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

Kees Cook
2021-09-28T23:10:09Z

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
---
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