Commit e322f0bc authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by Alexei Starovoitov
Browse files

bpf: preserve constant zero when doing partial register restore 



Similar to special handling of STACK_ZERO, when reading 1/2/4 bytes from
stack from slot that has register spilled into it and that register has
a constant value zero, preserve that zero and mark spilled register as
precise for that. This makes spilled const zero register and STACK_ZERO
cases equivalent in their behavior.

Acked-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231205184248.1502704-7-andrii@kernel.org


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent b33ceb6a

kernel/bpf/verifier.c

+21 −4
Original line number Diff line number Diff line
@@ -4767,23 +4767,40 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env, 
				copy_register_state(&state->regs[dst_regno], reg);

				state->regs[dst_regno].subreg_def = subreg_def;

			} else {

				int spill_cnt = 0, zero_cnt = 0;









				for (i = 0; i < size; i++) {









					type = stype[(slot - i) % BPF_REG_SIZE];









					if (type == STACK_SPILL)









					if (type == STACK_SPILL) {









						spill_cnt++;









						continue;









					}









					if (type == STACK_MISC)









						continue;









					if (type == STACK_ZERO)









					if (type == STACK_ZERO) {









						zero_cnt++;









						continue;









					}









					if (type == STACK_INVALID && env->allow_uninit_stack)









						continue;









					verbose(env, "invalid read from stack off %d+%d size %d\n",









						off, i, size);









					return -EACCES;









				}

















				if (spill_cnt == size &&









				    tnum_is_const(reg->var_off) && reg->var_off.value == 0) {









					__mark_reg_const_zero(&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]);









					insn_flags = 0; /* not restoring original register state */









				} else {









					mark_reg_unknown(env, state->regs, dst_regno);









					insn_flags = 0; /* not restoring original register state */









				}









			}









			state->regs[dst_regno].live |= REG_LIVE_WRITTEN;









		} else if (dst_regno >= 0) {









			/* restore register state from stack */