diff mbox series

[bpf-next,v1,3/4] bpf: Reject programs that try to load __percpu memory.

Message ID 20220304191657.981240-4-haoluo@google.com (mailing list archive)
State Accepted
Commit 5844101a1be9b8636024cb31c865ef13c7cc6db3
Delegated to: BPF
Headers show
Series bpf: add __percpu tagging in vmlinux BTF | expand

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Context Check Description
netdev/tree_selection success Clearly marked for bpf-next
netdev/fixes_present success Fixes tag not required for -next series
netdev/subject_prefix success Link
netdev/cover_letter success Series has a cover letter
netdev/patch_count success Link
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit success Errors and warnings before: 1449 this patch: 1449
netdev/cc_maintainers warning 4 maintainers not CCed: netdev@vger.kernel.org john.fastabend@gmail.com kafai@fb.com songliubraving@fb.com
netdev/build_clang success Errors and warnings before: 196 this patch: 196
netdev/module_param success Was 0 now: 0
netdev/verify_signedoff success Signed-off-by tag matches author and committer
netdev/verify_fixes success No Fixes tag
netdev/build_allmodconfig_warn success Errors and warnings before: 1466 this patch: 1466
netdev/checkpatch warning WARNING: line length of 101 exceeds 80 columns WARNING: line length of 83 exceeds 80 columns
netdev/kdoc success Errors and warnings before: 0 this patch: 0
netdev/source_inline success Was 0 now: 0
bpf/vmtest-bpf-next-PR success PR summary
bpf/vmtest-bpf-next success VM_Test

Commit Message

Hao Luo March 4, 2022, 7:16 p.m. UTC 
With the introduction of the btf_type_tag "percpu", we can add a
MEM_PERCPU to identify those pointers that point to percpu memory.
The ability of differetiating percpu pointers from regular memory
pointers have two benefits:

 1. It forbids unexpected use of percpu pointers, such as direct loads.
    In kernel, there are special functions used for accessing percpu
    memory. Directly loading percpu memory is meaningless. We already
    have BPF helpers like bpf_per_cpu_ptr() and bpf_this_cpu_ptr() that
    wrap the kernel percpu functions. So we can now convert percpu
    pointers into regular pointers in a safe way.

 2. Previously, bpf_per_cpu_ptr() and bpf_this_cpu_ptr() only work on
    PTR_TO_PERCPU_BTF_ID, a special reg_type which describes static
    percpu variables in kernel (we rely on pahole to encode them into
    vmlinux BTF). Now, since we can identify __percpu tagged pointers,
    we can also identify dynamically allocated percpu memory as well.
    It means we can use bpf_xxx_cpu_ptr() on dynamic percpu memory.
    This would be very convenient when accessing fields like
    "cgroup->rstat_cpu".

Cc: Yonghong Song <yhs@fb.com>
Signed-off-by: Hao Luo <haoluo@google.com>
---
 include/linux/bpf.h   | 11 +++++++++--
 kernel/bpf/btf.c      |  8 +++++++-
 kernel/bpf/verifier.c | 24 ++++++++++++++----------
 3 files changed, 30 insertions(+), 13 deletions(-)

Comments

Yonghong Song March 5, 2022, 9:15 p.m. UTC | #1 
On 3/4/22 11:16 AM, Hao Luo wrote:
> With the introduction of the btf_type_tag "percpu", we can add a
> MEM_PERCPU to identify those pointers that point to percpu memory.
> The ability of differetiating percpu pointers from regular memory
> pointers have two benefits:
> 
>   1. It forbids unexpected use of percpu pointers, such as direct loads.
>      In kernel, there are special functions used for accessing percpu
>      memory. Directly loading percpu memory is meaningless. We already
>      have BPF helpers like bpf_per_cpu_ptr() and bpf_this_cpu_ptr() that
>      wrap the kernel percpu functions. So we can now convert percpu
>      pointers into regular pointers in a safe way.
> 
>   2. Previously, bpf_per_cpu_ptr() and bpf_this_cpu_ptr() only work on
>      PTR_TO_PERCPU_BTF_ID, a special reg_type which describes static
>      percpu variables in kernel (we rely on pahole to encode them into
>      vmlinux BTF). Now, since we can identify __percpu tagged pointers,
>      we can also identify dynamically allocated percpu memory as well.
>      It means we can use bpf_xxx_cpu_ptr() on dynamic percpu memory.
>      This would be very convenient when accessing fields like
>      "cgroup->rstat_cpu".
> 
> Cc: Yonghong Song <yhs@fb.com>
> Signed-off-by: Hao Luo <haoluo@google.com>

Acked-by: Yonghong Song <yhs@fb.com>
diff mbox series

Patch

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index f19abc59b6cd..88449fbbe063 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -334,7 +334,15 @@  enum bpf_type_flag {
 	/* MEM is in user address space. */
 	MEM_USER		= BIT(3 + BPF_BASE_TYPE_BITS),
 
-	__BPF_TYPE_LAST_FLAG	= MEM_USER,
+	/* MEM is a percpu memory. MEM_PERCPU tags PTR_TO_BTF_ID. When tagged
+	 * with MEM_PERCPU, PTR_TO_BTF_ID _cannot_ be directly accessed. In
+	 * order to drop this tag, it must be passed into bpf_per_cpu_ptr()
+	 * or bpf_this_cpu_ptr(), which will return the pointer corresponding
+	 * to the specified cpu.
+	 */
+	MEM_PERCPU		= BIT(4 + BPF_BASE_TYPE_BITS),
+
+	__BPF_TYPE_LAST_FLAG	= MEM_PERCPU,
 };
 
 /* Max number of base types. */
@@ -516,7 +524,6 @@  enum bpf_reg_type {
 	 */
 	PTR_TO_MEM,		 /* reg points to valid memory region */
 	PTR_TO_BUF,		 /* reg points to a read/write buffer */
-	PTR_TO_PERCPU_BTF_ID,	 /* reg points to a percpu kernel variable */
 	PTR_TO_FUNC,		 /* reg points to a bpf program function */
 	__BPF_REG_TYPE_MAX,
 
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index b472cf0c8fdb..f9f2218b27fe 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -5057,6 +5057,8 @@  bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 		tag_value = __btf_name_by_offset(btf, t->name_off);
 		if (strcmp(tag_value, "user") == 0)
 			info->reg_type |= MEM_USER;
+		if (strcmp(tag_value, "percpu") == 0)
+			info->reg_type |= MEM_PERCPU;
 	}
 
 	/* skip modifiers */
@@ -5285,12 +5287,16 @@  static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
 				return -EACCES;
 			}
 
-			/* check __user tag */
+			/* check type tag */
 			t = btf_type_by_id(btf, mtype->type);
 			if (btf_type_is_type_tag(t)) {
 				tag_value = __btf_name_by_offset(btf, t->name_off);
+				/* check __user tag */
 				if (strcmp(tag_value, "user") == 0)
 					tmp_flag = MEM_USER;
+				/* check __percpu tag */
+				if (strcmp(tag_value, "percpu") == 0)
+					tmp_flag = MEM_PERCPU;
 			}
 
 			stype = btf_type_skip_modifiers(btf, mtype->type, &id);
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d63b1f40e029..093e47360d6d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -554,7 +554,6 @@  static const char *reg_type_str(struct bpf_verifier_env *env,
 		[PTR_TO_TP_BUFFER]	= "tp_buffer",
 		[PTR_TO_XDP_SOCK]	= "xdp_sock",
 		[PTR_TO_BTF_ID]		= "ptr_",
-		[PTR_TO_PERCPU_BTF_ID]	= "percpu_ptr_",
 		[PTR_TO_MEM]		= "mem",
 		[PTR_TO_BUF]		= "buf",
 		[PTR_TO_FUNC]		= "func",
@@ -562,8 +561,7 @@  static const char *reg_type_str(struct bpf_verifier_env *env,
 	};
 
 	if (type & PTR_MAYBE_NULL) {
-		if (base_type(type) == PTR_TO_BTF_ID ||
-		    base_type(type) == PTR_TO_PERCPU_BTF_ID)
+		if (base_type(type) == PTR_TO_BTF_ID)
 			strncpy(postfix, "or_null_", 16);
 		else
 			strncpy(postfix, "_or_null", 16);
@@ -575,6 +573,8 @@  static const char *reg_type_str(struct bpf_verifier_env *env,
 		strncpy(prefix, "alloc_", 32);
 	if (type & MEM_USER)
 		strncpy(prefix, "user_", 32);
+	if (type & MEM_PERCPU)
+		strncpy(prefix, "percpu_", 32);
 
 	snprintf(env->type_str_buf, TYPE_STR_BUF_LEN, "%s%s%s",
 		 prefix, str[base_type(type)], postfix);
@@ -697,8 +697,7 @@  static void print_verifier_state(struct bpf_verifier_env *env,
 			const char *sep = "";
 
 			verbose(env, "%s", reg_type_str(env, t));
-			if (base_type(t) == PTR_TO_BTF_ID ||
-			    base_type(t) == PTR_TO_PERCPU_BTF_ID)
+			if (base_type(t) == PTR_TO_BTF_ID)
 				verbose(env, "%s", kernel_type_name(reg->btf, reg->btf_id));
 			verbose(env, "(");
 /*
@@ -2783,7 +2782,6 @@  static bool is_spillable_regtype(enum bpf_reg_type type)
 	case PTR_TO_XDP_SOCK:
 	case PTR_TO_BTF_ID:
 	case PTR_TO_BUF:
-	case PTR_TO_PERCPU_BTF_ID:
 	case PTR_TO_MEM:
 	case PTR_TO_FUNC:
 	case PTR_TO_MAP_KEY:
@@ -4197,6 +4195,13 @@  static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 		return -EACCES;
 	}
 
+	if (reg->type & MEM_PERCPU) {
+		verbose(env,
+			"R%d is ptr_%s access percpu memory: off=%d\n",
+			regno, tname, off);
+		return -EACCES;
+	}
+
 	if (env->ops->btf_struct_access) {
 		ret = env->ops->btf_struct_access(&env->log, reg->btf, t,
 						  off, size, atype, &btf_id, &flag);
@@ -4803,7 +4808,7 @@  static int check_stack_range_initialized(
 		}
 
 		if (is_spilled_reg(&state->stack[spi]) &&
-		    state->stack[spi].spilled_ptr.type == PTR_TO_BTF_ID)
+		    base_type(state->stack[spi].spilled_ptr.type) == PTR_TO_BTF_ID)
 			goto mark;
 
 		if (is_spilled_reg(&state->stack[spi]) &&
@@ -5259,7 +5264,7 @@  static const struct bpf_reg_types alloc_mem_types = { .types = { PTR_TO_MEM | ME
 static const struct bpf_reg_types const_map_ptr_types = { .types = { CONST_PTR_TO_MAP } };
 static const struct bpf_reg_types btf_ptr_types = { .types = { PTR_TO_BTF_ID } };
 static const struct bpf_reg_types spin_lock_types = { .types = { PTR_TO_MAP_VALUE } };
-static const struct bpf_reg_types percpu_btf_ptr_types = { .types = { PTR_TO_PERCPU_BTF_ID } };
+static const struct bpf_reg_types percpu_btf_ptr_types = { .types = { PTR_TO_BTF_ID | MEM_PERCPU } };
 static const struct bpf_reg_types func_ptr_types = { .types = { PTR_TO_FUNC } };
 static const struct bpf_reg_types stack_ptr_types = { .types = { PTR_TO_STACK } };
 static const struct bpf_reg_types const_str_ptr_types = { .types = { PTR_TO_MAP_VALUE } };
@@ -9639,7 +9644,6 @@  static int check_ld_imm(struct bpf_verifier_env *env, struct bpf_insn *insn)
 			dst_reg->mem_size = aux->btf_var.mem_size;
 			break;
 		case PTR_TO_BTF_ID:
-		case PTR_TO_PERCPU_BTF_ID:
 			dst_reg->btf = aux->btf_var.btf;
 			dst_reg->btf_id = aux->btf_var.btf_id;
 			break;
@@ -11839,7 +11843,7 @@  static int check_pseudo_btf_id(struct bpf_verifier_env *env,
 	type = t->type;
 	t = btf_type_skip_modifiers(btf, type, NULL);
 	if (percpu) {
-		aux->btf_var.reg_type = PTR_TO_PERCPU_BTF_ID;
+		aux->btf_var.reg_type = PTR_TO_BTF_ID | MEM_PERCPU;
 		aux->btf_var.btf = btf;
 		aux->btf_var.btf_id = type;
 	} else if (!btf_type_is_struct(t)) {