From befae75856ab406a3f3fab2aa2118cf3b2dfe3e6 Mon Sep 17 00:00:00 2001 From: Eduard Zingerman Date: Wed, 16 Nov 2022 00:48:58 +0200 Subject: [PATCH] bpf: propagate nullness information for reg to reg comparisons Propagate nullness information for branches of register to register equality compare instructions. The following rules are used: - suppose register A maybe null - suppose register B is not null - for JNE A, B, ... - A is not null in the false branch - for JEQ A, B, ... - A is not null in the true branch E.g. for program like below: r6 = skb->sk; r7 = sk_fullsock(r6); r0 = sk_fullsock(r6); if (r0 == 0) return 0; (a) if (r0 != r7) return 0; (b) *r7->type; (c) return 0; It is safe to dereference r7 at point (c), because of (a) and (b). Signed-off-by: Eduard Zingerman Acked-by: Yonghong Song Link: https://lore.kernel.org/r/20221115224859.2452988-2-eddyz87@gmail.com Signed-off-by: Alexei Starovoitov --- kernel/bpf/verifier.c | 35 +++++++++++++++++++++++++++++++++-- 1 file changed, 33 insertions(+), 2 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index be24774961ab..0312d9ce292f 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -10267,6 +10267,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, struct bpf_verifier_state *other_branch; struct bpf_reg_state *regs = this_branch->frame[this_branch->curframe]->regs; struct bpf_reg_state *dst_reg, *other_branch_regs, *src_reg = NULL; + struct bpf_reg_state *eq_branch_regs; u8 opcode = BPF_OP(insn->code); bool is_jmp32; int pred = -1; @@ -10376,8 +10377,8 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, /* detect if we are comparing against a constant value so we can adjust * our min/max values for our dst register. * this is only legit if both are scalars (or pointers to the same - * object, I suppose, but we don't support that right now), because - * otherwise the different base pointers mean the offsets aren't + * object, I suppose, see the PTR_MAYBE_NULL related if block below), + * because otherwise the different base pointers mean the offsets aren't * comparable. */ if (BPF_SRC(insn->code) == BPF_X) { @@ -10426,6 +10427,36 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, find_equal_scalars(other_branch, &other_branch_regs[insn->dst_reg]); } + /* if one pointer register is compared to another pointer + * register check if PTR_MAYBE_NULL could be lifted. + * E.g. register A - maybe null + * register B - not null + * for JNE A, B, ... - A is not null in the false branch; + * for JEQ A, B, ... - A is not null in the true branch. + */ + if (!is_jmp32 && BPF_SRC(insn->code) == BPF_X && + __is_pointer_value(false, src_reg) && __is_pointer_value(false, dst_reg) && + type_may_be_null(src_reg->type) != type_may_be_null(dst_reg->type)) { + eq_branch_regs = NULL; + switch (opcode) { + case BPF_JEQ: + eq_branch_regs = other_branch_regs; + break; + case BPF_JNE: + eq_branch_regs = regs; + break; + default: + /* do nothing */ + break; + } + if (eq_branch_regs) { + if (type_may_be_null(src_reg->type)) + mark_ptr_not_null_reg(&eq_branch_regs[insn->src_reg]); + else + mark_ptr_not_null_reg(&eq_branch_regs[insn->dst_reg]); + } + } + /* detect if R == 0 where R is returned from bpf_map_lookup_elem(). * NOTE: these optimizations below are related with pointer comparison * which will never be JMP32.