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umtx: shm: Prevent reference counting overflow

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This hardens against provoked use-after-free occurences should there be
reference counting leaks in the future (which is currently not the
case).

At the deepest level, umtx_shm_find_reg_unlocked() now returns EOVERFLOW
when it cannot grant an additional reference to the registry object, and
so will umtx_shm_find_reg().  umtx_shm_create_reg() will fail if calling
umtx_shm_find_reg() returns EOVERFLOW (meaning a SHM object for the
passed key already exists, but we can't acquire another reference on
it), avoiding the creation of a duplicate registry entry for a given key
(this wouldn't pose problem for the rest of the code in its current
form, but is expressly avoided for intelligibility and hardening
purposes).

Since umtx_shm_find_reg*(), and consequently the whole _umtx_op() system
call, can only return EOVERFLOW on such a bug manifesting, we don't
document that return value.

Reviewed by:    kib, emaste
Approved by:    emaste (mentor)
Sponsored by:   The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D46126
This commit is contained in:
Olivier Certner 2024-09-04 14:38:12 +00:00 committed by Ed Maste
parent 62f40433ab
commit c3e6dfe55c
1 changed files with 54 additions and 22 deletions
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76
sys/kern/kern_umtx.c
View file

@ -4334,8 +4334,17 @@ umtx_shm_reg_delfree_tq(void *context __unused, int pending __unused)
static struct task umtx_shm_reg_delfree_task =
TASK_INITIALIZER(0, umtx_shm_reg_delfree_tq, NULL);
static struct umtx_shm_reg *
umtx_shm_find_reg_locked(const struct umtx_key *key)
/*
* Returns 0 if a SHM with the passed key is found in the registry, in which
* case it is returned through 'oreg'. Otherwise, returns an error among ESRCH
* (no corresponding SHM; ESRCH was chosen for compatibility, ENOENT would have
* been preferable) or EOVERFLOW (there is a corresponding SHM, but reference
* count would overflow, so can't return it), in which case '*oreg' is left
* unchanged.
*/
static int
umtx_shm_find_reg_locked(const struct umtx_key *key,
struct umtx_shm_reg **const oreg)
{
struct umtx_shm_reg *reg;
struct umtx_shm_reg_head *reg_head;
@ -4355,22 +4364,34 @@ umtx_shm_find_reg_locked(const struct umtx_key *key)
("reg %p refcnt 0 onlist", reg));
KASSERT((reg->ushm_flags & USHMF_LINKED) != 0,
("reg %p not linked", reg));
/*
* Don't let overflow happen, just deny a new reference
* (this is additional protection against some reference
* count leak, which is known not to be the case at the
* time of this writing).
*/
if (__predict_false(reg->ushm_refcnt == UINT_MAX))
return (EOVERFLOW);
reg->ushm_refcnt++;
return (reg);
*oreg = reg;
return (0);
}
}
return (NULL);
return (ESRCH);
}
static struct umtx_shm_reg *
umtx_shm_find_reg(const struct umtx_key *key)
/*
* Calls umtx_shm_find_reg_unlocked() under the 'umtx_shm_lock'.
*/
static int
umtx_shm_find_reg(const struct umtx_key *key, struct umtx_shm_reg **const oreg)
{
struct umtx_shm_reg *reg;
int error;
mtx_lock(&umtx_shm_lock);
reg = umtx_shm_find_reg_locked(key);
error = umtx_shm_find_reg_locked(key, oreg);
mtx_unlock(&umtx_shm_lock);
return (reg);
return (error);
}
static void
@ -4470,11 +4491,18 @@ umtx_shm_create_reg(struct thread *td, const struct umtx_key *key,
struct ucred *cred;
int error;
reg = umtx_shm_find_reg(key);
if (reg != NULL) {
*res = reg;
return (0);
error = umtx_shm_find_reg(key, res);
if (error != ESRCH) {
/*
* Either no error occured, and '*res' was filled, or EOVERFLOW
* was returned, indicating a reference count limit, and we
* won't create a duplicate registration. In both cases, we are
* done.
*/
return (error);
}
/* No entry, we will create one. */
cred = td->td_ucred;
if (!chgumtxcnt(cred->cr_ruidinfo, 1, lim_cur(td, RLIMIT_UMTXP)))
return (ENOMEM);
@ -4488,12 +4516,20 @@ umtx_shm_create_reg(struct thread *td, const struct umtx_key *key,
return (error);
}
mtx_lock(&umtx_shm_lock);
reg1 = umtx_shm_find_reg_locked(key);
if (reg1 != NULL) {
/* Re-lookup as 'umtx_shm_lock' has been temporarily released. */
error = umtx_shm_find_reg_locked(key, &reg1);
switch (error) {
case 0:
mtx_unlock(&umtx_shm_lock);
umtx_shm_free_reg(reg);
*res = reg1;
return (0);
case ESRCH:
break;
default:
mtx_unlock(&umtx_shm_lock);
umtx_shm_free_reg(reg);
return (error);
}
TAILQ_INSERT_TAIL(&umtx_shm_registry[key->hash], reg, ushm_reg_link);
LIST_INSERT_HEAD(USHM_OBJ_UMTX(key->info.shared.object), reg,
@ -4564,13 +4600,9 @@ umtx_shm(struct thread *td, void *addr, u_int flags)
if (error != 0)
return (error);
KASSERT(key.shared == 1, ("non-shared key"));
if ((flags & UMTX_SHM_CREAT) != 0) {
error = umtx_shm_create_reg(td, &key, &reg);
} else {
reg = umtx_shm_find_reg(&key);
if (reg == NULL)
error = ESRCH;
}
error = (flags & UMTX_SHM_CREAT) != 0 ?
umtx_shm_create_reg(td, &key, &reg) :
umtx_shm_find_reg(&key, &reg);
umtx_key_release(&key);
if (error != 0)
return (error);