linux-kernel - Re: [PATCH 1/2] x86/sgx: Resolve EAUG race where losing thread returns SIGBUS

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Message-Id: <D0WMM3MYQODE.3A89L7D6OVG3E@kernel.org>
Date: Mon, 29 Apr 2024 16:04:24 +0300
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Dmitrii Kuvaiskii" <dmitrii.kuvaiskii@...el.com>,
 <dave.hansen@...ux.intel.com>, <kai.huang@...el.com>,
 <haitao.huang@...ux.intel.com>, <reinette.chatre@...el.com>,
 <linux-sgx@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Cc: <mona.vij@...el.com>, <kailun.qin@...el.com>, <stable@...r.kernel.org>,
 Marcelina Kościelnicka <mwk@...isiblethingslab.com>
Subject: Re: [PATCH 1/2] x86/sgx: Resolve EAUG race where losing thread
 returns SIGBUS

On Mon Apr 29, 2024 at 1:43 PM EEST, Dmitrii Kuvaiskii wrote:
> Two enclave threads may try to access the same non-present enclave page
> simultaneously (e.g., if the SGX runtime supports lazy allocation). The
> threads will end up in sgx_encl_eaug_page(), racing to acquire the
> enclave lock. The winning thread will perform EAUG, set up the page
> table entry, and insert the page into encl->page_array. The losing
> thread will then get -EBUSY on xa_insert(&encl->page_array) and proceed
> to error handling path.

And that path removes page. Not sure I got gist of this tbh.

> This error handling path contains two bugs: (1) SIGBUS is sent to
> userspace even though the enclave page is correctly installed by another
> thread, and (2) sgx_encl_free_epc_page() is called that performs EREMOVE
> even though the enclave page was never intended to be removed. The first
> bug is less severe because it impacts only the user space; the second
> bug is more severe because it also impacts the OS state by ripping the
> page (added by the winning thread) from the enclave.
>
> Fix these two bugs (1) by returning VM_FAULT_NOPAGE to the generic Linux
> fault handler so that no signal is sent to userspace, and (2) by
> replacing sgx_encl_free_epc_page() with sgx_free_epc_page() so that no
> EREMOVE is performed.

What is the collateral damage caused by ENCLS[EREMOVE]?

>
> Fixes: 5a90d2c3f5ef ("x86/sgx: Support adding of pages to an initialized enclave")
> Cc: stable@...r.kernel.org
> Reported-by: Marcelina Kościelnicka <mwk@...isiblethingslab.com>
> Suggested-by: Reinette Chatre <reinette.chatre@...el.com>
> Signed-off-by: Dmitrii Kuvaiskii <dmitrii.kuvaiskii@...el.com>
> ---
>  arch/x86/kernel/cpu/sgx/encl.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> index 279148e72459..41f14b1a3025 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -382,8 +382,11 @@ static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
>  	 * If ret == -EBUSY then page was created in another flow while
>  	 * running without encl->lock
>  	 */
> -	if (ret)
> +	if (ret) {
> +		if (ret == -EBUSY)
> +			vmret = VM_FAULT_NOPAGE;
>  		goto err_out_shrink;
> +	}
>  
>  	pginfo.secs = (unsigned long)sgx_get_epc_virt_addr(encl->secs.epc_page);
>  	pginfo.addr = encl_page->desc & PAGE_MASK;
> @@ -419,7 +422,7 @@ static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
>  err_out_shrink:
>  	sgx_encl_shrink(encl, va_page);
>  err_out_epc:
> -	sgx_encl_free_epc_page(epc_page);
> +	sgx_free_epc_page(epc_page);

This ignores check for the page being reclaimer tracked, i.e. it does
changes that have been ignored in the commit message.

>  err_out_unlock:
>  	mutex_unlock(&encl->lock);
>  	kfree(encl_page);


BR, Jarkko