selftest/bpf: Verifier tests for var-off access

Add tests for the new functionality - reading and writing to the stack
through a variable-offset pointer.
Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210207011027.676572-4-andreimatei1@gmail.com

tools/testing/selftests/bpf/verifier/var_off.c

@@ -31,14 +31,109 @@
 	 * we don't know which
 	 */
 	BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_10),
-	/* dereference it */
+	/* dereference it for a stack read */
+	BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_2, 0),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.result = ACCEPT,
+	.result_unpriv = REJECT,
+	.errstr_unpriv = "R2 variable stack access prohibited for !root",
+	.prog_type = BPF_PROG_TYPE_CGROUP_SKB,
+},
+{
+	"variable-offset stack read, uninitialized",
+	.insns = {
+	/* Get an unknown value */
+	BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 0),
+	/* Make it small and 4-byte aligned */
+	BPF_ALU64_IMM(BPF_AND, BPF_REG_2, 4),
+	BPF_ALU64_IMM(BPF_SUB, BPF_REG_2, 8),
+	/* add it to fp.  We now have either fp-4 or fp-8, but
+	 * we don't know which
+	 */
+	BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_10),
+	/* dereference it for a stack read */
 	BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_2, 0),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
 	BPF_EXIT_INSN(),
 	},
-	.errstr = "variable stack access var_off=(0xfffffffffffffff8; 0x4)",
 	.result = REJECT,
+	.errstr = "invalid variable-offset read from stack R2",
 	.prog_type = BPF_PROG_TYPE_LWT_IN,
 },
+{
+	"variable-offset stack write, priv vs unpriv",
+	.insns = {
+	/* Get an unknown value */
+	BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 0),
+	/* Make it small and 8-byte aligned */
+	BPF_ALU64_IMM(BPF_AND, BPF_REG_2, 8),
+	BPF_ALU64_IMM(BPF_SUB, BPF_REG_2, 16),
+	/* Add it to fp.  We now have either fp-8 or fp-16, but
+	 * we don't know which
+	 */
+	BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_10),
+	/* Dereference it for a stack write */
+	BPF_ST_MEM(BPF_DW, BPF_REG_2, 0, 0),
+	/* Now read from the address we just wrote. This shows
+	 * that, after a variable-offset write, a priviledged
+	 * program can read the slots that were in the range of
+	 * that write (even if the verifier doesn't actually know
+	 * if the slot being read was really written to or not.
+	 */
+	BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_2, 0),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	/* Variable stack access is rejected for unprivileged.
+	 */
+	.errstr_unpriv = "R2 variable stack access prohibited for !root",
+	.result_unpriv = REJECT,
+	.result = ACCEPT,
+},
+{
+	"variable-offset stack write clobbers spilled regs",
+	.insns = {
+	/* Dummy instruction; needed because we need to patch the next one
+	 * and we can't patch the first instruction.
+	 */
+	BPF_MOV64_IMM(BPF_REG_6, 0),
+	/* Make R0 a map ptr */
+	BPF_LD_MAP_FD(BPF_REG_0, 0),
+	/* Get an unknown value */
+	BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 0),
+	/* Make it small and 8-byte aligned */
+	BPF_ALU64_IMM(BPF_AND, BPF_REG_2, 8),
+	BPF_ALU64_IMM(BPF_SUB, BPF_REG_2, 16),
+	/* Add it to fp. We now have either fp-8 or fp-16, but
+	 * we don't know which.
+	 */
+	BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_10),
+	/* Spill R0(map ptr) into stack */
+	BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_0, -8),
+	/* Dereference the unknown value for a stack write */
+	BPF_ST_MEM(BPF_DW, BPF_REG_2, 0, 0),
+	/* Fill the register back into R2 */
+	BPF_LDX_MEM(BPF_DW, BPF_REG_2, BPF_REG_10, -8),
+	/* Try to dereference R2 for a memory load */
+	BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_2, 8),
+	BPF_EXIT_INSN(),
+	},
+	.fixup_map_hash_8b = { 1 },
+	/* The unpriviledged case is not too interesting; variable
+	 * stack access is rejected.
+	 */
+	.errstr_unpriv = "R2 variable stack access prohibited for !root",
+	.result_unpriv = REJECT,
+	/* In the priviledged case, dereferencing a spilled-and-then-filled
+	 * register is rejected because the previous variable offset stack
+	 * write might have overwritten the spilled pointer (i.e. we lose track
+	 * of the spilled register when we analyze the write).
+	 */
+	.errstr = "R2 invalid mem access 'inv'",
+	.result = REJECT,
+},
 {
 	"indirect variable-offset stack access, unbounded",
 	.insns = {