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Date:   Fri, 20 Oct 2023 18:56:07 +0500
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Cc:     Muhammad Usama Anjum <usama.anjum@collabora.com>,
        jeffxu@google.com, jorgelo@chromium.org, groeck@chromium.org,
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Subject: Re: [RFC PATCH v2 6/8] mseal: Check seal flag for mremap(2)
Content-Language: en-US
To:     jeffxu@chromium.org, akpm@linux-foundation.org,
        keescook@chromium.org, jannh@google.com, sroettger@google.com,
        willy@infradead.org, gregkh@linuxfoundation.org,
        torvalds@linux-foundation.org
References: <20231017090815.1067790-1-jeffxu@chromium.org>
 <20231017090815.1067790-7-jeffxu@chromium.org>
From:   Muhammad Usama Anjum <usama.anjum@collabora.com>
In-Reply-To: <20231017090815.1067790-7-jeffxu@chromium.org>
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Precedence: bulk

On 10/17/23 2:08 PM, jeffxu@chromium.org wrote:
> From: Jeff Xu <jeffxu@google.com>
> 
> mremap(2) can shrink/expand a VMA, or move a VMA to a fixed
> address and overwriting or existing VMA. Sealing will
> prevent unintended mremap(2) call.
> 
> What this patch does:
> When a mremap(2) is invoked, if one of its VMAs has MM_SEAL_MREMAP
> set from previous mseal(2) call, this mremap(2) will fail, without
> any VMA modified.
> 
> This patch is based on following:
> 1. At syscall entry point: SYSCALL_DEFINE5(mremap,...)
> There are two cases:
Maybe we can reduce the code duplication by bringing the check if memory is
sealed before call to mremap_to().

> a. going into mremap_to().
> b. not going into mremap_to().
> 
> 2. For mremap_to() case.
> Since mremap_to() is called only from SYSCALL_DEFINE5(mremap,..),
> omit changing signature of mremap_to(), i.e. not passing
> checkSeals flag.
> In mremap_to(), it calls can_modify_mm() for src address and
> dst address (when MREMAP_FIXED is used), before any update is
> made to the VMAs.
> 
> 3. For non mremap_to() case.
> It is still part of SYSCALL_DEFINE5(mremap,...).
> It calls can_modify_mm() to check sealing in the src address,
> before any update is made to src VMAs.
> Check for dest address is not needed, because dest memory is
> allocated in current mremap(2) call.
> 
> Signed-off-by: Jeff Xu <jeffxu@google.com>
> ---
>  mm/mremap.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index ac363937f8c4..691fc32d37e4 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -836,7 +836,27 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
>  	if ((mm->map_count + 2) >= sysctl_max_map_count - 3)
>  		return -ENOMEM;
>  
> +	/*
> +	 * Check src address for sealing.
> +	 *
> +	 * Note: mremap_to() currently called from one place:
> +	 * SYSCALL_DEFINE4(pkey_mprotect, ...)
> +	 * and not in any other places.
> +	 * Therefore, omit changing the signature of mremap_to()
> +	 * Otherwise, we might need to add checkSeals and pass it
> +	 * from all callers of mremap_to().
> +	 */
> +	if (!can_modify_mm(mm, addr, addr + old_len, MM_SEAL_MREMAP))
> +		return -EACCES;
> +
>  	if (flags & MREMAP_FIXED) {
> +		/*
> +		 * Check dest address for sealing.
> +		 */
> +		if (!can_modify_mm(mm, new_addr, new_addr + new_len,
> +				   MM_SEAL_MREMAP))
> +			return -EACCES;
> +
Move these two checks to just before call to mremap_to() in sys_mremap() or
even earlier. Or even better move the first condition before mremap_to()
and second condition can be checked before call to mremap_to().

>  		ret = do_munmap(mm, new_addr, new_len, uf_unmap_early);
>  		if (ret)
>  			goto out;
> @@ -995,6 +1015,11 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
>  		goto out;
>  	}
>  
> +	if (!can_modify_mm(mm, addr, addr + old_len, MM_SEAL_MREMAP)) {
> +		ret = -EACCES;
> +		goto out;
> +	}
> +
>  	/*
>  	 * Always allow a shrinking remap: that just unmaps
>  	 * the unnecessary pages..

-- 
BR,
Muhammad Usama Anjum