[PATCH 2/6] ipc: reorganize initialization of kern_ipc_perm.seq

[Manfred Spraul]

ipc_addid() initializes kern_ipc_perm.seq after having called
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 ipc_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.

Reported-by: syzbot+2827ef6b3385deb07eaf@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
---
 Documentation/sysctl/kernel.txt |  3 ++-
 ipc/util.c                      | 45 +++++++++++++++++++++++----------
 2 files changed, 34 insertions(+), 14 deletions(-)

diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index eded671d55eb..b2d4a8f8fe97 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -440,7 +440,8 @@ 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 4e81182fa0ac..662c28c6c9fa 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -197,13 +197,24 @@ static struct kern_ipc_perm *ipc_findkey(struct ipc_ids *ids, key_t key)
 /*
  * Specify desired id for next allocated IPC object.
  */
-#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_idr_alloc(struct ipc_ids *ids,
+				struct kern_ipc_perm *new)
+{
+	int key;
 
-static inline int ipc_buildid(int id, struct ipc_ids *ids,
-			      struct kern_ipc_perm *new)
+	if (ids->next_id < 0) {
+		key = idr_alloc(&ids->ipcs_idr, new, 0, 0, GFP_NOWAIT);
+	} else {
+		key = idr_alloc(&ids->ipcs_idr, new,
+				ipcid_to_idx(ids->next_id),
+				0, GFP_NOWAIT);
+		ids->next_id = -1;
+	}
+	return key;
+}
+
+static inline void ipc_set_seq(struct ipc_ids *ids,
+				struct kern_ipc_perm *new)
 {
 	if (ids->next_id < 0) { /* default, behave as !CHECKPOINT_RESTORE */
 		new->seq = ids->seq++;
@@ -211,24 +222,19 @@ static inline int ipc_buildid(int id, struct ipc_ids *ids,
 			ids->seq = 0;
 	} else {
 		new->seq = ipcid_to_seqx(ids->next_id);
-		ids->next_id = -1;
 	}
-
-	return SEQ_MULTIPLIER * new->seq + id;
 }
 
 #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,
+static inline void ipc_set_seq(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 */
@@ -270,6 +276,19 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 	new->cuid = new->uid = euid;
 	new->gid = new->cgid = egid;
 
+	ipc_set_seq(ids, new);
+
+	/*
+	 * 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())
+	 */
 	id = ipc_idr_alloc(ids, new);
 	idr_preload_end();
 
@@ -291,7 +310,7 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 	if (id > ids->max_id)
 		ids->max_id = id;
 
-	new->id = ipc_buildid(id, ids, new);
+	new->id = SEQ_MULTIPLIER * new->seq + id;
 
 	return id;
 }
-- 
2.17.1