[PATCH] ipc: avoid overflow of semop undo (semadj) value

[Petr Mladek]

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.

Signed-off-by: Petr Mladek <pmladek@xxxxxxx>
---
 ipc/sem.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index db9d241af133..0d4375761449 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -599,7 +599,7 @@ SYSCALL_DEFINE3(semget, key_t, key, int, nsems, int, semflg)
 static int perform_atomic_semop(struct sem_array *sma, struct sembuf *sops,
 			     int nsops, struct sem_undo *un, int pid)
 {
-	int result, sem_op;
+	int result, undo, sem_op;
 	struct sembuf *sop;
 	struct sem * curr;
 
@@ -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.
-			 */
+			undo = un->semadj[sop->sem_num] - sem_op;
+			/* 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--;
 	}
 
-- 
1.8.4

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