bpf: Forbid XADD on spilled pointers for unprivileged users

When check_xadd() verifies an XADD operation on a pointer to a stack slot
containing a spilled pointer, check_stack_read() verifies that the read,
which is part of XADD, is valid. However, since the placeholder value -1 is
passed as `value_regno`, check_stack_read() can only return a binary
decision and can't return the type of the value that was read. The intent
here is to verify whether the value read from the stack slot may be used as
a SCALAR_VALUE; but since check_stack_read() doesn't check the type, and
the type information is lost when check_stack_read() returns, this is not
enforced, and a malicious user can abuse XADD to leak spilled kernel
pointers.

Fix it by letting check_stack_read() verify that the value is usable as a
SCALAR_VALUE if no type information is passed to the caller.

To be able to use __is_pointer_value() in check_stack_read(), move it up.

Fix up the expected unprivileged error message for a BPF selftest that,
until now, assumed that unprivileged users can use XADD on stack-spilled
pointers. This also gives us a test for the behavior introduced in this
patch for free.

In theory, this could also be fixed by forbidding XADD on stack spills
entirely, since XADD is a locked operation (for operations on memory with
concurrency) and there can't be any concurrency on the BPF stack; but
Alexei has said that he wants to keep XADD on stack slots working to avoid
changes to the test suite [1].

The following BPF program demonstrates how to leak a BPF map pointer as an
unprivileged user using this bug:

// r7 = map_pointer
BPF_LD_MAP_FD(BPF_REG_7, small_map),
// r8 = launder(map_pointer)
BPF_STX_MEM(BPF_DW, BPF_REG_FP, BPF_REG_7, -8),
BPF_MOV64_IMM(BPF_REG_1, 0),
((struct bpf_insn) {
.code = BPF_STX | BPF_DW | BPF_XADD,
.dst_reg = BPF_REG_FP,
.src_reg = BPF_REG_1,
.off = -8
}),
BPF_LDX_MEM(BPF_DW, BPF_REG_8, BPF_REG_FP, -8),

// store r8 into map
BPF_MOV64_REG(BPF_REG_ARG1, BPF_REG_7),
BPF_MOV64_REG(BPF_REG_ARG2, BPF_REG_FP),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_ARG2, -4),
BPF_ST_MEM(BPF_W, BPF_REG_ARG2, 0, 0),
BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 1),
BPF_EXIT_INSN(),
BPF_STX_MEM(BPF_DW, BPF_REG_0, BPF_REG_8, 0),

BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_EXIT_INSN()

[1] https://lore.kernel.org/bpf/[e-mail]/

Fixes: 17a5267067f3 ("bpf: verifier (add verifier core)")
Signed-off-by: Jann Horn <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

kernel/bpf/verifier.c

···
 	return reg->type == SCALAR_VALUE && tnum_is_const(reg->var_off);
 }
 
+static bool __is_pointer_value(bool allow_ptr_leaks,
+			       const struct bpf_reg_state *reg)
+{
+	if (allow_ptr_leaks)
+		return false;
+
+	return reg->type != SCALAR_VALUE;
+}
+
 static void save_register_state(struct bpf_func_state *state,
 				int spi, struct bpf_reg_state *reg)
 {
···
 			 * which resets stack/reg liveness for state transitions
 			 */
 			state->regs[value_regno].live |= REG_LIVE_WRITTEN;
+		} else if (__is_pointer_value(env->allow_ptr_leaks, reg)) {
+			/* If value_regno==-1, the caller is asking us whether
+			 * it is acceptable to use this value as a SCALAR_VALUE
+			 * (e.g. for XADD).
+			 * We must not allow unprivileged callers to do that
+			 * with spilled pointers.
+			 */
+			verbose(env, "leaking pointer from stack off %d\n",
+				off);
+			return -EACCES;
 		}
 		mark_reg_read(env, reg, reg->parent, REG_LIVE_READ64);
 	} else {
···
 		regno, reg_type_str[reg->type], off, size);
 
 	return -EACCES;
-}
-
-static bool __is_pointer_value(bool allow_ptr_leaks,
-			       const struct bpf_reg_state *reg)
-{
-	if (allow_ptr_leaks)
-		return false;
-
-	return reg->type != SCALAR_VALUE;
 }
 
 static struct bpf_reg_state *reg_state(struct bpf_verifier_env *env, int regno)

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

···
 	BPF_EXIT_INSN(),
 	},
 	.fixup_map_hash_48b = { 3 },
+	.errstr_unpriv = "leaking pointer from stack off -8",
 	.errstr = "R0 invalid mem access 'inv'",
 	.result = REJECT,
 	.flags = F_NEEDS_EFFICIENT_UNALIGNED_ACCESS,