sound: Revalidate after reacquiring locks when resizing buffers

sndbuf_resize() and sndbuf_remalloc() drop the channel lock in order to
allocate the new buffer(s) with M_WAITOK.  Before doing that they verify
that the buffer is not mapped, but this could change while the lock is
dropped, as there is no higher-level synchronization which prevents
buffer resize (e.g., via the SNDCTL_DSP_SETBLKSIZE ioctl) from happening
concurrently with mmap.  Thus, these checks are insufficient.

Modify sndbuf_resize() and sndbuf_remalloc() to revalidate after
reacquiring the lock.  Extend them to check for a couple of channel
flags, mirroring the check in chn_resizebuf().  Modify chn_resizebuf()
to revalidate state after reacquiring the channel lock.

We could also allocate new buffers with M_NOWAIT and so avoid dropping
the lock.  Then, I think we could go even further and invoke
CHANNEL_SETBLOCKSIZE and CHANNEL_SETFRAGMENTS with the channel lock
held.  But really it would be better to ensure that dsp operations are
serialized; there is no reason for mmap and chn_resizebuf() to be able
to run in parallel, we should not be relying on the channel lock for
this.  So, let's keep a more minimal approach here.

Add a simple regression test.

Approved by:	so
Security:	FreeBSD-SA-26:71.sound
Security:	CVE-2026-101308
Reviewed by:	christos
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D60554

sys/dev/sound/pcm/buffer.c

@@ -90,6 +90,24 @@ sndbuf_rele(struct snd_dbuf *b)
 	}
 }
 
+static bool
+sndbuf_busy(struct snd_dbuf *b)
+{
+	CHN_LOCKASSERT(b->channel);
+
+	return (refcount_load(&b->refcount) > 1 ||
+	    (b->flags & SNDBUF_F_DETACHED) != 0 ||
+	    /*
+	     * XXX-MJ CHN_F_MMAP is not reliable, but for software buffers the
+	     * refcount accurately reflects whether the buffer is mapped.
+	     * Ensure that there is no in-flight DMA transfer by checking
+	     * CHN_F_TRIGGERED.  This should really be checked at a higher
+	     * layer, but this is tricky since chn_resizebuf() and
+	     * sndbuf_resize() drop and reacquire the channel lock.
+	     */
+	    (b->channel->flags & (CHN_F_MMAP | CHN_F_TRIGGERED)) != 0);
+}
+
 static void
 sndbuf_setmap(void *arg, bus_dma_segment_t *segs, int nseg, int error)
 {
@@ -205,8 +223,7 @@ sndbuf_resize(struct snd_dbuf *b, unsigned int blkcnt, unsigned int blksz)
 
 	if (bufsize > b->allocsize ||
 	    bufsize < (b->allocsize >> SNDBUF_CACHE_SHIFT)) {
-		if (refcount_load(&b->refcount) > 1 ||
-		    (b->flags & SNDBUF_F_DETACHED) != 0) {
+		if (sndbuf_busy(b)) {
 			CHN_UNLOCK(b->channel);
 			return (EBUSY);
 		}
@@ -214,6 +231,11 @@ sndbuf_resize(struct snd_dbuf *b, unsigned int blkcnt, unsigned int blksz)
 		CHN_UNLOCK(b->channel);
 		tmpbuf = malloc(allocsize, M_DEVBUF, M_WAITOK);
 		CHN_LOCK(b->channel);
+		if (sndbuf_busy(b)) {
+			CHN_UNLOCK(b->channel);
+			free(tmpbuf, M_DEVBUF);
+			return (EBUSY);
+		}
 		if (snd_verbose > 3)
 			printf("%s(): b=%p %p -> %p [%d -> %d : %d]\n",
 			    __func__, b, b->tmpbuf, tmpbuf,
@@ -250,8 +272,7 @@ sndbuf_remalloc(struct snd_dbuf *b, unsigned int blkcnt, unsigned int blksz)
 
 	if (bufsize > b->allocsize ||
 	    bufsize < (b->allocsize >> SNDBUF_CACHE_SHIFT)) {
-		if (refcount_load(&b->refcount) > 1 ||
-		    (b->flags & SNDBUF_F_DETACHED) != 0)
+		if (sndbuf_busy(b))
 			return (EBUSY);
 		allocsize = round_page(bufsize);
 		CHN_UNLOCK(b->channel);
@@ -259,6 +280,12 @@ sndbuf_remalloc(struct snd_dbuf *b, unsigned int blkcnt, unsigned int blksz)
 		tmpbuf = malloc(allocsize, M_DEVBUF, M_WAITOK);
 		shadbuf = malloc(allocsize, M_DEVBUF, M_WAITOK);
 		CHN_LOCK(b->channel);
+		if (sndbuf_busy(b)) {
+			free(buf, M_DEVBUF);
+			free(tmpbuf, M_DEVBUF);
+			free(shadbuf, M_DEVBUF);
+			return (EBUSY);
+		}
 		free(b->buf, M_DEVBUF);
 		b->buf = buf;
 		free(b->tmpbuf, M_DEVBUF);

sys/dev/sound/pcm/channel.c

@@ -1841,8 +1841,9 @@ chn_resizebuf(struct pcm_channel *c, int latency,
 
 	CHN_LOCKASSERT(c);
 
-	if ((c->flags & (CHN_F_MMAP | CHN_F_TRIGGERED)) ||
-	    !(c->direction == PCMDIR_PLAY || c->direction == PCMDIR_REC))
+	if (!(c->direction == PCMDIR_PLAY || c->direction == PCMDIR_REC))
+		return EINVAL;
+	if ((c->flags & (CHN_F_MMAP | CHN_F_TRIGGERED)) != 0)
 		return EINVAL;
 
 	if (latency == -1) {
@@ -1900,6 +1901,9 @@ chn_resizebuf(struct pcm_channel *c, int latency,
 		chn_notify(c->parentchannel, CHN_N_BLOCKSIZE);
 		CHN_UNLOCK(c->parentchannel);
 		CHN_LOCK(c);
+		/* Revalidate. */
+		if ((c->flags & (CHN_F_MMAP | CHN_F_TRIGGERED)) != 0)
+			return EINVAL;
 		if (c->direction == PCMDIR_PLAY) {
 			limit = (pb != NULL) ?
 			    sndbuf_xbytes(pb->bufsize, pb, bs) : 0;
@@ -1965,6 +1969,9 @@ chn_resizebuf(struct pcm_channel *c, int latency,
 			b->blksz = nblksz;
 		}
 		CHN_LOCK(c);
+		/* Revalidate. */
+		if ((c->flags & (CHN_F_MMAP | CHN_F_TRIGGERED)) != 0)
+			return EINVAL;
 
 		if (!CHN_EMPTY(c, children)) {
 			nsblksz = round_blksz(

tests/sys/sound/mmap.c

@@ -129,11 +129,69 @@ ATF_TC_BODY(mmap_reject_prot_exec, tc)
 	close(fd);
 }
 
+/*
+ * Verify that an attempt to resize a channel's buffer while it is mmapped
+ * fails.
+ */
+ATF_TC(mmap_resize_fails);
+ATF_TC_HEAD(mmap_resize_fails, tc)
+{
+	atf_tc_set_md_var(tc, "descr",
+	    "Resizing a mmapped channel buffer fails");
+	atf_tc_set_md_var(tc, "require.kmods", "snd_dummy");
+}
+ATF_TC_BODY(mmap_resize_fails, tc)
+{
+	audio_buf_info abi;
+	uint8_t *buf;
+	size_t len;
+	int fd, arg, ret;
+
+	fd = open("/dev/dsp.dummy", O_RDWR);
+	ATF_REQUIRE_MSG(fd >= 0, FMT_ERR("open"));
+
+	arg = (2 << 16) | 14; /* 2 * 16KB */
+	ATF_REQUIRE_MSG(ioctl(fd, SNDCTL_DSP_SETFRAGMENT, &arg) == 0,
+	    FMT_ERR("SNDCTL_DSP_SETFRAGMENT"));
+	ATF_REQUIRE_MSG(ioctl(fd, SNDCTL_DSP_GETOSPACE, &abi) == 0,
+	    FMT_ERR("SNDCTL_DSP_GETOSPACE"));
+
+	len = abi.fragstotal * abi.fragsize;
+	ATF_REQUIRE_MSG(len >= PAGE_SIZE, "buffer too small: %zu", len);
+
+	buf = mmap(NULL, len, PROT_WRITE, MAP_SHARED, fd, 0);
+	ATF_REQUIRE_MSG(buf != MAP_FAILED, FMT_ERR("mmap"));
+
+	/* Try to resize the buffer; this should fail. */
+	arg = (4 << 16) | 12; /* 4 * 4KB */
+	ret = ioctl(fd, SNDCTL_DSP_SETFRAGMENT, &arg);
+	ATF_REQUIRE_MSG(ret != 0, "resize of mmapped buffer should fail");
+	ATF_REQUIRE_MSG(errno == EINVAL,
+	    "expected EINVAL, got %s", strerror(errno));
+
+	ATF_REQUIRE(munmap(buf, len) == 0);
+
+	/*
+	 * CHN_F_MMAP is only cleared when the fd is closed, so resizing still
+	 * fails after munmap.  This is probably a bug, but we can at least
+	 * verify that the resize succeeds after the fd is closed and reopened.
+	 */
+	ATF_REQUIRE(close(fd) == 0);
+	fd = open("/dev/dsp.dummy", O_RDWR);
+	ATF_REQUIRE_MSG(fd >= 0, FMT_ERR("open"));
+	arg = (4 << 16) | 12; /* 4 * 4KB */
+	ATF_REQUIRE_MSG(ioctl(fd, SNDCTL_DSP_SETFRAGMENT, &arg) == 0,
+	    FMT_ERR("SNDCTL_DSP_SETFRAGMENT after reopen"));
+
+	ATF_REQUIRE(close(fd) == 0);
+}
+
 ATF_TP_ADD_TCS(tp)
 {
 	ATF_TP_ADD_TC(tp, mmap_offset_overflow);
 	ATF_TP_ADD_TC(tp, mmap_buffer_lifetime);
 	ATF_TP_ADD_TC(tp, mmap_reject_prot_exec);
+	ATF_TP_ADD_TC(tp, mmap_resize_fails);
 
 	return (atf_no_error());
 }