dsp: Fix a potential use-after-free in dsp_oss_syncstart()

This function has a loop where it attempts to lock all channels in a
group.  If doing so would block, it releases all locks, sleeps for a
bit, and tries again.  However, once the syncgroup lock is dropped,
nothing prevents the syncgroup structure from being freed.

Fix the inner loop: after waking up, break out of it unconditionally and
start everything again.  I think the old code was also buggy and not
well-exercised: after waking up we'd continue to try and continue
locking channels.  Then we'd try again from the beginning and fail to
lock the channels we had already locked.

Approved by:	so
Security:	FreeBSD-SA-26:58.sound
Security:	CVE-2026-58091
Reported by:	Hazley Samsudin of GovTech CSG
Reviewed by:	christos
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D58912

2 files changed, 11 insertions, 15 deletions

diff --git a/sys/dev/sound/pcm/channel.h b/sys/dev/sound/pcm/channel.h
index 9ad21d219001..059dc23993cd 100644
--- a/sys/dev/sound/pcm/channel.h
+++ b/sys/dev/sound/pcm/channel.h
@@ -57,10 +57,11 @@ struct pcmchan_syncmember;
 extern struct mtx snd_pcm_syncgroups_mtx;
 extern SLIST_HEAD(pcm_synclist, pcmchan_syncgroup) snd_pcm_syncgroups;
 
-#define PCM_SG_LOCK()	    mtx_lock(&snd_pcm_syncgroups_mtx)
-#define PCM_SG_TRYLOCK()    mtx_trylock(&snd_pcm_syncgroups_mtx)
-#define PCM_SG_UNLOCK()	    mtx_unlock(&snd_pcm_syncgroups_mtx)
-#define PCM_SG_LOCKASSERT(arg)	mtx_assert(&snd_pcm_syncgroups_mtx, arg)
+#define PCM_SG_LOCKPTR()	(&snd_pcm_syncgroups_mtx)
+#define PCM_SG_LOCK()		mtx_lock(PCM_SG_LOCKPTR())
+#define PCM_SG_TRYLOCK()	mtx_trylock(PCM_SG_LOCKPTR())
+#define PCM_SG_UNLOCK()		mtx_unlock(PCM_SG_LOCKPTR())
+#define PCM_SG_LOCKASSERT(arg)	mtx_assert(PCM_SG_LOCKPTR(), arg)
 
 /**
  * @brief Specifies an audio device sync group
diff --git a/sys/dev/sound/pcm/dsp.c b/sys/dev/sound/pcm/dsp.c
index 4d69f176225c..96cd794c1fed 100644
--- a/sys/dev/sound/pcm/dsp.c
+++ b/sys/dev/sound/pcm/dsp.c
@@ -2720,14 +2720,11 @@ dsp_oss_syncstart(int sg_id)
 	struct pcmchan_syncmember *sm, *sm_tmp;
 	struct pcmchan_syncgroup *sg;
 	struct pcm_channel *c;
-	int ret, needlocks;
+	int ret;
 
-	/* Get the synclists lock */
 	PCM_SG_LOCK();
-
 	do {
 		ret = 0;
-		needlocks = 0;
 
 		/* Search for syncgroup by ID */
 		SLIST_FOREACH(sg, &snd_pcm_syncgroups, link) {
@@ -2764,16 +2761,14 @@ dsp_oss_syncstart(int sg_id)
 				}
 
 				/** @todo Is PRIBIO correct/ */
-				ret = msleep(sm, &snd_pcm_syncgroups_mtx,
+				ret = msleep(sm, PCM_SG_LOCKPTR(),
 				    PRIBIO | PCATCH, "pcmsg", timo);
-				if (ret == EINTR || ret == ERESTART)
-					break;
-
-				needlocks = 1;
-				ret = 0; /* Assumes ret == EAGAIN... */
+				if (ret == EAGAIN)
+					ret = 0;
+				break;
 			}
 		}
-	} while (needlocks && ret == 0);
+	} while (ret == 0 && sm != NULL);
 
 	/* Proceed only if no errors encountered. */
 	if (ret == 0) {