[PATCH bpf-next v2 0/3] bpf: Handle MEM_RCU type properly

[PATCH bpf-next v2 0/3] bpf: Handle MEM_RCU type properly

[Yonghong Song]

Patch set [1] added rcu support for bpf programs. In [1], a rcu
pointer is considered to be trusted and not null. This is actually
not true in some cases. The rcu pointer could be null, and for non-null
rcu pointer, it may have reference count of 0. This small patch set
fixed this problem. Patch 1 is the kernel fix. Patch 2 adjusted
selftests properly. Patch 3 added documentation for newly-introduced
KF_RCU flag.

  [1] https://lore.kernel.org/all/20221124053201.2372298-1-yhs@fb.com/

Changelogs:
  v1 -> v2:
    - rcu ptr could be NULL.
    - non_null_rcu_ptr->rcu_field can be marked as MEM_RCU as well.
    - Adjust the code to avoid existing error message change.

Yonghong Song (3):
  bpf: Handle MEM_RCU type properly
  selftests/bpf: Fix rcu_read_lock test with new MEM_RCU semantics
  docs/bpf: Add KF_RCU documentation

 Documentation/bpf/kfuncs.rst                  |  9 +++
 include/linux/bpf_verifier.h                  |  2 +-
 include/linux/btf.h                           |  1 +
 kernel/bpf/helpers.c                          | 14 +++++
 kernel/bpf/verifier.c                         | 45 ++++++++++-----
 .../selftests/bpf/progs/rcu_read_lock.c       | 55 +++++++++++++++----
 6 files changed, 102 insertions(+), 24 deletions(-)

-- 
2.30.2

[PATCH bpf-next v2 1/3] bpf: Handle MEM_RCU type properly

[Yonghong Song]

Commit 9bb00b2895cb ("bpf: Add kfunc bpf_rcu_read_lock/unlock()")
introduced MEM_RCU and bpf_rcu_read_lock/unlock() support. In that
commit, a rcu pointer is tagged with both MEM_RCU and PTR_TRUSTED
so that it can be passed into kfuncs or helpers as an argument.

Martin raised a good question in [1] such that the rcu pointer,
although being able to accessing the object, might have reference
count of 0. This might cause a problem if the rcu pointer is passed
to a kfunc which expects trusted arguments where ref count should
be greater than 0.

This patch makes the following changes related to MEM_RCU pointer:
  - MEM_RCU pointer might be NULL (PTR_MAYBE_NULL).
  - Introduce KF_RCU so MEM_RCU ptr can be acquired with
    a KF_RCU tagged kfunc which assumes ref count of rcu ptr
    could be zero.
  - For mem access 'b = ptr->a', say 'ptr' is a MEM_RCU ptr, and
    'a' is tagged with __rcu as well. Let us mark 'b' as
    MEM_RCU | PTR_MAYBE_NULL.

 [1] https://lore.kernel.org/bpf/ac70f574-4023-664e-b711-e0d3b18117fd@linux.dev/

Fixes: 9bb00b2895cb ("bpf: Add kfunc bpf_rcu_read_lock/unlock()")
Signed-off-by: Yonghong Song <yhs@fb.com>
---
 include/linux/bpf_verifier.h |  2 +-
 include/linux/btf.h          |  1 +
 kernel/bpf/helpers.c         | 14 +++++++++++
 kernel/bpf/verifier.c        | 45 +++++++++++++++++++++++++-----------
 4 files changed, 48 insertions(+), 14 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index b5090e89cb3f..c07b351a5bc7 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -682,7 +682,7 @@ static inline bool bpf_prog_check_recur(const struct bpf_prog *prog)
 	}
 }
 
-#define BPF_REG_TRUSTED_MODIFIERS (MEM_ALLOC | MEM_RCU | PTR_TRUSTED)
+#define BPF_REG_TRUSTED_MODIFIERS (MEM_ALLOC | PTR_TRUSTED)
 
 static inline bool bpf_type_has_unsafe_modifiers(u32 type)
 {
diff --git a/include/linux/btf.h b/include/linux/btf.h
index 9ed00077db6e..cbd6e4096f8c 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -70,6 +70,7 @@
 #define KF_TRUSTED_ARGS (1 << 4) /* kfunc only takes trusted pointer arguments */
 #define KF_SLEEPABLE    (1 << 5) /* kfunc may sleep */
 #define KF_DESTRUCTIVE  (1 << 6) /* kfunc performs destructive actions */
+#define KF_RCU          (1 << 7) /* kfunc only takes rcu pointer arguments */
 
 /*
  * Return the name of the passed struct, if exists, or halt the build if for
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index a5a511430f2a..cca642358e80 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1837,6 +1837,19 @@ struct task_struct *bpf_task_acquire(struct task_struct *p)
 	return p;
 }
 
+/**
+ * bpf_task_acquire_not_zero - Acquire a reference to a rcu task object. A task
+ * acquired by this kfunc which is not stored in a map as a kptr, must be
+ * released by calling bpf_task_release().
+ * @p: The task on which a reference is being acquired.
+ */
+struct task_struct *bpf_task_acquire_not_zero(struct task_struct *p)
+{
+	if (!refcount_inc_not_zero(&p->rcu_users))
+		return NULL;
+	return p;
+}
+
 /**
  * bpf_task_kptr_get - Acquire a reference on a struct task_struct kptr. A task
  * kptr acquired by this kfunc which is not subsequently stored in a map, must
@@ -2013,6 +2026,7 @@ BTF_ID_FLAGS(func, bpf_list_push_back)
 BTF_ID_FLAGS(func, bpf_list_pop_front, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_list_pop_back, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_task_acquire, KF_ACQUIRE | KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_task_acquire_not_zero, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_task_kptr_get, KF_ACQUIRE | KF_KPTR_GET | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_task_release, KF_RELEASE)
 #ifdef CONFIG_CGROUPS
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b0db9c10567b..66b82f52b5bc 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4275,7 +4275,7 @@ static bool is_trusted_reg(const struct bpf_reg_state *reg)
 		return true;
 
 	/* If a register is not referenced, it is trusted if it has the
-	 * MEM_ALLOC, MEM_RCU or PTR_TRUSTED type modifiers, and no others. Some of the
+	 * MEM_ALLOC or PTR_TRUSTED type modifiers, and no others. Some of the
 	 * other type modifiers may be safe, but we elect to take an opt-in
 	 * approach here as some (e.g. PTR_UNTRUSTED and PTR_MAYBE_NULL) are
 	 * not.
@@ -4287,6 +4287,11 @@ static bool is_trusted_reg(const struct bpf_reg_state *reg)
 	       !bpf_type_has_unsafe_modifiers(reg->type);
 }
 
+static bool is_rcu_reg(const struct bpf_reg_state *reg)
+{
+	return reg->type & MEM_RCU;
+}
+
 static int check_pkt_ptr_alignment(struct bpf_verifier_env *env,
 				   const struct bpf_reg_state *reg,
 				   int off, int size, bool strict)
@@ -4785,14 +4790,16 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 
 	if (flag & MEM_RCU) {
 		/* Mark value register as MEM_RCU only if it is protected by
-		 * bpf_rcu_read_lock() and the ptr reg is trusted. MEM_RCU
+		 * bpf_rcu_read_lock() and the ptr reg is rcu or trusted. MEM_RCU
 		 * itself can already indicate trustedness inside the rcu
-		 * read lock region. Also mark it as PTR_TRUSTED.
+		 * read lock region. Also mark rcu pointer as PTR_MAYBE_NULL since
+		 * it could be null in some cases.
 		 */
-		if (!env->cur_state->active_rcu_lock || !is_trusted_reg(reg))
+		if (!env->cur_state->active_rcu_lock ||
+		    !(is_trusted_reg(reg) || is_rcu_reg(reg)))
 			flag &= ~MEM_RCU;
 		else
-			flag |= PTR_TRUSTED;
+			flag |= PTR_MAYBE_NULL;
 	} else if (reg->type & MEM_RCU) {
 		/* ptr (reg) is marked as MEM_RCU, but the struct field is not tagged
 		 * with __rcu. Mark the flag as PTR_UNTRUSTED conservatively.
@@ -5957,7 +5964,7 @@ static const struct bpf_reg_types btf_ptr_types = {
 	.types = {
 		PTR_TO_BTF_ID,
 		PTR_TO_BTF_ID | PTR_TRUSTED,
-		PTR_TO_BTF_ID | MEM_RCU | PTR_TRUSTED,
+		PTR_TO_BTF_ID | MEM_RCU,
 	},
 };
 static const struct bpf_reg_types percpu_btf_ptr_types = {
@@ -6136,7 +6143,7 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
 	case PTR_TO_BTF_ID:
 	case PTR_TO_BTF_ID | MEM_ALLOC:
 	case PTR_TO_BTF_ID | PTR_TRUSTED:
-	case PTR_TO_BTF_ID | MEM_RCU | PTR_TRUSTED:
+	case PTR_TO_BTF_ID | MEM_RCU:
 	case PTR_TO_BTF_ID | MEM_ALLOC | PTR_TRUSTED:
 		/* When referenced PTR_TO_BTF_ID is passed to release function,
 		 * it's fixed offset must be 0.	In the other cases, fixed offset
@@ -8038,6 +8045,11 @@ static bool is_kfunc_destructive(struct bpf_kfunc_call_arg_meta *meta)
 	return meta->kfunc_flags & KF_DESTRUCTIVE;
 }
 
+static bool is_kfunc_rcu(struct bpf_kfunc_call_arg_meta *meta)
+{
+	return meta->kfunc_flags & KF_RCU;
+}
+
 static bool is_kfunc_arg_kptr_get(struct bpf_kfunc_call_arg_meta *meta, int arg)
 {
 	return arg == 0 && (meta->kfunc_flags & KF_KPTR_GET);
@@ -8722,13 +8734,20 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 		switch (kf_arg_type) {
 		case KF_ARG_PTR_TO_ALLOC_BTF_ID:
 		case KF_ARG_PTR_TO_BTF_ID:
-			if (!is_kfunc_trusted_args(meta))
+			if (!is_kfunc_trusted_args(meta) && !is_kfunc_rcu(meta))
 				break;
 
 			if (!is_trusted_reg(reg)) {
-				verbose(env, "R%d must be referenced or trusted\n", regno);
-				return -EINVAL;
+				if (!is_kfunc_rcu(meta)) {
+					verbose(env, "R%d must be referenced or trusted\n", regno);
+					return -EINVAL;
+				}
+				if (!is_rcu_reg(reg)) {
+					verbose(env, "R%d must be a rcu pointer\n", regno);
+					return -EINVAL;
+				}
 			}
+
 			fallthrough;
 		case KF_ARG_PTR_TO_CTX:
 			/* Trusted arguments have the same offset checks as release arguments */
@@ -8839,7 +8858,7 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 		case KF_ARG_PTR_TO_BTF_ID:
 			/* Only base_type is checked, further checks are done here */
 			if ((base_type(reg->type) != PTR_TO_BTF_ID ||
-			     bpf_type_has_unsafe_modifiers(reg->type)) &&
+			     (bpf_type_has_unsafe_modifiers(reg->type) && !is_rcu_reg(reg))) &&
 			    !reg2btf_ids[base_type(reg->type)]) {
 				verbose(env, "arg#%d is %s ", i, reg_type_str(env, reg->type));
 				verbose(env, "expected %s or socket\n",
@@ -8954,7 +8973,7 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		} else if (rcu_unlock) {
 			bpf_for_each_reg_in_vstate(env->cur_state, state, reg, ({
 				if (reg->type & MEM_RCU) {
-					reg->type &= ~(MEM_RCU | PTR_TRUSTED);
+					reg->type &= ~(MEM_RCU | PTR_MAYBE_NULL);
 					reg->type |= PTR_UNTRUSTED;
 				}
 			}));
@@ -11294,7 +11313,7 @@ static void mark_ptr_or_null_reg(struct bpf_func_state *state,
 				 bool is_null)
 {
 	if (type_may_be_null(reg->type) && reg->id == id &&
-	    !WARN_ON_ONCE(!reg->id)) {
+	    (is_rcu_reg(reg) || !WARN_ON_ONCE(!reg->id))) {
 		/* Old offset (both fixed and variable parts) should have been
 		 * known-zero, because we don't allow pointer arithmetic on
 		 * pointers that might be NULL. If we see this happening, don't
-- 
2.30.2

[PATCH bpf-next v2 2/3] selftests/bpf: Fix rcu_read_lock test with new MEM_RCU semantics

[Yonghong Song]

Add MEM_RCU pointer null checking for related tests. Also
modified task_acquire test so it takes a rcu ptr 'ptr' where
'ptr = rcu_ptr->rcu_field'.

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 .../selftests/bpf/progs/rcu_read_lock.c       | 55 +++++++++++++++----
 1 file changed, 45 insertions(+), 10 deletions(-)

diff --git a/tools/testing/selftests/bpf/progs/rcu_read_lock.c b/tools/testing/selftests/bpf/progs/rcu_read_lock.c
index 94a970076b98..cf06a34fcb02 100644
--- a/tools/testing/selftests/bpf/progs/rcu_read_lock.c
+++ b/tools/testing/selftests/bpf/progs/rcu_read_lock.c
@@ -23,13 +23,14 @@ struct bpf_key *bpf_lookup_user_key(__u32 serial, __u64 flags) __ksym;
 void bpf_key_put(struct bpf_key *key) __ksym;
 void bpf_rcu_read_lock(void) __ksym;
 void bpf_rcu_read_unlock(void) __ksym;
-struct task_struct *bpf_task_acquire(struct task_struct *p) __ksym;
+struct task_struct *bpf_task_acquire_not_zero(struct task_struct *p) __ksym;
 void bpf_task_release(struct task_struct *p) __ksym;
 
 SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
 int get_cgroup_id(void *ctx)
 {
 	struct task_struct *task;
+	struct css_set *cgroups;
 
 	task = bpf_get_current_task_btf();
 	if (task->pid != target_pid)
@@ -37,7 +38,11 @@ int get_cgroup_id(void *ctx)
 
 	/* simulate bpf_get_current_cgroup_id() helper */
 	bpf_rcu_read_lock();
-	cgroup_id = task->cgroups->dfl_cgrp->kn->id;
+	cgroups = task->cgroups;
+	if (!cgroups)
+		goto unlock;
+	cgroup_id = cgroups->dfl_cgrp->kn->id;
+unlock:
 	bpf_rcu_read_unlock();
 	return 0;
 }
@@ -56,6 +61,8 @@ int task_succ(void *ctx)
 	bpf_rcu_read_lock();
 	/* region including helper using rcu ptr real_parent */
 	real_parent = task->real_parent;
+	if (!real_parent)
+		goto out;
 	ptr = bpf_task_storage_get(&map_a, real_parent, &init_val,
 				   BPF_LOCAL_STORAGE_GET_F_CREATE);
 	if (!ptr)
@@ -92,7 +99,10 @@ int two_regions(void *ctx)
 	bpf_rcu_read_unlock();
 	bpf_rcu_read_lock();
 	real_parent = task->real_parent;
+	if (!real_parent)
+		goto out;
 	(void)bpf_task_storage_get(&map_a, real_parent, 0, 0);
+out:
 	bpf_rcu_read_unlock();
 	return 0;
 }
@@ -105,7 +115,10 @@ int non_sleepable_1(void *ctx)
 	task = bpf_get_current_task_btf();
 	bpf_rcu_read_lock();
 	real_parent = task->real_parent;
+	if (!real_parent)
+		goto out;
 	(void)bpf_task_storage_get(&map_a, real_parent, 0, 0);
+out:
 	bpf_rcu_read_unlock();
 	return 0;
 }
@@ -121,7 +134,10 @@ int non_sleepable_2(void *ctx)
 
 	bpf_rcu_read_lock();
 	real_parent = task->real_parent;
+	if (!real_parent)
+		goto out;
 	(void)bpf_task_storage_get(&map_a, real_parent, 0, 0);
+out:
 	bpf_rcu_read_unlock();
 	return 0;
 }
@@ -129,16 +145,28 @@ int non_sleepable_2(void *ctx)
 SEC("?fentry.s/" SYS_PREFIX "sys_nanosleep")
 int task_acquire(void *ctx)
 {
-	struct task_struct *task, *real_parent;
+	struct task_struct *task, *real_parent, *gparent;
 
 	task = bpf_get_current_task_btf();
 	bpf_rcu_read_lock();
 	real_parent = task->real_parent;
+	if (!real_parent)
+		goto out;
+
+	/* rcu_ptr->rcu_field */
+	gparent = real_parent->real_parent;
+	if (!gparent)
+		goto out;
+
 	/* acquire a reference which can be used outside rcu read lock region */
-	real_parent = bpf_task_acquire(real_parent);
+	gparent = bpf_task_acquire_not_zero(gparent);
+	if (!gparent)
+		goto out;
+
+	(void)bpf_task_storage_get(&map_a, gparent, 0, 0);
+	bpf_task_release(gparent);
+out:
 	bpf_rcu_read_unlock();
-	(void)bpf_task_storage_get(&map_a, real_parent, 0, 0);
-	bpf_task_release(real_parent);
 	return 0;
 }
 
@@ -181,9 +209,12 @@ int non_sleepable_rcu_mismatch(void *ctx)
 	/* non-sleepable: missing bpf_rcu_read_unlock() in one path */
 	bpf_rcu_read_lock();
 	real_parent = task->real_parent;
+	if (!real_parent)
+		goto out;
 	(void)bpf_task_storage_get(&map_a, real_parent, 0, 0);
 	if (real_parent)
 		bpf_rcu_read_unlock();
+out:
 	return 0;
 }
 
@@ -199,16 +230,17 @@ int inproper_sleepable_helper(void *ctx)
 	/* sleepable helper in rcu read lock region */
 	bpf_rcu_read_lock();
 	real_parent = task->real_parent;
+	if (!real_parent)
+		goto out;
 	regs = (struct pt_regs *)bpf_task_pt_regs(real_parent);
-	if (!regs) {
-		bpf_rcu_read_unlock();
-		return 0;
-	}
+	if (!regs)
+		goto out;
 
 	ptr = (void *)PT_REGS_IP(regs);
 	(void)bpf_copy_from_user_task(&value, sizeof(uint32_t), ptr, task, 0);
 	user_data = value;
 	(void)bpf_task_storage_get(&map_a, real_parent, 0, 0);
+out:
 	bpf_rcu_read_unlock();
 	return 0;
 }
@@ -239,7 +271,10 @@ int nested_rcu_region(void *ctx)
 	bpf_rcu_read_lock();
 	bpf_rcu_read_lock();
 	real_parent = task->real_parent;
+	if (!real_parent)
+		goto out;
 	(void)bpf_task_storage_get(&map_a, real_parent, 0, 0);
+out:
 	bpf_rcu_read_unlock();
 	bpf_rcu_read_unlock();
 	return 0;
-- 
2.30.2

[PATCH bpf-next v2 3/3] docs/bpf: Add KF_RCU documentation

[Yonghong Song]

Add proper KF_RCU documentation in kfuncs.rst.

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 Documentation/bpf/kfuncs.rst | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
index 90774479ab7a..b027fe16ee66 100644
--- a/Documentation/bpf/kfuncs.rst
+++ b/Documentation/bpf/kfuncs.rst
@@ -191,6 +191,15 @@ rebooting or panicking. Due to this additional restrictions apply to these
 calls. At the moment they only require CAP_SYS_BOOT capability, but more can be
 added later.
 
+2.4.8 KF_RCU flag
+-----------------
+
+The KF_RCU flag is used for kfuncs which have a rcu ptr as its argument.
+When used together with KF_ACQUIRE, it indicates the kfunc should have a
+single argument which must be a trusted argument or a MEM_RCU pointer.
+The argument may have reference count of 0 and the kfunc must take this
+into consideration.
+
 2.5 Registering the kfuncs
 --------------------------
 
-- 
2.30.2

Re: [PATCH bpf-next v2 0/3] bpf: Handle MEM_RCU type properly

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Sat, 3 Dec 2022 10:45:57 -0800 you wrote:
> Patch set [1] added rcu support for bpf programs. In [1], a rcu
> pointer is considered to be trusted and not null. This is actually
> not true in some cases. The rcu pointer could be null, and for non-null
> rcu pointer, it may have reference count of 0. This small patch set
> fixed this problem. Patch 1 is the kernel fix. Patch 2 adjusted
> selftests properly. Patch 3 added documentation for newly-introduced
> KF_RCU flag.
> 
> [...]

Here is the summary with links:
  - [bpf-next,v2,1/3] bpf: Handle MEM_RCU type properly
    https://git.kernel.org/bpf/bpf-next/c/fca1aa75518c
  - [bpf-next,v2,2/3] selftests/bpf: Fix rcu_read_lock test with new MEM_RCU semantics
    https://git.kernel.org/bpf/bpf-next/c/8723ec22a31d
  - [bpf-next,v2,3/3] docs/bpf: Add KF_RCU documentation
    https://git.kernel.org/bpf/bpf-next/c/f53625649888

You are awesome, thank you!
-- 
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