[PATCH bpf] bpf: Fix NULL pointer dereference in __bpf_sk_storage_map_seq_show

["Cen Zhang (Microsoft Security FORGE Labs)" <blbllhy@gmail.com>]

Iterating a sk_storage map is two stages:
bpf_sk_storage_map_seq_find_next()
returns a selem, then __bpf_sk_storage_map_seq_show() uses that selem.
__bpf_sk_storage_map_seq_show() re-reads selem->local_storage via
rcu_dereference() with no NULL check. A concurrent close() can run
bpf_selem_unlink_nofail() in between and set that field to NULL, causing
a NULL dereference of sk_storage->owner.

Oops: general protection fault, probably for non-canonical 
   address 0xdffffc0000000011
  net/core/bpf_sk_storage.c:809 __bpf_sk_storage_map_seq_show()
  bpf_seq_read+0x366/0x1120
  vfs_read+0x174/0xa50
  ksys_read+0xfc/0x1d0

Return if the re-read yields NULL.

Fixes: 5d800f87d0a5 ("bpf: Support lockless unlink when freeing map or local storage")
Reported-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
Cc: AutonomousCodeSecurity@microsoft.com
Assisted-by: Copilot (Grok 4.6)
Signed-off-by: Cen Zhang (Microsoft Security FORGE Labs) <blbllhy@gmail.com>
---
 net/core/bpf_sk_storage.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c
index 1d295a8769fa..7228520aa2b6 100644
--- a/net/core/bpf_sk_storage.c
+++ b/net/core/bpf_sk_storage.c
@@ -806,6 +806,8 @@ static int __bpf_sk_storage_map_seq_show(struct seq_file *seq,
 		ctx.map = info->map;
 		if (selem) {
 			sk_storage = rcu_dereference(selem->local_storage);
+			if (!sk_storage)
+				return 0;
 			ctx.sk = sk_storage->owner;
 			ctx.value = SDATA(selem)->data;
 		}
-- 
2.55.0