bpf: Ensure precise is reset to false in __mark_reg_const_zero()

It is safe to always start with imprecise SCALAR_VALUE register.
Previously __mark_reg_const_zero() relied on caller to reset precise
mark, but it's very error prone and we already missed it in a few
places. So instead make __mark_reg_const_zero() reset precision always,
as it's a safe default for SCALAR_VALUE. Explanation is basically the
same as for why we are resetting (or rather not setting) precision in
current state. If necessary, precision propagation will set it to
precise correctly.

As such, also remove a big comment about forward precision propagation
in mark_reg_stack_read() and avoid unnecessarily setting precision to
true after reading from STACK_ZERO stack. Again, precision propagation
will correctly handle this, if that SCALAR_VALUE register will ever be
needed to be precise.
Reported-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20231218173601.53047-1-andrii@kernel.org

kernel/bpf/verifier.c

@@ -1777,10 +1777,14 @@ static void __mark_reg_known_zero(struct bpf_reg_state *reg)
 	__mark_reg_known(reg, 0);
 }
 
-static void __mark_reg_const_zero(struct bpf_reg_state *reg)
+static void __mark_reg_const_zero(const struct bpf_verifier_env *env, struct bpf_reg_state *reg)
 {
 	__mark_reg_known(reg, 0);
 	reg->type = SCALAR_VALUE;
+	/* all scalars are assumed imprecise initially (unless unprivileged,
+	 * in which case everything is forced to be precise)
+	 */
+	reg->precise = !env->bpf_capable;
 }
 
 static void mark_reg_known_zero(struct bpf_verifier_env *env,
@@ -4706,21 +4710,10 @@ static void mark_reg_stack_read(struct bpf_verifier_env *env,
 		zeros++;
 	}
 	if (zeros == max_off - min_off) {
-		/* any access_size read into register is zero extended,
-		 * so the whole register == const_zero
-		 */
-		__mark_reg_const_zero(&state->regs[dst_regno]);
-		/* backtracking doesn't support STACK_ZERO yet,
-		 * so mark it precise here, so that later
-		 * backtracking can stop here.
-		 * Backtracking may not need this if this register
-		 * doesn't participate in pointer adjustment.
-		 * Forward propagation of precise flag is not
-		 * necessary either. This mark is only to stop
-		 * backtracking. Any register that contributed
-		 * to const 0 was marked precise before spill.
+		/* Any access_size read into register is zero extended,
+		 * so the whole register == const_zero.
 		 */
-		state->regs[dst_regno].precise = true;
+		__mark_reg_const_zero(env, &state->regs[dst_regno]);
 	} else {
 		/* have read misc data from the stack */
 		mark_reg_unknown(env, state->regs, dst_regno);
@@ -4803,11 +4796,11 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
 
 				if (spill_cnt == size &&
 				    tnum_is_const(reg->var_off) && reg->var_off.value == 0) {
-					__mark_reg_const_zero(&state->regs[dst_regno]);
+					__mark_reg_const_zero(env, &state->regs[dst_regno]);
 					/* this IS register fill, so keep insn_flags */
 				} else if (zero_cnt == size) {
 					/* similarly to mark_reg_stack_read(), preserve zeroes */
-					__mark_reg_const_zero(&state->regs[dst_regno]);
+					__mark_reg_const_zero(env, &state->regs[dst_regno]);
 					insn_flags = 0; /* not restoring original register state */
 				} else {
 					mark_reg_unknown(env, state->regs, dst_regno);
@@ -7963,7 +7956,7 @@ static int process_iter_next_call(struct bpf_verifier_env *env, int insn_idx,
 	/* switch to DRAINED state, but keep the depth unchanged */
 	/* mark current iter state as drained and assume returned NULL */
 	cur_iter->iter.state = BPF_ITER_STATE_DRAINED;
-	__mark_reg_const_zero(&cur_fr->regs[BPF_REG_0]);
+	__mark_reg_const_zero(env, &cur_fr->regs[BPF_REG_0]);
 
 	return 0;
 }

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

@@ -499,8 +499,14 @@ __success
 __msg("2: (7a) *(u64 *)(r10 -8) = 0          ; R10=fp0 fp-8_w=00000000")
 /* but fp-16 is spilled IMPRECISE zero const reg */
 __msg("4: (7b) *(u64 *)(r10 -16) = r0        ; R0_w=0 R10=fp0 fp-16_w=0")
-/* and now check that precision propagation works even for such tricky case */
-__msg("10: (71) r2 = *(u8 *)(r10 -9)         ; R2_w=P0 R10=fp0 fp-16_w=0")
+/* validate that assigning R2 from STACK_ZERO doesn't mark register
+ * precise immediately; if necessary, it will be marked precise later
+ */
+__msg("6: (71) r2 = *(u8 *)(r10 -1)          ; R2_w=0 R10=fp0 fp-8_w=00000000")
+/* similarly, when R2 is assigned from spilled register, it is initially
+ * imprecise, but will be marked precise later once it is used in precise context
+ */
+__msg("10: (71) r2 = *(u8 *)(r10 -9)         ; R2_w=0 R10=fp0 fp-16_w=0")
 __msg("11: (0f) r1 += r2")
 __msg("mark_precise: frame0: last_idx 11 first_idx 0 subseq_idx -1")
 __msg("mark_precise: frame0: regs=r2 stack= before 10: (71) r2 = *(u8 *)(r10 -9)")