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Commit e2652ae6 authored by Manfred Spraul's avatar Manfred Spraul Committed by Linus Torvalds
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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: default avatar <syzbot+2827ef6b3385deb07eaf@syzkaller.appspotmail.com>
Signed-off-by: default avatarManfred 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 615c999c

Documentation/sysctl/kernel.txt

+2 −1
Original line number Diff line number Diff line
@@ -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.



==============================================================

ipc/util.c

+48 −43
Original line number Diff line number Diff line
@@ -194,45 +194,53 @@ 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 */

		new->seq = ids->seq++;

		if (ids->seq > IPCID_SEQ_MAX)

			ids->seq = 0;

	} else {

		new->seq = ipcid_to_seqx(ids->next_id);

		ids->next_id = -1;

	}

	int idx, next_id = -1;



	return SEQ_MULTIPLIER * new->seq + id;

}

#ifdef CONFIG_CHECKPOINT_RESTORE

	next_id = ids->next_id;

	ids->next_id = -1;

#endif



#else

#define ipc_idr_alloc(ids, new)					\

	idr_alloc(&(ids)->ipcs_idr, (new), 0, 0, GFP_NOWAIT)

	/*

	 * 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())

	 */



static inline int ipc_buildid(int id, struct ipc_ids *ids,

			      struct kern_ipc_perm *new)

{

	if (next_id < 0) { /* !CHECKPOINT_RESTORE or next_id is unset */

		new->seq = ids->seq++;

		if (ids->seq > IPCID_SEQ_MAX)

			ids->seq = 0;



	return SEQ_MULTIPLIER * new->seq + id;

		idx = idr_alloc(&ids->ipcs_idr, new, 0, 0, GFP_NOWAIT);

	} else {

		new->seq = ipcid_to_seqx(next_id);

		idx = idr_alloc(&ids->ipcs_idr, new, ipcid_to_idx(next_id),

				0, GFP_NOWAIT);

	}

	if (idx >= 0)

		new->id = SEQ_MULTIPLIER * new->seq + idx;

	return idx;

}



#endif /* CONFIG_CHECKPOINT_RESTORE */



/**

 * ipc_addid - add an ipc identifier
@@ -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;

}



/**