[PATCH 1/1] bpf: Drop unprotected find_vpid() in favour of find_get_pid()

[Lee Jones]

The documentation for find_pid() clearly states:

  "Must be called with the tasklist_lock or rcu_read_lock() held."

Presently we do neither.

In an ideal world we would wrap the in-lined call to find_vpid() along
with get_pid_task() in the suggested rcu_read_lock() and have done.
However, looking at get_pid_task()'s internals, it already does that
independently, so this would lead to deadlock.

Instead, we'll use find_get_pid() which searches for the vpid, then
takes a reference to it preventing early free, all within the safety
of rcu_read_lock().  Once we have our reference we can safely make use
of it up until the point it is put.

Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: John Fastabend <john.fastabend@xxxxxxxxx>
Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
Cc: Martin KaFai Lau <martin.lau@xxxxxxxxx>
Cc: Song Liu <song@xxxxxxxxxx>
Cc: Yonghong Song <yhs@xxxxxx>
Cc: KP Singh <kpsingh@xxxxxxxxxx>
Cc: Stanislav Fomichev <sdf@xxxxxxxxxx>
Cc: Hao Luo <haoluo@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: bpf@xxxxxxxxxxxxxxx
Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
Signed-off-by: Lee Jones <lee@xxxxxxxxxx>
---
 kernel/bpf/syscall.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 83c7136c5788d..c20cff30581c4 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -4385,6 +4385,7 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
 	const struct perf_event *event;
 	struct task_struct *task;
 	struct file *file;
+	struct pid *ppid;
 	int err;
 
 	if (CHECK_ATTR(BPF_TASK_FD_QUERY))
@@ -4396,7 +4397,9 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
 	if (attr->task_fd_query.flags != 0)
 		return -EINVAL;
 
-	task = get_pid_task(find_vpid(pid), PIDTYPE_PID);
+	ppid = find_get_pid(pid);
+	task = get_pid_task(ppid, PIDTYPE_PID);
+	put_pid(ppid);
 	if (!task)
 		return -ENOENT;
 
-- 
2.37.0.170.g444d1eabd0-goog