Re: [PATCH] x86/sgx: Avoid returning NULL in __sgx_alloc_epc_page()

[Kai Huang]

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@xxxxxxxxx>
> > ---
> >  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?