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

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Date:   Fri, 19 Mar 2021 22:12:01 +1300
From:   Kai Huang <kai.huang@...el.com>
To:     Kai Huang <kai.huang@...el.com>
Cc:     Borislav Petkov <bp@...en8.de>, <x86@...nel.org>,
        <linux-sgx@...r.kernel.org>, <jarkko@...nel.org>,
        <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, 19 Mar 2021 22:01:41 +1300 Kai Huang wrote:
> On Fri, 19 Mar 2021 09:45:23 +0100 Borislav Petkov wrote:
> > 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>
> > > ---
> > >  arch/x86/kernel/cpu/sgx/main.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > I was on the verge whether to merge that into the original patch since
> > it is the top patch on the branch or create a new one but opted for
> > former because this way it won't break bisection and people won't have
> > to pay attention whether there's a fix patch to the NUMA patch too, in
> > case they wanna backport and whatnot.
> 
> Sure.
> 
> [...]
> 
> > +
> > +	/* Fall back to the non-local NUMA nodes: */
> > +	while (true) {
> > +		nid = next_node_in(nid, sgx_numa_mask);
> > +		if (nid == nid_of_current)
> > +			break;
> >  
> > -		page = __sgx_alloc_epc_page_from_section(section);
> > +		page = __sgx_alloc_epc_page_from_node(nid);
> >  		if (page)
> >  			return page;
> >  	}
> 
> It seems "return ERR_PTR(-ENOMEM)" is missing at the bottom of this function?

Oh my fault. This line is not shown in patch probably due to it is not changed
by this patch. Please ignore this.