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

[Kai Huang <kai.huang@intel.com>]

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@intel.com>
---
 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