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+2827ef6b3385deb07...@syzkaller.appspotmail.com
Signed-off-by: Manfred Spraul <manf...@colorfullife.com>
Cc: Dmitry Vyukov <dvyu...@google.com>
Cc: Kees Cook <keesc...@chromium.org>
Cc: Davidlohr Bueso <d...@stgolabs.net>
Cc: Michael Kerrisk <mtk.manpa...@gmail.com>
---
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
