Re: [PATCH v3 3/7] mm/mprotect: Replace can_modify_mm with can_modify_vma

[Lorenzo Stoakes]

On Sat, Aug 17, 2024 at 01:18:30AM GMT, Pedro Falcato wrote:
> Avoid taking an extra trip down the mmap tree by checking the vmas
> directly. mprotect (per POSIX) tolerates partial failure.

Pretty sure this also applies to any such mXXX() operation, though I
haven't read the formalised POSIX spec. But in practice, this is how it is
:)

>
> Signed-off-by: Pedro Falcato <pedro.falcato@xxxxxxxxx>
> ---
>  mm/mprotect.c | 12 +++---------
>  1 file changed, 3 insertions(+), 9 deletions(-)
>
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 446f8e5f10d9..0c5d6d06107d 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -611,6 +611,9 @@ mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb,
>  	unsigned long charged = 0;
>  	int error;
>
> +	if (!can_modify_vma(vma))
> +		return -EPERM;
> +

I'm glad to get rid of the unlikely() too, imo these should _only_ be added
based on actual data to back them up rather than because the programmer
instinctively 'feels' that something is unlikely from the compiler's point
of view.

>  	if (newflags == oldflags) {
>  		*pprev = vma;
>  		return 0;
> @@ -769,15 +772,6 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
>  		}
>  	}
>
> -	/*
> -	 * checking if memory is sealed.
> -	 * can_modify_mm assumes we have acquired the lock on MM.
> -	 */
> -	if (unlikely(!can_modify_mm(current->mm, start, end))) {
> -		error = -EPERM;
> -		goto out;
> -	}
> -

This will allow the vm_ops->mprotect() caller to run on the vma before
initiating the mprotect() fixup, a quick survey suggests that sgx uses this
to see if mprotect() should be permitted in sgx_vma_mprotect() (so fine),
and um uses it to actually do an mprotect() call on host memory (honestly
fine too).

Looking at the struct vm_operations_struct declaration I see:

	/*
	 * Called by mprotect() to make driver-specific permission
	 * checks before mprotect() is finalised.   The VMA must not
	 * be modified.  Returns 0 if mprotect() can proceed.
	 */
	int (*mprotect)(struct vm_area_struct *vma, unsigned long start,
			unsigned long end, unsigned long newflags);


Which explicitly says DO NOT MODIFY THE VMA.

So we're good.

>  	prev = vma_prev(&vmi);
>  	if (start > vma->vm_start)
>  		prev = vma;
>
> --
> 2.46.0
>

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