Date:   Mon, 25 Oct 2021 15:08:09 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
Subject: [PATCH bpf v2] bpf: fix potential race in tail call compatibility check

Lorenzo noticed that the code testing for program type compatibility of
tail call maps is potentially racy in that two threads could encounter a
map with an unset type simultaneously and both return true even though they
are inserting incompatible programs.

The race window is quite small, but artificially enlarging it by adding a
usleep_range() inside the check in bpf_prog_array_compatible() makes it
trivial to trigger from userspace with a program that does, essentially:

        map_fd = bpf_create_map(BPF_MAP_TYPE_PROG_ARRAY, 4, 4, 2, 0);
        pid = fork();
        if (pid) {
                key = 0;
                value = xdp_fd;
        } else {
                key = 1;
                value = tc_fd;
        }
        err = bpf_map_update_elem(map_fd, &key, &value, 0);

While the race window is small, it has potentially serious ramifications in
that triggering it would allow a BPF program to tail call to a program of a
different type. So let's get rid of it by protecting the update with a
spinlock. The commit in the Fixes tag is the last commit that touches the
code in question.

v2:
- Use a spinlock instead of an atomic variable and cmpxchg() (Alexei)

Fixes: 3324b584b6f6 ("ebpf: misc core cleanup")
Reported-by: Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
 include/linux/bpf.h   |  1 +
 kernel/bpf/arraymap.c |  1 +
 kernel/bpf/core.c     | 14 ++++++++++----
 kernel/bpf/syscall.c  |  2 ++
 4 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 020a7d5bf470..98d906176d89 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -929,6 +929,7 @@ struct bpf_array_aux {
 	 * stored in the map to make sure that all callers and callees have
 	 * the same prog type and JITed flag.
 	 */
+	spinlock_t type_check_lock;
 	enum bpf_prog_type type;
 	bool jited;
 	/* Programs with direct jumps into programs part of this array. */
diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index cebd4fb06d19..da9b1e96cadc 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c
@@ -1072,6 +1072,7 @@ static struct bpf_map *prog_array_map_alloc(union bpf_attr *attr)
 	INIT_WORK(&aux->work, prog_array_map_clear_deferred);
 	INIT_LIST_HEAD(&aux->poke_progs);
 	mutex_init(&aux->poke_mutex);
+	spin_lock_init(&aux->type_check_lock);
 
 	map = array_map_alloc(attr);
 	if (IS_ERR(map)) {
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index c1e7eb3f1876..9439c839d279 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1823,20 +1823,26 @@ static unsigned int __bpf_prog_ret0_warn(const void *ctx,
 bool bpf_prog_array_compatible(struct bpf_array *array,
 			       const struct bpf_prog *fp)
 {
+	bool ret;
+
 	if (fp->kprobe_override)
 		return false;
 
+	spin_lock(&array->aux->type_check_lock);
+
 	if (!array->aux->type) {
 		/* There's no owner yet where we could check for
 		 * compatibility.
 		 */
 		array->aux->type  = fp->type;
 		array->aux->jited = fp->jited;
-		return true;
+		ret = true;
+	} else {
+		ret = array->aux->type  == fp->type &&
+		      array->aux->jited == fp->jited;
 	}
-
-	return array->aux->type  == fp->type &&
-	       array->aux->jited == fp->jited;
+	spin_unlock(&array->aux->type_check_lock);
+	return ret;
 }
 
 static int bpf_check_tail_call(const struct bpf_prog *fp)
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 4e50c0bfdb7d..955011c7df29 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -543,8 +543,10 @@ static void bpf_map_show_fdinfo(struct seq_file *m, struct file *filp)
 
 	if (map->map_type == BPF_MAP_TYPE_PROG_ARRAY) {
 		array = container_of(map, struct bpf_array, map);
+		spin_lock(&array->aux->type_check_lock);
 		type  = array->aux->type;
 		jited = array->aux->jited;
+		spin_unlock(&array->aux->type_check_lock);
 	}
 
 	seq_printf(m,
-- 
2.33.0

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