git: 47d2d509c6b3 - main - sound: Revalidate after reacquiring locks when resizing buffers
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Date: Wed, 14 Oct 2026 16:02:19 UTC
The branch main has been updated by markj:
URL: https://cgit.FreeBSD.org/src/commit/?id=47d2d509c6b32b3c2245748f755fbf2344351d94
commit 47d2d509c6b32b3c2245748f755fbf2344351d94
Author: Mark Johnston <markj@FreeBSD.org>
AuthorDate: 2026-10-13 15:54:07 +0000
Commit: Mark Johnston <markj@FreeBSD.org>
CommitDate: 2026-10-14 16:00:47 +0000
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 | 35 +++++++++++++++++++++++----
sys/dev/sound/pcm/channel.c | 11 +++++++--
tests/sys/sound/mmap.c | 58 +++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 98 insertions(+), 6 deletions(-)
diff --git a/sys/dev/sound/pcm/buffer.c b/sys/dev/sound/pcm/buffer.c
index 14c035281db5..2b3021480700 100644
--- a/sys/dev/sound/pcm/buffer.c
+++ b/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);
diff --git a/sys/dev/sound/pcm/channel.c b/sys/dev/sound/pcm/channel.c
index 24877c319b35..c79761ccc22e 100644
--- a/sys/dev/sound/pcm/channel.c
+++ b/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(
diff --git a/tests/sys/sound/mmap.c b/tests/sys/sound/mmap.c
index 6ebfd2f7dec5..95ed9a483274 100644
--- a/tests/sys/sound/mmap.c
+++ b/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());
}