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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: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231205184248.1502704-7-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Andrii Nakryiko 2023-12-05 10:42:44 -08:00 committed by Alexei Starovoitov
parent b33ceb6a3d
commit e322f0bcb8
1 changed files with 21 additions and 4 deletions
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25
kernel/bpf/verifier.c
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@ -4767,22 +4767,39 @@ 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;
}
mark_reg_unknown(env, state->regs, dst_regno);
insn_flags = 0; /* not restoring original register state */
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) {