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

[Dominique Martinet]

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@xxxxxxx>
Closes: https://lore.kernel.org/16271.1734448631@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
---
 net/9p/client.c | 28 +++++++++++++++-------------
 1 file changed, 15 insertions(+), 13 deletions(-)

diff --git a/net/9p/client.c b/net/9p/client.c
index 09f8ced9f8bb..c290d96e1bb3 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);
+		size_t 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 %zu (/%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 > %zu)\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;
@@ -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);

---
base-commit: cdd30ebb1b9f36159d66f088b61aee264e649d7a
change-id: 20241222-9p_unsigned_rw-03f95da525a0

Best regards,
-- 
Dominique Martinet | Asmadeus