Re: [PATCH 17/39] bpf: resolve_pseudo_ldimm64(): take handling of a single ldimm64 insn into helper

Andrii Nakryiko
2024-08-06T22:32:32Z

On Mon, Jul 29, 2024 at 10:20 PM <viro@kernel.org> wrote:
>
> From: Al Viro <viro@zeniv.linux.org.uk>
>
> Equivalent transformation. For one thing, it's easier to follow that way.
> For another, that simplifies the control flow in the vicinity of struct fd
> handling in there, which will allow a switch to CLASS(fd) and make the
> thing much easier to verify wrt leaks.
>
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
> kernel/bpf/verifier.c | 342 +++++++++++++++++++++---------------------
> 1 file changed, 172 insertions(+), 170 deletions(-)
>

This looks unnecessarily intrusive. I think it's best to extract the
logic of fetching and adding bpf_map by fd into a helper and that way
contain fdget + fdput logic nicely. Something like below, which I can
send to bpf-next.

commit b5eec08241cc0263e560551de91eda73ccc5987d
Author: Andrii Nakryiko <andrii@kernel.org>
Date: Tue Aug 6 14:31:34 2024 -0700

bpf: factor out fetching bpf_map from FD and adding it to used_maps list

Factor out the logic to extract bpf_map instances from FD embedded in
bpf_insns, adding it to the list of used_maps (unless it's already
there, in which case we just reuse map's index). This simplifies the
logic in resolve_pseudo_ldimm64(), especially around `struct fd`
handling, as all that is now neatly contained in the helper and doesn't
leak into a dozen error handling paths.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index df3be12096cf..14e4ef687a59 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -18865,6 +18865,58 @@ static bool bpf_map_is_cgroup_storage(struct
bpf_map *map)
map->map_type == BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE);
}

+/* Add map behind fd to used maps list, if it's not already there, and return
+ * its index. Also set *reused to true if this map was already in the list of
+ * used maps.
+ * Returns <0 on error, or >= 0 index, on success.
+ */
+static int add_used_map_from_fd(struct bpf_verifier_env *env, int fd,
bool *reused)
+{
+ struct fd f = fdget(fd);
+ struct bpf_map *map;
+ int i;
+
+ map = __bpf_map_get(f);
+ if (IS_ERR(map)) {
+ verbose(env, "fd %d is not pointing to valid bpf_map\n", fd);
+ return PTR_ERR(map);
+ }
+
+ /* check whether we recorded this map already */
+ for (i = 0; i < env->used_map_cnt; i++) {
+ if (env->used_maps[i] == map) {
+ *reused = true;
+ fdput(f);
+ return i;
+ }
+ }
+
+ if (env->used_map_cnt >= MAX_USED_MAPS) {
+ verbose(env, "The total number of maps per program has
reached the limit of %u\n",
+ MAX_USED_MAPS);
+ fdput(f);
+ return -E2BIG;
+ }
+
+ if (env->prog->sleepable)
+ atomic64_inc(&map->sleepable_refcnt);
+
+ /* hold the map. If the program is rejected by verifier,
+ * the map will be released by release_maps() or it
+ * will be used by the valid program until it's unloaded
+ * and all maps are released in bpf_free_used_maps()
+ */
+ bpf_map_inc(map);
+
+ *reused = false;
+ env->used_maps[env->used_map_cnt++] = map;
+
+ fdput(f);
+
+ return env->used_map_cnt - 1;
+
+}
+
/* find and rewrite pseudo imm in ld_imm64 instructions:
*
* 1. if it accesses map FD, replace it with actual map pointer.
@@ -18876,7 +18928,7 @@ static int resolve_pseudo_ldimm64(struct
bpf_verifier_env *env)
{
struct bpf_insn *insn = env->prog->insnsi;
int insn_cnt = env->prog->len;
- int i, j, err;
+ int i, err;

err = bpf_prog_calc_tag(env->prog);
if (err)
@@ -18893,9 +18945,10 @@ static int resolve_pseudo_ldimm64(struct
bpf_verifier_env *env)
if (insn[0].code == (BPF_LD | BPF_IMM | BPF_DW)) {
struct bpf_insn_aux_data *aux;
struct bpf_map *map;
- struct fd f;
+ int map_idx;
u64 addr;
u32 fd;
+ bool reused;

if (i == insn_cnt - 1 || insn[1].code != 0 ||
insn[1].dst_reg != 0 || insn[1].src_reg != 0 ||
@@ -18956,20 +19009,18 @@ static int resolve_pseudo_ldimm64(struct
bpf_verifier_env *env)
break;
}

- f = fdget(fd);
- map = __bpf_map_get(f);
- if (IS_ERR(map)) {
- verbose(env, "fd %d is not pointing to valid bpf_map\n", fd);
- return PTR_ERR(map);
- }
+ map_idx = add_used_map_from_fd(env, fd, &reused);
+ if (map_idx < 0)
+ return map_idx;
+ map = env->used_maps[map_idx];
+
+ aux = &env->insn_aux_data[i];
+ aux->map_index = map_idx;

err = check_map_prog_compatibility(env, map, env->prog);
- if (err) {
- fdput(f);
+ if (err)
return err;
- }

- aux = &env->insn_aux_data[i];
if (insn[0].src_reg == BPF_PSEUDO_MAP_FD ||
insn[0].src_reg == BPF_PSEUDO_MAP_IDX) {
addr = (unsigned long)map;
@@ -18978,13 +19029,11 @@ static int resolve_pseudo_ldimm64(struct
bpf_verifier_env *env)

if (off >= BPF_MAX_VAR_OFF) {
verbose(env, "direct value offset of %u is not
allowed\n", off);
- fdput(f);
return -EINVAL;
}

if (!map->ops->map_direct_value_addr) {
verbose(env, "no direct value access support for
this map type\n");
- fdput(f);
return -EINVAL;
}

@@ -18992,7 +19041,6 @@ static int resolve_pseudo_ldimm64(struct
bpf_verifier_env *env)
if (err) {
verbose(env, "invalid access to map value
pointer, value_size=%u off=%u\n",
map->value_size, off);
- fdput(f);
return err;
}

@@ -19003,70 +19051,39 @@ static int resolve_pseudo_ldimm64(struct
bpf_verifier_env *env)
insn[0].imm = (u32)addr;
insn[1].imm = addr >> 32;

- /* check whether we recorded this map already */
- for (j = 0; j < env->used_map_cnt; j++) {
- if (env->used_maps[j] == map) {
- aux->map_index = j;
- fdput(f);
- goto next_insn;
- }
- }
-
- if (env->used_map_cnt >= MAX_USED_MAPS) {
- verbose(env, "The total number of maps per program
has reached the limit of %u\n",
- MAX_USED_MAPS);
- fdput(f);
- return -E2BIG;
- }
-
- if (env->prog->sleepable)
- atomic64_inc(&map->sleepable_refcnt);
- /* hold the map. If the program is rejected by verifier,
- * the map will be released by release_maps() or it
- * will be used by the valid program until it's unloaded
- * and all maps are released in bpf_free_used_maps()
- */
- bpf_map_inc(map);
-
- aux->map_index = env->used_map_cnt;
- env->used_maps[env->used_map_cnt++] = map;
+ /* proceed with extra checks only if its newly added used map */
+ if (reused)
+ goto next_insn;

if (bpf_map_is_cgroup_storage(map) &&
bpf_cgroup_storage_assign(env->prog->aux, map)) {
verbose(env, "only one cgroup storage of each type is
allowed\n");
- fdput(f);
return -EBUSY;
}
if (map->map_type == BPF_MAP_TYPE_ARENA) {
if (env->prog->aux->arena) {
verbose(env, "Only one arena per program\n");
- fdput(f);
return -EBUSY;
}
if (!env->allow_ptr_leaks || !env->bpf_capable) {
verbose(env, "CAP_BPF and CAP_PERFMON are
required to use arena\n");
- fdput(f);
return -EPERM;
}
if (!env->prog->jit_requested) {
verbose(env, "JIT is required to use arena\n");
- fdput(f);
return -EOPNOTSUPP;
}
if (!bpf_jit_supports_arena()) {
verbose(env, "JIT doesn't support arena\n");
- fdput(f);
return -EOPNOTSUPP;
}
env->prog->aux->arena = (void *)map;
if (!bpf_arena_get_user_vm_start(env->prog->aux->arena)) {
verbose(env, "arena's user address must be set
via map_extra or mmap()\n");
- fdput(f);
return -EINVAL;
}
}

- fdput(f);
next_insn:
insn++;
i++; …