Date: Wed, 4 Sep 2024 14:53:57 GMT From: Ed Maste <emaste@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: git: c3e6dfe55c0e - main - umtx: shm: Prevent reference counting overflow Message-ID: <202409041453.484ErvrB017376@gitrepo.freebsd.org>
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The branch main has been updated by emaste: URL: https://cgit.FreeBSD.org/src/commit/?id=c3e6dfe55c0e81d0717b0458bc95128384c3ebe8 commit c3e6dfe55c0e81d0717b0458bc95128384c3ebe8 Author: Olivier Certner <olce@FreeBSD.org> AuthorDate: 2024-09-04 14:38:12 +0000 Commit: Ed Maste <emaste@FreeBSD.org> CommitDate: 2024-09-04 14:38:12 +0000 umtx: shm: Prevent reference counting overflow 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 --- sys/kern/kern_umtx.c | 76 +++++++++++++++++++++++++++++++++++++--------------- 1 file changed, 54 insertions(+), 22 deletions(-) diff --git a/sys/kern/kern_umtx.c b/sys/kern/kern_umtx.c index 37c44f969a27..a6c160ab6283 100644 --- a/sys/kern/kern_umtx.c +++ b/sys/kern/kern_umtx.c @@ -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, ®1); + 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, ®); - } else { - reg = umtx_shm_find_reg(&key); - if (reg == NULL) - error = ESRCH; - } + error = (flags & UMTX_SHM_CREAT) != 0 ? + umtx_shm_create_reg(td, &key, ®) : + umtx_shm_find_reg(&key, ®); umtx_key_release(&key); if (error != 0) return (error);
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