linux-kernel - Re: [PATCH v3 4/7] mm/mremap: Replace can_modify_mm with can_modify

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Message-ID: <2c7a5fcb-198a-42b3-98ec-ab4e81259b52@lucifer.local>
Date: Wed, 21 Aug 2024 07:53:43 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Pedro Falcato <pedro.falcato@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Shuah Khan <shuah@...nel.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, jeffxu@...omium.org,
        oliver.sang@...el.com, torvalds@...ux-foundation.org,
        Michael Ellerman <mpe@...erman.id.au>, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v3 4/7] mm/mremap: Replace can_modify_mm with
 can_modify_vma

On Sat, Aug 17, 2024 at 01:18:31AM GMT, Pedro Falcato wrote:
> Delegate all can_modify checks to the proper places. Unmap checks are
> done in do_unmap (et al). The source VMA check is done purposefully
> before unmapping, to keep the original mseal semantics.
>
> Signed-off-by: Pedro Falcato <pedro.falcato@...il.com>
> ---
>  mm/mremap.c | 32 ++++++--------------------------
>  1 file changed, 6 insertions(+), 26 deletions(-)
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index e7ae140fc640..24712f8dbb6b 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -902,19 +902,6 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
>  	if ((mm->map_count + 2) >= sysctl_max_map_count - 3)
>  		return -ENOMEM;
>
> -	/*
> -	 * In mremap_to().
> -	 * Move a VMA to another location, check if src addr is sealed.
> -	 *
> -	 * Place can_modify_mm here because mremap_to()
> -	 * does its own checking for address range, and we only
> -	 * check the sealing after passing those checks.
> -	 *
> -	 * can_modify_mm assumes we have acquired the lock on MM.
> -	 */
> -	if (unlikely(!can_modify_mm(mm, addr, addr + old_len)))
> -		return -EPERM;
> -

I'm honestly confused as to why the original implementation felt it
necessary to split the checks. I guess for the purposes of efficiency? But
doesn't seem efficient to me.

>  	if (flags & MREMAP_FIXED) {
>  		/*
>  		 * In mremap_to().
> @@ -1052,6 +1039,12 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
>  		goto out;
>  	}
>
> +	/* Don't allow remapping vmas when they have already been sealed */
> +	if (!can_modify_vma(vma)) {
> +		ret = -EPERM;
> +		goto out;
> +	}
> +

This is much better, and having it be a VMA check is so obviously correct
here. Again confused as to why this implemented at an mm granularity
anyway...

>  	if (is_vm_hugetlb_page(vma)) {
>  		struct hstate *h __maybe_unused = hstate_vma(vma);
>
> @@ -1079,19 +1072,6 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
>  		goto out;
>  	}
>
> -	/*
> -	 * Below is shrink/expand case (not mremap_to())
> -	 * Check if src address is sealed, if so, reject.
> -	 * In other words, prevent shrinking or expanding a sealed VMA.
> -	 *
> -	 * Place can_modify_mm here so we can keep the logic related to
> -	 * shrink/expand together.
> -	 */
> -	if (unlikely(!can_modify_mm(mm, addr, addr + old_len))) {
> -		ret = -EPERM;
> -		goto out;
> -	}
> -
>  	/*
>  	 * Always allow a shrinking remap: that just unmaps
>  	 * the unnecessary pages..
>
> --
> 2.46.0
>

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