Re: [PATCH v3 5/7] mseal: Replace can_modify_mm_madv with a vma variant

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Sat, Aug 17, 2024 at 01:18:32AM GMT, Pedro Falcato wrote:
> 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.
>
> This patch allows for partial madvise success when finding a sealed VMA,
> which historically has been allowed in Linux.
>
> Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
> ---
>  mm/internal.h |  2 --
>  mm/madvise.c  | 13 +++----------
>  mm/mseal.c    | 17 ++++-------------
>  mm/vma.h      |  7 +++++++
>  4 files changed, 14 insertions(+), 25 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index ca422aede342..1db320650539 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1363,8 +1363,6 @@ static inline int can_do_mseal(unsigned long flags)
>
>  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);
>  #else
>  static inline int can_do_mseal(unsigned long flags)
>  {
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 89089d84f8df..4e64770be16c 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;
> -	}
> -
>  	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 2170e2139ca0..fdd1666344fa 100644
> --- a/mm/mseal.c
> +++ b/mm/mseal.c
> @@ -75,24 +75,15 @@ 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;

Not your fault, but I find it extremely irritating that something this subtle
has literally zero comments.

mseal()'d + user does not have permission to modify pages = potentially
discards, as per the original message:

   6> Some destructive madvice() behaviors (e.g. MADV_DONTNEED) for anonymous
      memory, when users don't have write permission to the memory. Those
      behaviors can alter region contents by discarding pages, effectively a
      memset(0) for anonymous memory.

For something so invasive to just leave this as implied + needing to look
up the commit message to understand is just... yeah. But again, not your
fault...

>
>  	/* Allow by default. */
>  	return true;
> diff --git a/mm/vma.h b/mm/vma.h
> index e979015cc7fc..da31d0f62157 100644
> --- a/mm/vma.h
> +++ b/mm/vma.h
> @@ -380,6 +380,8 @@ static inline bool can_modify_vma(struct vm_area_struct *vma)
>  	return true;
>  }
>
> +bool can_modify_vma_madv(struct vm_area_struct *vma, int behavior);
> +
>  #else
>
>  static inline bool can_modify_vma(struct vm_area_struct *vma)
> @@ -387,6 +389,11 @@ static inline bool can_modify_vma(struct vm_area_struct *vma)
>  	return true;
>  }
>
> +static inline bool can_modify_vma_madv(struct vm_area_struct *vma, int behavior)
> +{
> +	return true;
> +}
> +
>  #endif
>
>  #endif	/* __MM_VMA_H */
>
> --
> 2.46.0
>

I remain baffled that the original implementation tried to do these things
at an mm- granularity.

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>