ksmbd: fix null pointer dereference in proc_show_files()

When a SMB2 client opens a file with a durable v2 handle and then issues
SMB2 SESSION_LOGOFF, session_fd_check() clears fp->tcon = NULL on the
reconnectable file pointer but leaves the fp registered in global_ft.idr
until the durable scavenger fires (up to fp->durable_timeout seconds
later).

During that window any read of /proc/fs/ksmbd/files (mode 0400) panics
the kernel because proc_show_files() walks global_ft.idr and
unconditionally dereferences fp->tcon->id with no NULL guard.

Reproducer requires only a successful SMB2 SESSION_SETUP and a share
configured with 'durable handles = yes'. KASAN report on mainline
70390501d194:

  general protection fault, probably for non-canonical address
  0xdffffc0000000000: 0000 [#1] SMP KASAN PTI
  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  RIP: 0010:proc_show_files+0x118/0x740
  Call Trace:
   proc_show_files+0x118/0x740
   seq_read_iter+0x4ef/0xe10
   proc_reg_read_iter+0x1b7/0x280
   ...

Guard the dereference. A durable-disconnected fp legitimately has no
tcon; report its tree id as 0 rather than oopsing.

Fixes: b38f99c1217a ("ksmbd: add procfs interface for runtime monitoring and statistics")
Cc: stable@vger.kernel.org
Signed-off-by: Jeremy Laratro <research@aradex.io>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

diff --git a/fs/smb/server/vfs_cache.c b/fs/smb/server/vfs_cache.c
index 354c4d8a1cfbd13e716e3659149b95b1070f78a2..913164c958b1527ad054d6e6f5035d87c9b19e88 100644 (file)
--- a/fs/smb/server/vfs_cache.c
+++ b/fs/smb/server/vfs_cache.c
@@ -81,7 +81,7 @@ static int proc_show_files(struct seq_file *m, void *v)
        read_lock(&global_ft.lock);
        idr_for_each_entry(global_ft.idr, fp, id) {
                seq_printf(m, "%#-10x %#-10llx %#-10llx %#-10x",
-                          fp->tcon->id,
+                          fp->tcon ? fp->tcon->id : 0,
                           fp->persistent_id,
                           fp->volatile_id,
                           atomic_read(&fp->refcount));