[PATCH AUTOSEL 6.12 4/7] 9p/net: fix improper handling of bogus negative read/write replies

[PATCH AUTOSEL 6.12 4/7] 9p/net: fix improper handling of bogus negative read/write replies

[Sasha Levin]

From: Dominique Martinet <asmadeus@codewreck.org>

[ Upstream commit d0259a856afca31d699b706ed5e2adf11086c73b ]

In p9_client_write() and p9_client_read_once(), if the server
incorrectly replies with success but a negative write/read count then we
would consider written (negative) <= rsize (positive) because both
variables were signed.

Make variables unsigned to avoid this problem.

The reproducer linked below now fails with the following error instead
of a null pointer deref:
9pnet: bogus RWRITE count (4294967295 > 3)

Reported-by: Robert Morris <rtm@mit.edu>
Closes: https://lore.kernel.org/16271.1734448631@26-5-164.dynamic.csail.mit.edu
Message-ID: <20250319-9p_unsigned_rw-v3-1-71327f1503d0@codewreck.org>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/9p/client.c | 30 ++++++++++++++++--------------
 1 file changed, 16 insertions(+), 14 deletions(-)

diff --git a/net/9p/client.c b/net/9p/client.c
index 09f8ced9f8bb7..52a5497cfca79 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -1548,7 +1548,8 @@ p9_client_read_once(struct p9_fid *fid, u64 offset, struct iov_iter *to,
 	struct p9_client *clnt = fid->clnt;
 	struct p9_req_t *req;
 	int count = iov_iter_count(to);
-	int rsize, received, non_zc = 0;
+	u32 rsize, received;
+	bool non_zc = false;
 	char *dataptr;
 
 	*err = 0;
@@ -1571,7 +1572,7 @@ p9_client_read_once(struct p9_fid *fid, u64 offset, struct iov_iter *to,
 				       0, 11, "dqd", fid->fid,
 				       offset, rsize);
 	} else {
-		non_zc = 1;
+		non_zc = true;
 		req = p9_client_rpc(clnt, P9_TREAD, "dqd", fid->fid, offset,
 				    rsize);
 	}
@@ -1592,11 +1593,11 @@ p9_client_read_once(struct p9_fid *fid, u64 offset, struct iov_iter *to,
 		return 0;
 	}
 	if (rsize < received) {
-		pr_err("bogus RREAD count (%d > %d)\n", received, rsize);
+		pr_err("bogus RREAD count (%u > %u)\n", received, rsize);
 		received = rsize;
 	}
 
-	p9_debug(P9_DEBUG_9P, "<<< RREAD count %d\n", received);
+	p9_debug(P9_DEBUG_9P, "<<< RREAD count %u\n", received);
 
 	if (non_zc) {
 		int n = copy_to_iter(dataptr, received, to);
@@ -1623,9 +1624,9 @@ p9_client_write(struct p9_fid *fid, u64 offset, struct iov_iter *from, int *err)
 	*err = 0;
 
 	while (iov_iter_count(from)) {
-		int count = iov_iter_count(from);
-		int rsize = fid->iounit;
-		int written;
+		size_t count = iov_iter_count(from);
+		u32 rsize = fid->iounit;
+		u32 written;
 
 		if (!rsize || rsize > clnt->msize - P9_IOHDRSZ)
 			rsize = clnt->msize - P9_IOHDRSZ;
@@ -1633,7 +1634,7 @@ p9_client_write(struct p9_fid *fid, u64 offset, struct iov_iter *from, int *err)
 		if (count < rsize)
 			rsize = count;
 
-		p9_debug(P9_DEBUG_9P, ">>> TWRITE fid %d offset %llu count %d (/%d)\n",
+		p9_debug(P9_DEBUG_9P, ">>> TWRITE fid %d offset %llu count %u (/%zu)\n",
 			 fid->fid, offset, rsize, count);
 
 		/* Don't bother zerocopy for small IO (< 1024) */
@@ -1659,11 +1660,11 @@ p9_client_write(struct p9_fid *fid, u64 offset, struct iov_iter *from, int *err)
 			break;
 		}
 		if (rsize < written) {
-			pr_err("bogus RWRITE count (%d > %d)\n", written, rsize);
+			pr_err("bogus RWRITE count (%u > %u)\n", written, rsize);
 			written = rsize;
 		}
 
-		p9_debug(P9_DEBUG_9P, "<<< RWRITE count %d\n", written);
+		p9_debug(P9_DEBUG_9P, "<<< RWRITE count %u\n", written);
 
 		p9_req_put(clnt, req);
 		iov_iter_revert(from, count - written - iov_iter_count(from));
@@ -2098,7 +2099,8 @@ EXPORT_SYMBOL_GPL(p9_client_xattrcreate);
 
 int p9_client_readdir(struct p9_fid *fid, char *data, u32 count, u64 offset)
 {
-	int err, rsize, non_zc = 0;
+	int err, non_zc = 0;
+	u32 rsize;
 	struct p9_client *clnt;
 	struct p9_req_t *req;
 	char *dataptr;
@@ -2107,7 +2109,7 @@ int p9_client_readdir(struct p9_fid *fid, char *data, u32 count, u64 offset)
 
 	iov_iter_kvec(&to, ITER_DEST, &kv, 1, count);
 
-	p9_debug(P9_DEBUG_9P, ">>> TREADDIR fid %d offset %llu count %d\n",
+	p9_debug(P9_DEBUG_9P, ">>> TREADDIR fid %d offset %llu count %u\n",
 		 fid->fid, offset, count);
 
 	clnt = fid->clnt;
@@ -2142,11 +2144,11 @@ int p9_client_readdir(struct p9_fid *fid, char *data, u32 count, u64 offset)
 		goto free_and_error;
 	}
 	if (rsize < count) {
-		pr_err("bogus RREADDIR count (%d > %d)\n", count, rsize);
+		pr_err("bogus RREADDIR count (%u > %u)\n", count, rsize);
 		count = rsize;
 	}
 
-	p9_debug(P9_DEBUG_9P, "<<< RREADDIR count %d\n", count);
+	p9_debug(P9_DEBUG_9P, "<<< RREADDIR count %u\n", count);
 
 	if (non_zc)
 		memmove(data, dataptr, count);
-- 
2.39.5

[PATCH AUTOSEL 6.12 5/7] 9p/trans_fd: mark concurrent read and writes to p9_conn->err

[Sasha Levin]

From: Ignacio Encinas <ignacio@iencinas.com>

[ Upstream commit fbc0283fbeae27b88448c90305e738991457fee2 ]

Writes for the error value of a connection are spinlock-protected inside
p9_conn_cancel, but lockless reads are present elsewhere to avoid
performing unnecessary work after an error has been met.

Mark the write and lockless reads to make KCSAN happy. Mark the write as
exclusive following the recommendation in "Lock-Protected Writes with
Lockless Reads" in tools/memory-model/Documentation/access-marking.txt
while we are at it.

Mark p9_fd_request and p9_conn_cancel m->err reads despite the fact that
they do not race with concurrent writes for stylistic reasons.

Reported-by: syzbot+d69a7cc8c683c2cb7506@syzkaller.appspotmail.com
Reported-by: syzbot+483d6c9b9231ea7e1851@syzkaller.appspotmail.com
Signed-off-by: Ignacio Encinas <ignacio@iencinas.com>
Message-ID: <20250318-p9_conn_err_benign_data_race-v3-1-290bb18335cc@iencinas.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/9p/trans_fd.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
index 196060dc6138a..791e4868f2d4e 100644
--- a/net/9p/trans_fd.c
+++ b/net/9p/trans_fd.c
@@ -191,12 +191,13 @@ static void p9_conn_cancel(struct p9_conn *m, int err)
 
 	spin_lock(&m->req_lock);
 
-	if (m->err) {
+	if (READ_ONCE(m->err)) {
 		spin_unlock(&m->req_lock);
 		return;
 	}
 
-	m->err = err;
+	WRITE_ONCE(m->err, err);
+	ASSERT_EXCLUSIVE_WRITER(m->err);
 
 	list_for_each_entry_safe(req, rtmp, &m->req_list, req_list) {
 		list_move(&req->req_list, &cancel_list);
@@ -283,7 +284,7 @@ static void p9_read_work(struct work_struct *work)
 
 	m = container_of(work, struct p9_conn, rq);
 
-	if (m->err < 0)
+	if (READ_ONCE(m->err) < 0)
 		return;
 
 	p9_debug(P9_DEBUG_TRANS, "start mux %p pos %zd\n", m, m->rc.offset);
@@ -450,7 +451,7 @@ static void p9_write_work(struct work_struct *work)
 
 	m = container_of(work, struct p9_conn, wq);
 
-	if (m->err < 0) {
+	if (READ_ONCE(m->err) < 0) {
 		clear_bit(Wworksched, &m->wsched);
 		return;
 	}
@@ -622,7 +623,7 @@ static void p9_poll_mux(struct p9_conn *m)
 	__poll_t n;
 	int err = -ECONNRESET;
 
-	if (m->err < 0)
+	if (READ_ONCE(m->err) < 0)
 		return;
 
 	n = p9_fd_poll(m->client, NULL, &err);
@@ -665,6 +666,7 @@ static void p9_poll_mux(struct p9_conn *m)
 static int p9_fd_request(struct p9_client *client, struct p9_req_t *req)
 {
 	__poll_t n;
+	int err;
 	struct p9_trans_fd *ts = client->trans;
 	struct p9_conn *m = &ts->conn;
 
@@ -673,9 +675,10 @@ static int p9_fd_request(struct p9_client *client, struct p9_req_t *req)
 
 	spin_lock(&m->req_lock);
 
-	if (m->err < 0) {
+	err = READ_ONCE(m->err);
+	if (err < 0) {
 		spin_unlock(&m->req_lock);
-		return m->err;
+		return err;
 	}
 
 	WRITE_ONCE(req->status, REQ_STATUS_UNSENT);
-- 
2.39.5