fs/9p: avoid accessing utsname after namespace has been torn down

During trinity fuzzing in a kvmtool guest, I stumbled across the
following:

Unable to handle kernel NULL pointer dereference at virtual address 00000004
PC is at v9fs_file_do_lock+0xc8/0x1a0
LR is at v9fs_file_do_lock+0x48/0x1a0
[<c01e2ed0>] (v9fs_file_do_lock+0xc8/0x1a0) from [<c0119154>] (locks_remove_flock+0x8c/0x124)
[<c0119154>] (locks_remove_flock+0x8c/0x124) from [<c00d9bf0>] (__fput+0x58/0x1e4)
[<c00d9bf0>] (__fput+0x58/0x1e4) from [<c0044340>] (task_work_run+0xac/0xe8)
[<c0044340>] (task_work_run+0xac/0xe8) from [<c002e36c>] (do_exit+0x6bc/0x8d8)
[<c002e36c>] (do_exit+0x6bc/0x8d8) from [<c002e674>] (do_group_exit+0x3c/0xb0)
[<c002e674>] (do_group_exit+0x3c/0xb0) from [<c002e6f8>] (__wake_up_parent+0x0/0x18)

I believe this is due to an attempt to access utsname()->nodename, after
exit_task_namespaces() has been called, leaving current->nsproxy->uts_ns
as NULL and causing the above dereference.

A similar issue was fixed for lockd in 9a1b6bf818e7 ("LOCKD: Don't call
utsname()->nodename from nlmclnt_setlockargs"), so this patch attempts
something similar for 9pfs.

Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/net/9p/client.c b/net/9p/client.c
index 8b93cae2d11d..0e49b288e574 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -992,6 +992,7 @@ struct p9_client *p9_client_create(const char *dev_name, char *options)
 {
         int err;
         struct p9_client *clnt;
+        char *client_id;
 
         err = 0;
         clnt = kmalloc(sizeof(struct p9_client), GFP_KERNEL);
@@ -1000,6 +1001,10 @@ struct p9_client *p9_client_create(const char *dev_name, char *options)
 
         clnt->trans_mod = NULL;
         clnt->trans = NULL;
+
+        client_id = utsname()->nodename;
+        memcpy(clnt->name, client_id, strlen(client_id) + 1);
+
         spin_lock_init(&clnt->lock);
         INIT_LIST_HEAD(&clnt->fidlist);