linux-kernel - Re: [PATCH] x86/sgx: Avoid returning NULL in __sgx_alloc_epc

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Date:   Fri, 19 Mar 2021 07:42:31 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Kai Huang <kai.huang@...el.com>
Cc:     x86@...nel.org, linux-sgx@...r.kernel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, dave.hansen@...el.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/sgx: Avoid returning NULL in __sgx_alloc_epc_page()

On Fri, Mar 19, 2021 at 05:06:02PM +1300, Kai Huang wrote:
> Below kernel bug happened when running simple SGX application when EPC
> is under pressure.  The root cause is with commit 5b8719504e3a
> ("x86/sgx: Add a basic NUMA allocation scheme to sgx_alloc_epc_page()"),
> __sgx_alloc_epc_page() returns NULL when there's no free EPC page can be
> allocated, while old behavior was it always returned ERR_PTR(-ENOMEM) in
> such case.
> 
> Fix by directly returning the page if __sgx_alloc_epc_page_from_node()
> allocates a valid page in fallback to non-local allocation, and always
> returning ERR_PTR(-ENOMEM) if no EPC page can be allocated.
> 
> [  253.474764] BUG: kernel NULL pointer dereference, address: 0000000000000008
> [  253.500101] #PF: supervisor write access in kernel mode
> [  253.525462] #PF: error_code(0x0002) - not-present page
> ...
> [  254.102041] Call Trace:
> [  254.126699]  sgx_ioc_enclave_add_pages+0x241/0x770
> [  254.151305]  sgx_ioctl+0x194/0x4b0
> [  254.174976]  ? handle_mm_fault+0xd0/0x260
> [  254.198470]  ? do_user_addr_fault+0x1ef/0x570
> [  254.221827]  __x64_sys_ioctl+0x91/0xc0
> [  254.244546]  do_syscall_64+0x38/0x90
> [  254.266728]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> [  254.289232] RIP: 0033:0x7fdc4cf4031b
> ...
> [  254.711480] CR2: 0000000000000008
> [  254.735494] ---[ end trace 970dce6d4cdf7f64 ]---
> [  254.759915] RIP: 0010:sgx_alloc_epc_page+0x46/0x152
> ...
> 
> Fixes: 5b8719504e3a("x86/sgx: Add a basic NUMA allocation scheme to sgx_alloc_epc_page()")
> Signed-off-by: Kai Huang <kai.huang@...el.com>


Reviewed-by: Jarkko Sakkinen <jarkko@...nel.org>

> ---
>  arch/x86/kernel/cpu/sgx/main.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index fe26e7e91c25..7105e34da530 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -508,10 +508,10 @@ struct sgx_epc_page *__sgx_alloc_epc_page(void)
>  
>  		page = __sgx_alloc_epc_page_from_node(nid);
>  		if (page)
> -			break;
> +			return page;
>  	}
>  
> -	return page;
> +	return ERR_PTR(-ENOMEM);
>  }
>  
>  /**
> -- 
> 2.30.2
> 
> 

/Jarkko