selftests/bpf: Add test for Spectre v1 mitigation

This is based on the gadget from the description of commit 9183671af6db
("bpf: Fix leakage under speculation on mispredicted branches").

Signed-off-by: Luis Gerhorst <[email protected]>
Acked-by: Kumar Kartikeya Dwivedi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: luisgerhorst

tools/testing/selftests/bpf/progs/verifier_unpriv.c

@@ -744,4 +744,61 @@ l0_%=:	r0 = 0;						\
 "	::: __clobber_all);
 }
 
+SEC("socket")
+__description("unpriv: Spectre v1 path-based type confusion of scalar as stack-ptr")
+__success __success_unpriv __retval(0)
+#ifdef SPEC_V1
+__xlated_unpriv("if r0 != 0x1 goto pc+2")
+/* This nospec prevents the exploit because it forces the mispredicted (not
+ * taken) `if r0 != 0x0 goto l0_%=` to resolve before using r6 as a pointer.
+ * This causes the CPU to realize that `r6 = r9` should have never executed. It
+ * ensures that r6 always contains a readable stack slot ptr when the insn after
+ * the nospec executes.
+ */
+__xlated_unpriv("nospec")
+__xlated_unpriv("r9 = *(u8 *)(r6 +0)")
+#endif
+__naked void unpriv_spec_v1_type_confusion(void)
+{
+	asm volatile ("					\
+	r1 = 0;						\
+	*(u64*)(r10 - 8) = r1;				\
+	r2 = r10;					\
+	r2 += -8;					\
+	r1 = %[map_hash_8b] ll;				\
+	call %[bpf_map_lookup_elem];			\
+	if r0 == 0 goto l2_%=;				\
+	/* r0: pointer to a map array entry */		\
+	r2 = r10;					\
+	r2 += -8;					\
+	r1 = %[map_hash_8b] ll;				\
+	/* r1, r2: prepared call args */		\
+	r6 = r10;					\
+	r6 += -8;					\
+	/* r6: pointer to readable stack slot */	\
+	r9 = 0xffffc900;				\
+	r9 <<= 32;					\
+	/* r9: scalar controlled by attacker */		\
+	r0 = *(u64 *)(r0 + 0); /* cache miss */		\
+	if r0 != 0x0 goto l0_%=;			\
+	r6 = r9;					\
+l0_%=:	if r0 != 0x1 goto l1_%=;			\
+	r9 = *(u8 *)(r6 + 0);				\
+l1_%=:  /* leak r9 */					\
+	r9 &= 1;					\
+	r9 <<= 9;					\
+	*(u64*)(r10 - 8) = r9;				\
+	call %[bpf_map_lookup_elem];			\
+	if r0 == 0 goto l2_%=;				\
+	/* leak secret into is_cached(map[0|512]): */	\
+	r0 = *(u64 *)(r0 + 0);				\
+l2_%=:							\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm(bpf_map_lookup_elem),
+	  __imm_addr(map_hash_8b)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";