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Date:   Thu, 26 Nov 2020 17:57:47 +0100
From:   Florent Revest <revest@...omium.org>
To:     bpf@...r.kernel.org
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kpsingh@...omium.org, revest@...gle.com,
        linux-kernel@...r.kernel.org
Subject: [PATCH bpf-next 1/2] bpf: Add a bpf_kallsyms_lookup helper

This helper exposes the kallsyms_lookup function to eBPF tracing
programs. This can be used to retrieve the name of the symbol at an
address. For example, when hooking into nf_register_net_hook, one can
audit the name of the registered netfilter hook and potentially also
the name of the module in which the symbol is located.

Signed-off-by: Florent Revest <revest@...gle.com>
---
 include/uapi/linux/bpf.h       | 16 +++++++++++++
 kernel/trace/bpf_trace.c       | 41 ++++++++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h | 16 +++++++++++++
 3 files changed, 73 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index c3458ec1f30a..670998635eac 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -3817,6 +3817,21 @@ union bpf_attr {
  *		The **hash_algo** is returned on success,
  *		**-EOPNOTSUP** if IMA is disabled or **-EINVAL** if
  *		invalid arguments are passed.
+ *
+ * long bpf_kallsyms_lookup(u64 address, char *symbol, u32 symbol_size, char *module, u32 module_size)
+ *	Description
+ *		Uses kallsyms to write the name of the symbol at *address*
+ *		into *symbol* of size *symbol_sz*. This is guaranteed to be
+ *		zero terminated.
+ *		If the symbol is in a module, up to *module_size* bytes of
+ *		the module name is written in *module*. This is also
+ *		guaranteed to be zero-terminated. Note: a module name
+ *		is always shorter than 64 bytes.
+ *	Return
+ *		On success, the strictly positive length of the full symbol
+ *		name, If this is greater than *symbol_size*, the written
+ *		symbol is truncated.
+ *		On error, a negative value.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -3981,6 +3996,7 @@ union bpf_attr {
 	FN(bprm_opts_set),		\
 	FN(ktime_get_coarse_ns),	\
 	FN(ima_inode_hash),		\
+	FN(kallsyms_lookup),	\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index d255bc9b2bfa..9d86e20c2b13 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -17,6 +17,7 @@
 #include <linux/error-injection.h>
 #include <linux/btf_ids.h>
 #include <linux/bpf_lsm.h>
+#include <linux/kallsyms.h>
 
 #include <net/bpf_sk_storage.h>
 
@@ -1260,6 +1261,44 @@ const struct bpf_func_proto bpf_snprintf_btf_proto = {
 	.arg5_type	= ARG_ANYTHING,
 };
 
+BPF_CALL_5(bpf_kallsyms_lookup, u64, address, char *, symbol, u32, symbol_size,
+	   char *, module, u32, module_size)
+{
+	char buffer[KSYM_SYMBOL_LEN];
+	unsigned long offset, size;
+	const char *name;
+	char *modname;
+	long ret;
+
+	name = kallsyms_lookup(address, &size, &offset, &modname, buffer);
+	if (!name)
+		return -EINVAL;
+
+	ret = strlen(name) + 1;
+	if (symbol_size) {
+		strncpy(symbol, name, symbol_size);
+		symbol[symbol_size - 1] = '\0';
+	}
+
+	if (modname && module_size) {
+		strncpy(module, modname, module_size);
+		module[module_size - 1] = '\0';
+	}
+
+	return ret;
+}
+
+const struct bpf_func_proto bpf_kallsyms_lookup_proto = {
+	.func		= bpf_kallsyms_lookup,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_ANYTHING,
+	.arg2_type	= ARG_PTR_TO_MEM,
+	.arg3_type	= ARG_CONST_SIZE,
+	.arg4_type	= ARG_PTR_TO_MEM,
+	.arg5_type	= ARG_CONST_SIZE,
+};
+
 const struct bpf_func_proto *
 bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 {
@@ -1356,6 +1395,8 @@ bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_per_cpu_ptr_proto;
 	case BPF_FUNC_bpf_this_cpu_ptr:
 		return &bpf_this_cpu_ptr_proto;
+	case BPF_FUNC_kallsyms_lookup:
+		return &bpf_kallsyms_lookup_proto;
 	default:
 		return NULL;
 	}
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index c3458ec1f30a..670998635eac 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -3817,6 +3817,21 @@ union bpf_attr {
  *		The **hash_algo** is returned on success,
  *		**-EOPNOTSUP** if IMA is disabled or **-EINVAL** if
  *		invalid arguments are passed.
+ *
+ * long bpf_kallsyms_lookup(u64 address, char *symbol, u32 symbol_size, char *module, u32 module_size)
+ *	Description
+ *		Uses kallsyms to write the name of the symbol at *address*
+ *		into *symbol* of size *symbol_sz*. This is guaranteed to be
+ *		zero terminated.
+ *		If the symbol is in a module, up to *module_size* bytes of
+ *		the module name is written in *module*. This is also
+ *		guaranteed to be zero-terminated. Note: a module name
+ *		is always shorter than 64 bytes.
+ *	Return
+ *		On success, the strictly positive length of the full symbol
+ *		name, If this is greater than *symbol_size*, the written
+ *		symbol is truncated.
+ *		On error, a negative value.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -3981,6 +3996,7 @@ union bpf_attr {
 	FN(bprm_opts_set),		\
 	FN(ktime_get_coarse_ns),	\
 	FN(ima_inode_hash),		\
+	FN(kallsyms_lookup),	\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
-- 
2.29.2.454.gaff20da3a2-goog

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