[PATCH bpf-next 2/4] bpf: Give vmlinux BTF init its own mutex

[Daniel Borkmann <daniel@iogearbox.net>]

bpf_get_btf_vmlinux() serializes the lazy vmlinux BTF parse with
bpf_verifier_lock, the same mutex bpf_check() holds across the whole
verification of an unprivileged program (if enabled; it's disabled
by default). The latter can potentially stall the mutex holder for
a long time (e.g. via userfaultfd), and therefore block first-time
bpf_get_btf_vmlinux() caller from any context, including privileged
program loads.

Give the vmlinux BTF initialization a dedicated btf_vmlinux_lock so
it is independent of the unprivileged verification mutex. The parse
only needs mutual exclusion against itself.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 kernel/bpf/btf.c      | 2 +-
 kernel/bpf/verifier.c | 7 ++++---
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index dff5c0d91641..8c04c340f499 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6451,7 +6451,7 @@ struct btf *btf_parse_vmlinux(void)
 	if (IS_ERR(btf))
 		goto err_out;
 
-	/* btf_parse_vmlinux() runs under bpf_verifier_lock */
+	/* btf_parse_vmlinux() runs under btf_vmlinux_lock */
 	bpf_ctx_convert.t = btf_type_by_id(btf, bpf_ctx_convert_btf_id[0]);
 	err = btf_alloc_id(btf);
 	if (err) {
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 9217e0f87cb5..40e20dfa3212 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -324,6 +324,7 @@ static const char *btf_type_name(const struct btf *btf, u32 id)
 }
 
 static DEFINE_MUTEX(bpf_verifier_lock);
+static DEFINE_MUTEX(btf_vmlinux_lock);
 static DEFINE_MUTEX(bpf_percpu_ma_lock);
 
 __printf(2, 3) static void verbose(void *private_data, const char *fmt, ...)
@@ -19563,7 +19564,7 @@ struct btf *bpf_get_btf_vmlinux(void)
 	struct btf *btf = smp_load_acquire(&btf_vmlinux);
 
 	if (!btf && IS_ENABLED(CONFIG_DEBUG_INFO_BTF)) {
-		mutex_lock(&bpf_verifier_lock);
+		mutex_lock(&btf_vmlinux_lock);
 		btf = btf_vmlinux;
 		if (!btf) {
 			btf = btf_parse_vmlinux();
@@ -19575,7 +19576,7 @@ struct btf *bpf_get_btf_vmlinux(void)
 			 */
 			smp_store_release(&btf_vmlinux, btf);
 		}
-		mutex_unlock(&bpf_verifier_lock);
+		mutex_unlock(&btf_vmlinux_lock);
 	}
 	return btf;
 }
@@ -20089,7 +20090,7 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr, bpfptr_t uattr,
 
 	bpf_get_btf_vmlinux();
 
-	/* grab the mutex to protect few globals used by verifier */
+	/* Serialize verification of unprivileged programs. */
 	if (!is_priv)
 		mutex_lock(&bpf_verifier_lock);
 
-- 
2.43.0