[PATCH 5.19 026/155] bpf: Allow helpers to accept pointers with a fixed size

Greg Kroah-Hartman
2022-09-06T14:01:57Z

From: Maxim Mikityanskiy <maximmi@nvidia.com>

[ Upstream commit 508362ac66b0478affb4e52cb8da98478312d72d ]

Before this commit, the BPF verifier required ARG_PTR_TO_MEM arguments
to be followed by ARG_CONST_SIZE holding the size of the memory region.
The helpers had to check that size in runtime.

There are cases where the size expected by a helper is a compile-time
constant. Checking it in runtime is an unnecessary overhead and waste of
BPF registers.

This commit allows helpers to accept pointers to memory without the
corresponding ARG_CONST_SIZE, given that they define the memory region
size in struct bpf_func_proto and use ARG_PTR_TO_FIXED_SIZE_MEM type.

arg_size is unionized with arg_btf_id to reduce the kernel image size,
and it's valid because they are used by different argument types.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/20220615134847.3753567-3-maximmi@nvidia.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/bpf.h | 13 +++++++++++++
kernel/bpf/verifier.c | 43 ++++++++++++++++++++++++++++++++-----------
2 files changed, 45 insertions(+), 11 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 7424cf234ae03..ed352c00330cd 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -398,6 +398,9 @@ enum bpf_type_flag {
/* DYNPTR points to a ringbuf record. */
DYNPTR_TYPE_RINGBUF = BIT(9 + BPF_BASE_TYPE_BITS),

+ /* Size is known at compile time. */
+ MEM_FIXED_SIZE = BIT(10 + BPF_BASE_TYPE_BITS),
+
__BPF_TYPE_FLAG_MAX,
__BPF_TYPE_LAST_FLAG = __BPF_TYPE_FLAG_MAX - 1,
};
@@ -461,6 +464,8 @@ enum bpf_arg_type {
* all bytes or clear them in error case.
*/
ARG_PTR_TO_UNINIT_MEM = MEM_UNINIT | ARG_PTR_TO_MEM,
+ /* Pointer to valid memory of size known at compile time. */
+ ARG_PTR_TO_FIXED_SIZE_MEM = MEM_FIXED_SIZE | ARG_PTR_TO_MEM,

/* This must be the last entry. Its purpose is to ensure the enum is
* wide enough to hold the higher bits reserved for bpf_type_flag.
@@ -526,6 +531,14 @@ struct bpf_func_proto {
u32 *arg5_btf_id;
};
u32 *arg_btf_id[5];
+ struct {
+ size_t arg1_size;
+ size_t arg2_size;
+ size_t arg3_size;
+ size_t arg4_size;
+ size_t arg5_size;
+ };
+ size_t arg_size[5];
};
int *ret_btf_id; /* return value btf_id */
bool (*allowed)(const struct bpf_prog *prog);
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 0e45d405f151c..f0dd73bf69ddf 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5847,6 +5847,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
enum bpf_arg_type arg_type = fn->arg_type[arg];
enum bpf_reg_type type = reg->type;
+ u32 *arg_btf_id = NULL;
int err = 0;

if (arg_type == ARG_DONTCARE)
@@ -5883,7 +5884,11 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
*/
goto skip_type_check;

- err = check_reg_type(env, regno, arg_type, fn->arg_btf_id[arg], meta);
+ /* arg_btf_id and arg_size are in a union. */
+ if (base_type(arg_type) == ARG_PTR_TO_BTF_ID)
+ arg_btf_id = fn->arg_btf_id[arg];
+
+ err = check_reg_type(env, regno, arg_type, arg_btf_id, meta);
if (err)
return err;

@@ -6010,6 +6015,11 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
* next is_mem_size argument below.
*/
meta->raw_mode = arg_type & MEM_UNINIT;
+ if (arg_type & MEM_FIXED_SIZE) {
+ err = check_helper_mem_access(env, regno,
+ fn->arg_size[arg], false,
+ meta);
+ }
} else if (arg_type_is_mem_size(arg_type)) {
bool zero_size_allowed = (arg_type == ARG_CONST_SIZE_OR_ZERO);

@@ -6400,11 +6410,19 @@ static bool check_raw_mode_ok(const struct bpf_func_proto *fn)
return count <= 1;
}

-static bool check_args_pair_invalid(enum bpf_arg_type arg_curr,
- enum bpf_arg_type arg_next)
+static bool check_args_pair_invalid(const struct bpf_func_proto *fn, int arg)
{
- return (base_type(arg_curr) == ARG_PTR_TO_MEM) !=
- arg_type_is_mem_size(arg_next);
+ bool is_fixed = fn->arg_type[arg] & MEM_FIXED_SIZE;
+ bool has_size = fn->arg_size[arg] != 0;
+ bool is_next_size = false;
+
+ if (arg + 1 < ARRAY_SIZE(fn->arg_type))
+ is_next_size = arg_type_is_mem_size(fn->arg_type[arg + 1]);
+
+ if (base_type(fn->arg_type[arg]) != ARG_PTR_TO_MEM)
+ return is_next_size;
+
+ return has_size == is_next_size || is_next_size == is_fixed;
}

static bool check_arg_pair_ok(const struct bpf_func_proto *fn)
@@ -6415,11 +6433,11 @@ static bool check_arg_pair_ok(const struct bpf_func_proto *fn)
* helper function specification.
*/
if (arg_type_is_mem_size(fn->arg1_type) ||
- base_type(fn->arg5_type) == ARG_PTR_TO_MEM ||
- check_args_pair_invalid(fn->arg1_type, fn->arg2_type) ||
- check_args_pair_invalid(fn->arg2_type, fn->arg3_type) ||
- check_args_pair_invalid(fn->arg3_type, fn->arg4_type) ||
- check_args_pair_invalid(fn->arg4_type, fn->arg5_type))
+ check_args_pair_invalid(fn, 0) ||
+ check_args_pair_invalid(fn, 1) ||
+ check_args_pair_invalid(fn, 2) ||
+ check_args_pair_invalid(fn, 3) ||
+ check_args_pair_invalid(fn, 4))
return false;

return true;
@@ -6460,7 +6478,10 @@ static bool check_btf_id_ok(const struct bpf_func_proto *fn)
if (base_type(fn->arg_type[i]) == ARG_PTR_TO_BTF_ID && !fn->arg_btf_id[i])
return false;

- if (base_type(fn->arg_type[i]) != ARG_PTR_TO_BTF_ID && fn->arg_btf_id[i])
+ if (base_type(fn->arg_type[i]) != ARG_PTR_TO_BTF_ID && fn->arg_btf_id[i] &&
+ /* arg_btf_id and arg_size are in a union. */
+ (base_type(fn->arg_type[i]) != ARG_PTR_TO_MEM ||
+ !(fn->arg_type[i] & MEM_FIXED_SIZE)))
return false;
}

--
2.35.1