[PATCH bpf] bpf: Skip unsettled links in link iterator

[Weiming Shi <bestswngs@gmail.com>]

bpf_link_prime() inserts a link into link_idr before anon_inode_getfile()
succeeds and before bpf_link_settle() publishes the ID in link->id.
bpf_link_by_id() treats such an ID-zero link as unsettled, but the link
iterator takes a reference without this check.

If anon_inode_getfile() then fails, the creator removes the ID and frees
its still-private link directly.  The iterator is left with a dangling
reference and its next bpf_link_put() accesses freed memory.

Treat ID-zero entries as transient in bpf_link_get_curr_or_next(), just as
bpf_link_by_id() does.

  BUG: KASAN: slab-use-after-free in bpf_link_put
  Write of size 8 by task exp/384
  Call Trace:
  bpf_link_put                    kernel/bpf/syscall.c:3372
  bpf_link_seq_next               kernel/bpf/link_iter.c:33
  bpf_seq_read                    kernel/bpf/bpf_iter.c:158
  vfs_read                        fs/read_write.c:572
  ksys_read                       fs/read_write.c:716
  do_syscall_64                   arch/x86/entry/syscall_64.c:84
  entry_SYSCALL_64_after_hwframe  arch/x86/entry/entry_64.S:121
  Kernel panic - not syncing: KASAN: panic_on_warn set ...

Fixes: 9f8836127308 ("bpf: Add bpf_link iterator")
Reported-by: Xiang Mei <xmei5@asu.edu>
Assisted-by: LLM
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 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 a8c9b2865c5df..2870ff3167d42 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -6044,7 +6044,10 @@ struct bpf_link *bpf_link_get_curr_or_next(u32 *id)
 again:
 	link = idr_get_next(&link_idr, id);
 	if (link) {
-		link = bpf_link_inc_not_zero(link);
+		if (link->id)
+			link = bpf_link_inc_not_zero(link);
+		else
+			link = ERR_PTR(-EAGAIN);
 		if (IS_ERR(link)) {
 			(*id)++;
 			goto again;
-- 
2.55.0