From 78f5009cc35eb5e52d276a046d90ee2f41b60f8c Mon Sep 17 00:00:00 2001 From: Petr Mladek Date: Mon, 27 Jan 2014 17:07:00 -0800 Subject: ipc/sem.c: avoid overflow of semop undo (semadj) value When trying to understand semop code, I found a small mistake in the check for semadj (undo) value overflow. The new undo value is not stored immediately and next potential checks are done against the old value. The failing scenario is not much practical. One semop call has to do more operations on the same semaphore. Also semval and semadj must have different values, so there has to be some operations without SEM_UNDO flag. For example: struct sembuf depositor_op[1]; struct sembuf collector_op[2]; depositor_op[0].sem_num = 0; depositor_op[0].sem_op = 20000; depositor_op[0].sem_flg = 0; collector_op[0].sem_num = 0; collector_op[0].sem_op = -10000; collector_op[0].sem_flg = SEM_UNDO; collector_op[1].sem_num = 0; collector_op[1].sem_op = -10000; collector_op[1].sem_flg = SEM_UNDO; if (semop(semid, depositor_op, 1) == -1) { perror("Failed to do 1st deposit"); return 1; } if (semop(semid, collector_op, 2) == -1) { perror("Failed to do 1st collect"); return 1; } if (semop(semid, depositor_op, 1) == -1) { perror("Failed to do 2nd deposit"); return 1; } if (semop(semid, collector_op, 2) == -1) { perror("Failed to do 2nd collect"); return 1; } return 0; It passes without error now but the semadj value has overflown in the 2nd collector operation. [akpm@linux-foundation.org: restore lessened scope of local `undo'] [davidlohr@hp.com: correct header comment for perform_atomic_semop] Signed-off-by: Petr Mladek Acked-by: Davidlohr Bueso Acked-by: Manfred Spraul Cc: Jiri Kosina Signed-off-by: Davidlohr Bueso Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- ipc/sem.c | 24 +++++++++++++----------- 1 file changed, 13 insertions(+), 11 deletions(-) (limited to 'ipc/sem.c') diff --git a/ipc/sem.c b/ipc/sem.c index db9d241af133..cc9ac35b793c 100644 --- a/ipc/sem.c +++ b/ipc/sem.c @@ -584,10 +584,11 @@ SYSCALL_DEFINE3(semget, key_t, key, int, nsems, int, semflg) return ipcget(ns, &sem_ids(ns), &sem_ops, &sem_params); } -/** perform_atomic_semop - Perform (if possible) a semaphore operation +/** + * perform_atomic_semop - Perform (if possible) a semaphore operation * @sma: semaphore array * @sops: array with operations that should be checked - * @nsems: number of sops + * @nsops: number of operations * @un: undo array * @pid: pid that did the change * @@ -595,7 +596,6 @@ SYSCALL_DEFINE3(semget, key_t, key, int, nsems, int, semflg) * Returns 1 if the operation is impossible, the caller must sleep. * Negative values are error codes. */ - static int perform_atomic_semop(struct sem_array *sma, struct sembuf *sops, int nsops, struct sem_undo *un, int pid) { @@ -607,7 +607,7 @@ static int perform_atomic_semop(struct sem_array *sma, struct sembuf *sops, curr = sma->sem_base + sop->sem_num; sem_op = sop->sem_op; result = curr->semval; - + if (!sem_op && result) goto would_block; @@ -616,25 +616,24 @@ static int perform_atomic_semop(struct sem_array *sma, struct sembuf *sops, goto would_block; if (result > SEMVMX) goto out_of_range; + if (sop->sem_flg & SEM_UNDO) { int undo = un->semadj[sop->sem_num] - sem_op; - /* - * Exceeding the undo range is an error. - */ + /* Exceeding the undo range is an error. */ if (undo < (-SEMAEM - 1) || undo > SEMAEM) goto out_of_range; + un->semadj[sop->sem_num] = undo; } + curr->semval = result; } sop--; while (sop >= sops) { sma->sem_base[sop->sem_num].sempid = pid; - if (sop->sem_flg & SEM_UNDO) - un->semadj[sop->sem_num] -= sop->sem_op; sop--; } - + return 0; out_of_range: @@ -650,7 +649,10 @@ would_block: undo: sop--; while (sop >= sops) { - sma->sem_base[sop->sem_num].semval -= sop->sem_op; + sem_op = sop->sem_op; + sma->sem_base[sop->sem_num].semval -= sem_op; + if (sop->sem_flg & SEM_UNDO) + un->semadj[sop->sem_num] += sem_op; sop--; } -- cgit v1.2.3-59-g8ed1b