[PATCH bpf-next] bpf: fix use-after-free of bpf_link when priming half-fails

[Andrii Nakryiko <andriin@xxxxxx>]

If bpf_link_prime() succeeds to allocate new anon file, but then fails to
allocate ID for it, link priming is considered to be failed and user is
supposed ot be able to directly kfree() bpf_link, because it was never exposed
to user-space.

But at that point file already keeps a pointer to bpf_link and will eventually
call bpf_link_release(), so if bpf_link was kfree()'d by caller, that would
lead to use-after-free.

Fix this by creating file with NULL private_data until ID allocation succeeds.
Only then set private_data to bpf_link. Teach bpf_link_release() to recognize
such situation and do nothing.

Fixes: a3b80e107894 ("bpf: Allocate ID for bpf_link")
Reported-by: syzbot+39b64425f91b5aab714d@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
---
 kernel/bpf/syscall.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index c75b2dd2459c..ce00df64a4d4 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2267,7 +2267,12 @@ static int bpf_link_release(struct inode *inode, struct file *filp)
 {
 	struct bpf_link *link = filp->private_data;
 
-	bpf_link_put(link);
+	/* if bpf_link_prime() allocated file, but failed to allocate ID,
+	 * file->private_data will be null and by now link itself is kfree()'d
+	 * directly, so just do nothing in such case.
+	 */
+	if (link)
+		bpf_link_put(link);
 	return 0;
 }
 
@@ -2348,7 +2353,7 @@ int bpf_link_prime(struct bpf_link *link, struct bpf_link_primer *primer)
 	if (fd < 0)
 		return fd;
 
-	file = anon_inode_getfile("bpf_link", &bpf_link_fops, link, O_CLOEXEC);
+	file = anon_inode_getfile("bpf_link", &bpf_link_fops, NULL, O_CLOEXEC);
 	if (IS_ERR(file)) {
 		put_unused_fd(fd);
 		return PTR_ERR(file);
@@ -2357,10 +2362,15 @@ int bpf_link_prime(struct bpf_link *link, struct bpf_link_primer *primer)
 	id = bpf_link_alloc_id(link);
 	if (id < 0) {
 		put_unused_fd(fd);
-		fput(file);
+		fput(file); /* won't put link, so user can kfree() it */
 		return id;
 	}
 
+	/* Link priming succeeded, point file's private data to link now.
+	 * After this caller has to call bpf_link_cleanup() to free link.
+	 */
+	file->private_data = link;
+
 	primer->link = link;
 	primer->file = file;
 	primer->fd = fd;
-- 
2.24.1