[PATCH bpf-next v6 1/2] bpf: Fix attaching fentry/fexit/fmod_ret/lsm to modules

[Viktor Malik <vmalik@redhat.com>]

This resolves two problems with attachment of fentry/fexit/fmod_ret/lsm
to functions located in modules:

1. The verifier tries to find the address to attach to in kallsyms. This
   is always done by searching the entire kallsyms, not respecting the
   module in which the function is located. Such approach causes an
   incorrect attachment address to be computed if the function to attach
   to is shadowed by a function of the same name located earlier in
   kallsyms.

2. If the address to attach to is located in a module, the module
   reference is only acquired in register_fentry. If the module is
   unloaded between the place where the address is found
   (bpf_check_attach_target in the verifier) and register_fentry, it is
   possible that another module is loaded to the same address which may
   lead to potential errors.

Since the attachment must contain the BTF of the program to attach to,
we extract the module from it and search for the function address in the
correct module (resolving problem no. 1). Then, the module reference is
taken directly in bpf_check_attach_target and stored in the bpf program
(in bpf_prog_aux). The reference is only released when the program is
unloaded (resolving problem no. 2).

Signed-off-by: Viktor Malik <vmalik@redhat.com>
---
 include/linux/bpf.h      |  1 +
 kernel/bpf/syscall.c     |  2 ++
 kernel/bpf/trampoline.c  | 27 ---------------------------
 kernel/bpf/verifier.c    | 20 +++++++++++++++++++-
 kernel/module/internal.h |  5 +++++
 5 files changed, 27 insertions(+), 28 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 4385418118f6..2aadc78fe3c1 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1330,6 +1330,7 @@ struct bpf_prog_aux {
 	 * main prog always has linfo_idx == 0
 	 */
 	u32 linfo_idx;
+	struct module *mod;
 	u32 num_exentries;
 	struct exception_table_entry *extable;
 	union {
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index cda8d00f3762..5b8227e08182 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2064,6 +2064,8 @@ static void bpf_prog_put_deferred(struct work_struct *work)
 	prog = aux->prog;
 	perf_event_bpf_event(prog, PERF_BPF_EVENT_PROG_UNLOAD, 0);
 	bpf_audit_prog(prog, BPF_AUDIT_UNLOAD);
+	if (aux->mod)
+		module_put(aux->mod);
 	bpf_prog_free_id(prog);
 	__bpf_prog_put_noref(prog, true);
 }
diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index d0ed7d6f5eec..ebb20bf252c7 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -172,26 +172,6 @@ static struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
 	return tr;
 }
 
-static int bpf_trampoline_module_get(struct bpf_trampoline *tr)
-{
-	struct module *mod;
-	int err = 0;
-
-	preempt_disable();
-	mod = __module_text_address((unsigned long) tr->func.addr);
-	if (mod && !try_module_get(mod))
-		err = -ENOENT;
-	preempt_enable();
-	tr->mod = mod;
-	return err;
-}
-
-static void bpf_trampoline_module_put(struct bpf_trampoline *tr)
-{
-	module_put(tr->mod);
-	tr->mod = NULL;
-}
-
 static int unregister_fentry(struct bpf_trampoline *tr, void *old_addr)
 {
 	void *ip = tr->func.addr;
@@ -202,8 +182,6 @@ static int unregister_fentry(struct bpf_trampoline *tr, void *old_addr)
 	else
 		ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, old_addr, NULL);
 
-	if (!ret)
-		bpf_trampoline_module_put(tr);
 	return ret;
 }
 
@@ -238,9 +216,6 @@ static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
 		tr->func.ftrace_managed = true;
 	}
 
-	if (bpf_trampoline_module_get(tr))
-		return -ENOENT;
-
 	if (tr->func.ftrace_managed) {
 		ftrace_set_filter_ip(tr->fops, (unsigned long)ip, 0, 1);
 		ret = register_ftrace_direct_multi(tr->fops, (long)new_addr);
@@ -248,8 +223,6 @@ static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
 		ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, NULL, new_addr);
 	}
 
-	if (ret)
-		bpf_trampoline_module_put(tr);
 	return ret;
 }
 
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 388245e8826e..6a19bd450558 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -24,6 +24,7 @@
 #include <linux/bpf_lsm.h>
 #include <linux/btf_ids.h>
 #include <linux/poison.h>
+#include "../module/internal.h"
 
 #include "disasm.h"
 
@@ -16868,6 +16869,7 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 	const char *tname;
 	struct btf *btf;
 	long addr = 0;
+	struct module *mod = NULL;
 
 	if (!btf_id) {
 		bpf_log(log, "Tracing programs must provide btf_id\n");
@@ -17041,7 +17043,17 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 			else
 				addr = (long) tgt_prog->aux->func[subprog]->bpf_func;
 		} else {
-			addr = kallsyms_lookup_name(tname);
+			if (btf_is_module(btf)) {
+				preempt_disable();
+				mod = btf_try_get_module(btf);
+				if (mod)
+					addr = find_kallsyms_symbol_value(mod, tname);
+				else
+					addr = 0;
+				preempt_enable();
+			} else {
+				addr = kallsyms_lookup_name(tname);
+			}
 			if (!addr) {
 				bpf_log(log,
 					"The address of function %s cannot be found\n",
@@ -17105,6 +17117,12 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 	tgt_info->tgt_addr = addr;
 	tgt_info->tgt_name = tname;
 	tgt_info->tgt_type = t;
+	if (mod) {
+		if (!prog->aux->mod)
+			prog->aux->mod = mod;
+		else
+			module_put(mod);
+	}
 	return 0;
 }
 
diff --git a/kernel/module/internal.h b/kernel/module/internal.h
index 2e2bf236f558..5cb103a46018 100644
--- a/kernel/module/internal.h
+++ b/kernel/module/internal.h
@@ -256,6 +256,11 @@ static inline bool sect_empty(const Elf_Shdr *sect)
 static inline void init_build_id(struct module *mod, const struct load_info *info) { }
 static inline void layout_symtab(struct module *mod, struct load_info *info) { }
 static inline void add_kallsyms(struct module *mod, const struct load_info *info) { }
+static inline unsigned long find_kallsyms_symbol_value(struct module *mod
+						       const char *name)
+{
+	return 0;
+}
 #endif /* CONFIG_KALLSYMS */
 
 #ifdef CONFIG_SYSFS
-- 
2.39.1