git: 253b98f749cf - main - sound: Fix KASSERT panics in chn_read() and chn_write()

The branch main has been updated by christos:

URL: https://cgit.FreeBSD.org/src/commit/?id=253b98f749cf93a9a682f46925c43cbbd04e1110

commit 253b98f749cf93a9a682f46925c43cbbd04e1110
Author:     Christos Margiolis <christos@FreeBSD.org>
AuthorDate: 2025-11-20 15:23:09 +0000
Commit:     Christos Margiolis <christos@FreeBSD.org>
CommitDate: 2025-11-20 15:23:09 +0000

    sound: Fix KASSERT panics in chn_read() and chn_write()
    
    INVARIANTS kernels may trigger a KASSERT panic from sndbuf_acquire(),
    when fuzzing write(2) using stress2, because of a race in chn_write().
    
    In the case of chn_write(), what sndbuf_acquire() does is extend the
    ready-to-read area of the buffer by a specified amount of bytes. The
    KASSERT in question makes sure the number of bytes we want to extend the
    ready area by, is less than or equal to the number of free bytes in the
    buffer. This makes sense, because we cannot extend the ready area to
    something larger than what is available (i.e., free) in the first place.
    
    What chn_write() currently does for every write is; calculate the
    appropriate write size, let's say X, unlock the channel, uiomove() X
    bytes to the channel's buffer, lock the channel, and call
    sndbuf_acquire() to extend the ready area by X bytes. The problem with
    this approach, however, is the following.
    
    Suppose an empty channel buffer with a length of 1024 bytes, and 2
    threads, (A) and (B), where (B) is a higher-priority one. Suppose thread
    (A) wants to write 1024 bytes. It unlocks the channel and uiomove()s
    1024 bytes to the channel buffer. At the same time, thread (B) picks up
    the lock, and because it is higher priority, it keeps dominating the
    lock for a few iterations. By the time thread (A) picks up the lock
    again, it tries to call sndbuf_acquire() with a size of 1024 bytes,
    which was calculated before it performed the uiomove(). In this case,
    there is a very high chance that the buffer will not be empty, that is,
    have a free area of 1024 bytes, as was the case when thread (A) started
    executing, and so the KASSERT will trigger a panic because the condition
    (bytes <= free) is not met.
    
    Another scenario that can trigger a panic is the following: suppose a
    buffer with a size of 4 bytes, and two threads: (A) and (B). In the
    first iteration, thread (A) wants to write 2 bytes, while the buffer is
    empty, BUT the pointer (sndbuf_getfreeptr()) is at the end (i.e.,
    buf[3]). In the first iteration of the loop, because of the way we
    calculate t, we'll end up writing only 1 byte, so after sz -= t, sz will
    be 1, and so we'll need one more iteration in the inner loop, to write
    the remaining 1 byte. Now we're at the end of the first loop, thread (A)
    unlocks the channel, it has written 1 byte, it needs to write 1 more,
    and the buffer is left with 3 empty slots. Now thread (B) picks up the
    lock, and it wants to write 3 (or more) bytes. Eventually it writes the
    3 bytes, and it leaves the buffer with 0 free slots. By the time thread
    (A) picks up the lock again, and continues with the second iteration of
    the inner loop, it will try to write the last byte, but sndbuf_acquire()
    will panic because there is no free space anymore.
    
    To fix this, get rid of the inner loop and calculate the write size on
    each iteration. Also, call sndbuf_acquire() before unlocking the
    channel. In the scenarios explained above, we'll end up entering the
    chn_sleep() case. Modify it as well, so that we do not kill the channel
    if we need to sleep more.
    
    Do the same for chn_read() to avoid possible similar panics from
    sndbuf_dispose().
    
    Reported by:    pho
    Tested by:      christos, pho
    Sponsored by:   The FreeBSD Foundation
    MFC after:      1 week
    Reviewed by:    pho, kib
    Differential Revision:  https://reviews.freebsd.org/D53666
---
 sys/dev/sound/pcm/channel.c | 70 ++++++++++++++-------------------------------
 1 file changed, 22 insertions(+), 48 deletions(-)

diff --git a/sys/dev/sound/pcm/channel.c b/sys/dev/sound/pcm/channel.c
index f29a819ce0ae..e92181d74e19 100644
--- a/sys/dev/sound/pcm/channel.c
+++ b/sys/dev/sound/pcm/channel.c
@@ -438,7 +438,7 @@ chn_write(struct pcm_channel *c, struct uio *buf)
 {
 	struct snd_dbuf *bs = c->bufsoft;
 	void *off;
-	int ret, timeout, sz, t, p;
+	int ret, timeout, sz, p;
 
 	CHN_LOCKASSERT(c);
 
@@ -446,24 +446,17 @@ chn_write(struct pcm_channel *c, struct uio *buf)
 	timeout = chn_timeout * hz;
 
 	while (ret == 0 && buf->uio_resid > 0) {
+		p = sndbuf_getfreeptr(bs);
 		sz = min(buf->uio_resid, sndbuf_getfree(bs));
+		sz = min(sz, bs->bufsize - p);
 		if (sz > 0) {
-			/*
-			 * The following assumes that the free space in
-			 * the buffer can never be less around the
-			 * unlock-uiomove-lock sequence.
-			 */
-			while (ret == 0 && sz > 0) {
-				p = sndbuf_getfreeptr(bs);
-				t = min(sz, bs->bufsize - p);
-				off = sndbuf_getbufofs(bs, p);
-				CHN_UNLOCK(c);
-				ret = uiomove(off, t, buf);
-				CHN_LOCK(c);
-				sz -= t;
-				sndbuf_acquire(bs, NULL, t);
-			}
-			ret = 0;
+			off = sndbuf_getbufofs(bs, p);
+			sndbuf_acquire(bs, NULL, sz);
+			CHN_UNLOCK(c);
+			ret = uiomove(off, sz, buf);
+			CHN_LOCK(c);
+			if (ret != 0)
+				break;
 			if (CHN_STOPPED(c) && !(c->flags & CHN_F_NOTRIGGER)) {
 				ret = chn_start(c, 0);
 				if (ret != 0)
@@ -483,13 +476,7 @@ chn_write(struct pcm_channel *c, struct uio *buf)
 			ret = EAGAIN;
 		} else {
    			ret = chn_sleep(c, timeout);
-			if (ret == EAGAIN) {
-				ret = EINVAL;
-				c->flags |= CHN_F_DEAD;
-				device_printf(c->dev, "%s(): %s: "
-				    "play interrupt timeout, channel dead\n",
-				    __func__, c->name);
-			} else if (ret == ERESTART || ret == EINTR)
+			if (ret == ERESTART || ret == EINTR)
 				c->flags |= CHN_F_ABORTING;
 		}
 	}
@@ -552,7 +539,7 @@ chn_read(struct pcm_channel *c, struct uio *buf)
 {
 	struct snd_dbuf *bs = c->bufsoft;
 	void *off;
-	int ret, timeout, sz, t, p;
+	int ret, timeout, sz, p;
 
 	CHN_LOCKASSERT(c);
 
@@ -568,35 +555,22 @@ chn_read(struct pcm_channel *c, struct uio *buf)
 	timeout = chn_timeout * hz;
 
 	while (ret == 0 && buf->uio_resid > 0) {
+		p = sndbuf_getreadyptr(bs);
 		sz = min(buf->uio_resid, sndbuf_getready(bs));
+		sz = min(sz, bs->bufsize - p);
 		if (sz > 0) {
-			/*
-			 * The following assumes that the free space in
-			 * the buffer can never be less around the
-			 * unlock-uiomove-lock sequence.
-			 */
-			while (ret == 0 && sz > 0) {
-				p = sndbuf_getreadyptr(bs);
-				t = min(sz, bs->bufsize - p);
-				off = sndbuf_getbufofs(bs, p);
-				CHN_UNLOCK(c);
-				ret = uiomove(off, t, buf);
-				CHN_LOCK(c);
-				sz -= t;
-				sndbuf_dispose(bs, NULL, t);
-			}
-			ret = 0;
+			off = sndbuf_getbufofs(bs, p);
+			sndbuf_dispose(bs, NULL, sz);
+			CHN_UNLOCK(c);
+			ret = uiomove(off, sz, buf);
+			CHN_LOCK(c);
+			if (ret != 0)
+				break;
 		} else if (c->flags & (CHN_F_NBIO | CHN_F_NOTRIGGER))
 			ret = EAGAIN;
 		else {
    			ret = chn_sleep(c, timeout);
-			if (ret == EAGAIN) {
-				ret = EINVAL;
-				c->flags |= CHN_F_DEAD;
-				device_printf(c->dev, "%s(): %s: "
-				    "record interrupt timeout, channel dead\n",
-				    __func__, c->name);
-			} else if (ret == ERESTART || ret == EINTR)
+			if (ret == ERESTART || ret == EINTR)
 				c->flags |= CHN_F_ABORTING;
 		}
 	}