[Pal/Linux-SGX] Detect signal stack overflow in "enclave_entry.S"

Creating a frame with CPU context on signal stack when handling an
exception was done without any bound checks. Malicious host could
generate any amount of signals and cause these frames to overflow the
stack. This commit fixes it by adding bound checks, effectively
disallowing more than ~9 nested exceptions (which should be more than
enough in normal case).

Signed-off-by: Borys Popławski <borysp@invisiblethingslab.com>

Pal/src/host/Linux-SGX/enclave_entry.S

@@ -384,6 +384,16 @@ enclave_entry:
 	addq $STACK_FRAME_SUB, %rax
 	subq %rax, %rsi
 
+	# Disallow too many nested exceptions. In normal Gramine flow, this should never happen.
+	# Since addresses need to be canonical, this addition does not overflow.
+	movq %gs:SGX_SIG_STACK_HIGH, %rax
+	addq %gs:SGX_SIG_STACK_LOW, %rax
+	shrq $1, %rax
+	cmp %rax, %rsi
+	jae .Lno_signal_stack_overflow
+	FAIL_LOOP
+.Lno_signal_stack_overflow:
+
 	# Align xsave area to 64 bytes after sgx_cpu_context_t
 	andq $~(PAL_XSTATE_ALIGN - 1), %rsi
 	subq $SGX_CPU_CONTEXT_XSTATE_ALIGN_SUB, %rsi