ipc: reorganize initialization of kern_ipc_perm.seq

ipc_addid() initializes kern_ipc_perm.seq after having called idr_alloc()
(within ipc_idr_alloc()).

Thus a parallel semop() or msgrcv() that uses ipc_obtain_object_check()
may see an uninitialized value.

The patch moves the initialization of kern_ipc_perm.seq before the calls
of idr_alloc().

Notes:
1) This patch has a user space visible side effect:
If /proc/sys/kernel/*_next_id is used (i.e.: checkpoint/restore) and
if semget()/msgget()/shmget() fails in the final step of adding the id
to the rhash tree, then .._next_id is cleared. Before the patch, is
remained unmodified.

There is no change of the behavior after a successful ..get() call: It
always clears .._next_id, there is no impact to non checkpoint/restore
code as that code does not use .._next_id.

2) The patch correctly documents that after a call to ipc_idr_alloc(),
the full tear-down sequence must be used. The callers of ipc_addid()
do not fullfill that, i.e. more bugfixes are required.

The patch is a squash of a patch from Dmitry and my own changes.

Link: http://lkml.kernel.org/r/20180712185241.4017-3-manfred@colorfullife.com
Reported-by: syzbot+2827ef6b3385deb07eaf@syzkaller.appspotmail.com
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 50 insertions, 44 deletions

diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index 9d38e75b19a0..37a679501ddc 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -464,7 +464,8 @@ Notes:
 1) kernel doesn't guarantee, that new object will have desired id. So,
 it's up to userspace, how to handle an object with "wrong" id.
 2) Toggle with non-default value will be set back to -1 by kernel after
-successful IPC object allocation.
+successful IPC object allocation. If an IPC object allocation syscall
+fails, it is undefined if the value remains unmodified or is reset to -1.
 
 ==============================================================
 
diff --git a/ipc/util.c b/ipc/util.c
index fdffff41f65b..e5c9e2b2e4c4 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -194,46 +194,54 @@ static struct kern_ipc_perm *ipc_findkey(struct ipc_ids *ids, key_t key)
         return NULL;
 }
 
-#ifdef CONFIG_CHECKPOINT_RESTORE
 /*
- * Specify desired id for next allocated IPC object.
+ * Insert new IPC object into idr tree, and set sequence number and id
+ * in the correct order.
+ * Especially:
+ * - the sequence number must be set before inserting the object into the idr,
+ *   because the sequence number is accessed without a lock.
+ * - the id can/must be set after inserting the object into the idr.
+ *   All accesses must be done after getting kern_ipc_perm.lock.
+ *
+ * The caller must own kern_ipc_perm.lock.of the new object.
+ * On error, the function returns a (negative) error code.
  */
-#define ipc_idr_alloc(ids, new)                                         \
-        idr_alloc(&(ids)->ipcs_idr, (new),                              \
-                  (ids)->next_id < 0 ? 0 : ipcid_to_idx((ids)->next_id),\
-                  0, GFP_NOWAIT)
-
-static inline int ipc_buildid(int id, struct ipc_ids *ids,
-                              struct kern_ipc_perm *new)
+static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
 {
-        if (ids->next_id < 0) { /* default, behave as !CHECKPOINT_RESTORE */
+        int idx, next_id = -1;
+
+#ifdef CONFIG_CHECKPOINT_RESTORE
+        next_id = ids->next_id;
+        ids->next_id = -1;
+#endif
+
+        /*
+         * As soon as a new object is inserted into the idr,
+         * ipc_obtain_object_idr() or ipc_obtain_object_check() can find it,
+         * and the lockless preparations for ipc operations can start.
+         * This means especially: permission checks, audit calls, allocation
+         * of undo structures, ...
+         *
+         * Thus the object must be fully initialized, and if something fails,
+         * then the full tear-down sequence must be followed.
+         * (i.e.: set new->deleted, reduce refcount, call_rcu())
+         */
+
+        if (next_id < 0) { /* !CHECKPOINT_RESTORE or next_id is unset */
                 new->seq = ids->seq++;
                 if (ids->seq > IPCID_SEQ_MAX)
                         ids->seq = 0;
+                idx = idr_alloc(&ids->ipcs_idr, new, 0, 0, GFP_NOWAIT);
         } else {
-                new->seq = ipcid_to_seqx(ids->next_id);
-                ids->next_id = -1;
+                new->seq = ipcid_to_seqx(next_id);
+                idx = idr_alloc(&ids->ipcs_idr, new, ipcid_to_idx(next_id),
+                                0, GFP_NOWAIT);
         }
-
-        return SEQ_MULTIPLIER * new->seq + id;
+        if (idx >= 0)
+                new->id = SEQ_MULTIPLIER * new->seq + idx;
+        return idx;
 }
 
-#else
-#define ipc_idr_alloc(ids, new)                                 \
-        idr_alloc(&(ids)->ipcs_idr, (new), 0, 0, GFP_NOWAIT)
-
-static inline int ipc_buildid(int id, struct ipc_ids *ids,
-                              struct kern_ipc_perm *new)
-{
-        new->seq = ids->seq++;
-        if (ids->seq > IPCID_SEQ_MAX)
-                ids->seq = 0;
-
-        return SEQ_MULTIPLIER * new->seq + id;
-}
-
-#endif /* CONFIG_CHECKPOINT_RESTORE */
-
 /**
  * ipc_addid - add an ipc identifier
  * @ids: ipc identifier set
@@ -251,7 +259,7 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 {
         kuid_t euid;
         kgid_t egid;
-        int id, err;
+        int idx, err;
 
         if (limit > IPCMNI)
                 limit = IPCMNI;
@@ -271,30 +279,27 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
         new->cuid = new->uid = euid;
         new->gid = new->cgid = egid;
 
-        id = ipc_idr_alloc(ids, new);
+        idx = ipc_idr_alloc(ids, new);
         idr_preload_end();
 
-        if (id >= 0 && new->key != IPC_PRIVATE) {
+        if (idx >= 0 && new->key != IPC_PRIVATE) {
                 err = rhashtable_insert_fast(&ids->key_ht, &new->khtnode,
                                              ipc_kht_params);
                 if (err < 0) {
-                        idr_remove(&ids->ipcs_idr, id);
-                        id = err;
+                        idr_remove(&ids->ipcs_idr, idx);
+                        idx = err;
                 }
         }
-        if (id < 0) {
+        if (idx < 0) {
                 spin_unlock(&new->lock);
                 rcu_read_unlock();
-                return id;
+                return idx;
         }
 
         ids->in_use++;
-        if (id > ids->max_id)
-                ids->max_id = id;
-
-        new->id = ipc_buildid(id, ids, new);
-
-        return id;
+        if (idx > ids->max_id)
+                ids->max_id = idx;
+        return idx;
 }
 
 /**