Re: [PATCH bpf-next v2 1/2] bpf: Replace "want address" users of BPF_CAST_CALL with BPF_CALL_IMM

Gustavo A. R. Silva
2021-09-28T23:21:15Z

On Tue, Sep 28, 2021 at 04:09:45PM -0700, Kees Cook wrote:
> In order to keep ahead of cases in the kernel where Control Flow
> Integrity (CFI) may trip over function call casts, enabling
> -Wcast-function-type is helpful. To that end, BPF_CAST_CALL causes
> various warnings and is one of the last places in the kernel triggering
> this warning.
>
> Most places using BPF_CAST_CALL actually just want a void * to perform
> math on. It's not actually performing a call, so just use a different
> helper to get the void *, by way of the new BPF_CALL_IMM() helper, which
> can clean up a common copy/paste idiom as well.
>
> This change results in no object code difference.
>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Martin KaFai Lau <kafai@fb.com>
> Cc: Song Liu <songliubraving@fb.com>
> Cc: Yonghong Song <yhs@fb.com>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: KP Singh <kpsingh@kernel.org>
> Cc: netdev@vger.kernel.org
> Cc: bpf@vger.kernel.org
> Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
> Link: https://github.com/KSPP/linux/issues/20
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
> Link: https://lore.kernel.org/lkml/CAEf4Bzb46=-J5Fxc3mMZ8JQPtK1uoE0q6+g6WPz53Cvx=CBEhw@mail.gmail.com

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Thanks
--
Gustavo

> ---
> include/linux/filter.h | 6 +++++-
> kernel/bpf/hashtab.c | 6 +++---
> kernel/bpf/verifier.c | 26 +++++++++-----------------
> lib/test_bpf.c | 2 +-
> 4 files changed, 18 insertions(+), 22 deletions(-)
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 4a93c12543ee..6c247663d4ce 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -365,13 +365,17 @@ static inline bool insn_is_zext(const struct bpf_insn *insn)
> #define BPF_CAST_CALL(x) \
> ((u64 (*)(u64, u64, u64, u64, u64))(x))
>
> +/* Convert function address to BPF immediate */
> +
> +#define BPF_CALL_IMM(x) ((void *)(x) - (void *)__bpf_call_base)
> +
> #define BPF_EMIT_CALL(FUNC) \
> ((struct bpf_insn) { \
> .code = BPF_JMP | BPF_CALL, \
> .dst_reg = 0, \
> .src_reg = 0, \
> .off = 0, \
> - .imm = ((FUNC) - __bpf_call_base) })
> + .imm = BPF_CALL_IMM(FUNC) })
>
> /* Raw code statement block */
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 32471ba02708..3d8f9d6997d5 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -668,7 +668,7 @@ static int htab_map_gen_lookup(struct bpf_map *map, struct bpf_insn *insn_buf)
>
> BUILD_BUG_ON(!__same_type(&__htab_map_lookup_elem,
> (void *(*)(struct bpf_map *map, void *key))NULL));
> - *insn++ = BPF_EMIT_CALL(BPF_CAST_CALL(__htab_map_lookup_elem));
> + *insn++ = BPF_EMIT_CALL(__htab_map_lookup_elem);
> *insn++ = BPF_JMP_IMM(BPF_JEQ, ret, 0, 1);
> *insn++ = BPF_ALU64_IMM(BPF_ADD, ret,
> offsetof(struct htab_elem, key) +
> @@ -709,7 +709,7 @@ static int htab_lru_map_gen_lookup(struct bpf_map *map,
>
> BUILD_BUG_ON(!__same_type(&__htab_map_lookup_elem,
> (void *(*)(struct bpf_map *map, void *key))NULL));
> - *insn++ = BPF_EMIT_CALL(BPF_CAST_CALL(__htab_map_lookup_elem));
> + *insn++ = BPF_EMIT_CALL(__htab_map_lookup_elem);
> *insn++ = BPF_JMP_IMM(BPF_JEQ, ret, 0, 4);
> *insn++ = BPF_LDX_MEM(BPF_B, ref_reg, ret,
> offsetof(struct htab_elem, lru_node) +
> @@ -2397,7 +2397,7 @@ static int htab_of_map_gen_lookup(struct bpf_map *map,
>
> BUILD_BUG_ON(!__same_type(&__htab_map_lookup_elem,
> (void *(*)(struct bpf_map *map, void *key))NULL));
> - *insn++ = BPF_EMIT_CALL(BPF_CAST_CALL(__htab_map_lookup_elem));
> + *insn++ = BPF_EMIT_CALL(__htab_map_lookup_elem);
> *insn++ = BPF_JMP_IMM(BPF_JEQ, ret, 0, 2);
> *insn++ = BPF_ALU64_IMM(BPF_ADD, ret,
> offsetof(struct htab_elem, key) +
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 7a8351604f67..1433752db740 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1744,7 +1744,7 @@ static int add_kfunc_call(struct bpf_verifier_env *env, u32 func_id)
>
> desc = &tab->descs[tab->nr_descs++];
> desc->func_id = func_id;
> - desc->imm = BPF_CAST_CALL(addr) - __bpf_call_base;
> + desc->imm = BPF_CALL_IMM(addr);
> err = btf_distill_func_proto(&env->log, btf_vmlinux,
> func_proto, func_name,
> &desc->func_model);
> @@ -12514,8 +12514,7 @@ static int jit_subprogs(struct bpf_verifier_env *env)
> if (!bpf_pseudo_call(insn))
> continue;
> subprog = insn->off;
> - insn->imm = BPF_CAST_CALL(func[subprog]->bpf_func) -
> - __bpf_call_base;
> + insn->imm = BPF_CALL_IMM(func[subprog]->bpf_func);
> }
>
> /* we use the aux data to keep a list of the start addresses
> @@ -12995,32 +12994,25 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
> patch_map_ops_generic:
> switch (insn->imm) {
> case BPF_FUNC_map_lookup_elem:
> - insn->imm = BPF_CAST_CALL(ops->map_lookup_elem) -
> - __bpf_call_base;
> + insn->imm = BPF_CALL_IMM(ops->map_lookup_elem);
> continue;
> case BPF_FUNC_map_update_elem:
> - insn->imm = BPF_CAST_CALL(ops->map_update_elem) -
> - __bpf_call_base;
> + insn->imm = BPF_CALL_IMM(ops->map_update_elem);
> continue;
> case BPF_FUNC_map_delete_elem:
> - insn->imm = BPF_CAST_CALL(ops->map_delete_elem) -
> - __bpf_call_base;
> + insn->imm = BPF_CALL_IMM(ops->map_delete_elem);
> continue;
> case BPF_FUNC_map_push_elem:
> - insn->imm = BPF_CAST_CALL(ops->map_push_elem) -
> - __bpf_call_base;
> + insn->imm = BPF_CALL_IMM(ops->map_push_elem);
> continue;
> case BPF_FUNC_map_pop_elem:
> - insn->imm = BPF_CAST_CALL(ops->map_pop_elem) -
> - __bpf_call_base;
> + insn->imm = BPF_CALL_IMM(ops->map_pop_elem);
> continue;
> case BPF_FUNC_map_peek_elem:
> - insn->imm = BPF_CAST_CALL(ops->map_peek_elem) -
> - __bpf_call_base;
> + insn->imm = BPF_CALL_IMM(ops->map_peek_elem);
> continue;
> case BPF_FUNC_redirect_map:
> - insn->imm = BPF_CAST_CALL(ops->map_redirect) -
> - __bpf_call_base;
> + insn->imm = BPF_CALL_IMM(ops->map_redirect);
> continue;
> }
>
> diff --git a/lib/test_bpf.c b/lib/test_bpf.c
> index 08f438e6fe9e..21ea1ab253a1 100644
> --- a/lib/test_bpf.c
> +++ b/lib/test_bpf.c
> @@ -12439,7 +12439,7 @@ static __init int prepare_tail_call_tests(struct bpf_array **pprogs)
> err = -EFAULT;
> goto out_err;
> }
> - *insn = BPF_EMIT_CALL(BPF_CAST_CALL(addr));
> + *insn = BPF_EMIT_CALL(addr);
> if ((long)__bpf_call_base + insn->imm != addr)
> *insn = BPF_JMP_A(0); /* Skip: NOP */
> break;
> --
> 2.30.2
>