
>  	switch (behavior) {
>  	case MADV_REMOVE:
>  		return madvise_remove(vma, prev, start, end);
> @@ -1448,15 +1451,6 @@ int do_madvise(struct mm_struct *mm, unsigned long start, size_t len_in, int beh
>  	start = untagged_addr_remote(mm, start);
>  	end = start + len;
>  
> -	/*
> -	 * Check if the address range is sealed for do_madvise().
> -	 * can_modify_mm_madv assumes we have acquired the lock on MM.
> -	 */
> -	if (unlikely(!can_modify_mm_madv(mm, start, end, behavior))) {
> -		error = -EPERM;
> -		goto out;
> -	}
> -

Funny, this check stopped populate madvise operations.  The new code
does not, which is probably better and fine.

>  	blk_start_plug(&plug);
>  	switch (behavior) {
>  	case MADV_POPULATE_READ:
> @@ -1470,7 +1464,6 @@ int do_madvise(struct mm_struct *mm, unsigned long start, size_t len_in, int beh
>  	}
>  	blk_finish_plug(&plug);
>  
> -out:
>  	if (write)
>  		mmap_write_unlock(mm);
>  	else
> diff --git a/mm/mseal.c b/mm/mseal.c
> index 4591ae8d29c..6559242dd05 100644
> --- a/mm/mseal.c
> +++ b/mm/mseal.c
> @@ -67,24 +67,15 @@ bool can_modify_mm(struct mm_struct *mm, unsigned long start, unsigned long end)
>  }
>  
>  /*
> - * Check if the vmas of a memory range are allowed to be modified by madvise.
> - * the memory ranger can have a gap (unallocated memory).
> - * return true, if it is allowed.
> + * Check if a vma is allowed to be modified by madvise.
>   */
> -bool can_modify_mm_madv(struct mm_struct *mm, unsigned long start, unsigned long end,
> -		int behavior)
> +bool can_modify_vma_madv(struct vm_area_struct *vma, int behavior)
>  {
> -	struct vm_area_struct *vma;
> -
> -	VMA_ITERATOR(vmi, mm, start);
> -
>  	if (!is_madv_discard(behavior))
>  		return true;
>  
> -	/* going through each vma to check. */
> -	for_each_vma_range(vmi, vma, end)
> -		if (unlikely(is_ro_anon(vma) && !can_modify_vma(vma)))
> -			return false;
> +	if (unlikely(!can_modify_vma(vma) && is_ro_anon(vma)))
> +		return false;
>  
>  	/* Allow by default. */
>  	return true;
> -- 
> 2.46.0
>