Re: [PATCH v13 02/10] btf: Handle dynamic pointer parameter in kfuncs

On Tue, 23 Aug 2022 at 19:27, Roberto Sassu roberto.sassu@huaweicloud.com wrote:

From: Roberto Sassu roberto.sassu@huawei.com

Allow the bpf_dynptr_kern parameter to be specified in kfuncs. Also, ensure that the dynamic pointer is valid and initialized.

To properly detect whether a parameter is of the desired type, introduce the stringify_struct() macro to compare the returned structure name with the desired name. In addition, protect against structure renames, by halting the build with BUILD_BUG_ON(), so that developers have to revisit the code.

Cc: Joanne Koong joannelkoong@gmail.com Signed-off-by: Roberto Sassu roberto.sassu@huawei.com

---

include/linux/bpf_verifier.h | 3 +++ include/linux/btf.h | 9 +++++++++ kernel/bpf/btf.c | 18 ++++++++++++++++++ kernel/bpf/verifier.c | 4 ++-- 4 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h index 2e3bad8640dc..55876fbdbae2 100644 --- a/include/linux/bpf_verifier.h +++ b/include/linux/bpf_verifier.h @@ -560,6 +560,9 @@ int check_kfunc_mem_size_reg(struct bpf_verifier_env *env, struct bpf_reg_state u32 regno); int check_mem_reg(struct bpf_verifier_env *env, struct bpf_reg_state *reg, u32 regno, u32 mem_size); +bool is_dynptr_reg_valid_init(struct bpf_verifier_env *env,

•                         struct bpf_reg_state *reg,
  

•                         enum bpf_arg_type arg_type);
  
  

/* this lives here instead of in bpf.h because it needs to dereference tgt_prog */ static inline u64 bpf_trampoline_compute_key(const struct bpf_prog *tgt_prog, diff --git a/include/linux/btf.h b/include/linux/btf.h index ad93c2d9cc1c..f546d368ac5d 100644 --- a/include/linux/btf.h +++ b/include/linux/btf.h @@ -52,6 +52,15 @@ #define KF_SLEEPABLE (1 << 5) /* kfunc may sleep */ #define KF_DESTRUCTIVE (1 << 6) /* kfunc performs destructive actions */

+/*

• • Return the name of the passed struct, if exists, or halt the build if for
• • example the structure gets renamed. In this way, developers have to revisit
• • the code using that structure name, and update it accordingly.
• */

+#define stringify_struct(x) \

•   ({ BUILD_BUG_ON(sizeof(struct x) < 0);  \
  

•      __stringify(x); })
  

struct btf; struct btf_member; struct btf_type; diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c index e49b3b6d48ad..26cb548420af 100644 --- a/kernel/bpf/btf.c +++ b/kernel/bpf/btf.c @@ -6362,15 +6362,20 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,

                    if (is_kfunc) {
                            bool arg_mem_size = i + 1 < nargs && is_kfunc_arg_mem_size(btf, &args[i + 1], &regs[regno + 1]);

•                           bool arg_dynptr = btf_type_is_struct(ref_t) &&
  

•                                             !strcmp(ref_tname,
  

•                                                     stringify_struct(bpf_dynptr_kern));
  
                            /* Permit pointer to mem, but only when argument
                             * type is pointer to scalar, or struct composed
                             * (recursively) of scalars.
                             * When arg_mem_size is true, the pointer can be
                             * void *.
  

•                            * Also permit initialized dynamic pointers.
                             */
                            if (!btf_type_is_scalar(ref_t) &&
                                !__btf_type_is_scalar_struct(log, btf, ref_t, 0) &&
  

•                               !arg_dynptr &&
                                (arg_mem_size ? !btf_type_is_void(ref_t) : 1)) {
                                    bpf_log(log,
                                            "arg#%d pointer type %s %s must point to %sscalar, or struct with scalar\n",
  

@@ -6378,6 +6383,19 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env, return -EINVAL; }

•                           if (arg_dynptr) {
  

•                                   if (!is_dynptr_reg_valid_init(env, reg,
  

•                                                   ARG_PTR_TO_DYNPTR)) {
  

Do you intend to really accept all kinds of dynptr here? In the future we will get more, and by default ARG_PTR_TO_DYNPTR accepts all, so it seems better to start with a small strict subset.

Secondly, you need to also check whether reg is a PTR_TO_STACK inside this arg_dynptr branch. It is incorrect to call is_dynptr_reg_valid_init for any other type of register. It would also be nice to include some tests for that.

•                                           bpf_log(log,
  

•                                                   "arg#%d pointer type %s %s must be initialized\n",
  

•                                                   i, btf_type_str(ref_t),
  

•                                                   ref_tname);
  

•                                           return -EINVAL;
  

•                                   }
  

•                                   continue;
  

•                           }
  

•                           /* Check for mem, len pair */
                            if (arg_mem_size) {
                                    if (check_kfunc_mem_size_reg(env, &regs[regno + 1], regno + 1)) {
  

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 2c1f8069f7b7..aa834e7bb296 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -779,8 +779,8 @@ static bool is_dynptr_reg_valid_uninit(struct bpf_verifier_env *env, struct bpf_ return true; }

-static bool is_dynptr_reg_valid_init(struct bpf_verifier_env *env, struct bpf_reg_state *reg,

•                                enum bpf_arg_type arg_type)
  

+bool is_dynptr_reg_valid_init(struct bpf_verifier_env *env, struct bpf_reg_state *reg,

•                         enum bpf_arg_type arg_type)
  

{ struct bpf_func_state *state = func(env, reg); int spi = get_spi(reg->off); -- 2.25.1