[PATCH 10/11] fs: don't allow kernel reads and writes using ->read and ->write

[Christoph Hellwig <hch@xxxxxx>]

Don't allow calling ->read or ->write with set_fs as a preparation for
killing off set_fs.  While I've not triggered any of these cases in my
setups as all the usual suspect (file systems, pipes, sockets, block
devices, system character devices) use the iter ops this is almost
going to be guaranteed to eventuall break something, so print a detailed
error message helping to debug such cases.  The fix will be to switch the
affected driver to use ->read_uptr / ->write_uptr or ->read_iter /
->write_iter.

Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
 fs/read_write.c | 29 ++++++++++++++++-------------
 1 file changed, 16 insertions(+), 13 deletions(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index b92c222ca886ca..1b813d9bcf08b7 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -420,6 +420,18 @@ ssize_t iter_read(struct file *filp, char __user *buf, size_t len, loff_t *ppos,
 	return ret;
 }
 
+static void warn_unsupported(struct file *file, const char *op)
+{
+	char pathname[128], *path;
+
+	path = file_path(file, pathname, sizeof(pathname));
+	if (IS_ERR(path))
+		path = "(unknown)";
+	pr_warn_ratelimited(
+		"kernel space %s not supported for file %s (pid: %d comm: %.20s)\n",
+		op, path, current->pid, current->comm);
+}
+
 ssize_t __kernel_read(struct file *file, void *buf, size_t count, loff_t *pos)
 {
 	ssize_t ret;
@@ -433,12 +445,6 @@ ssize_t __kernel_read(struct file *file, void *buf, size_t count, loff_t *pos)
 		count =  MAX_RW_COUNT;
 	if (file->f_op->read_uptr) {
 		ret = file->f_op->read_uptr(file, KERNEL_UPTR(buf), count, pos);
-	} else if (file->f_op->read) {
-		mm_segment_t old_fs = get_fs();
-
-		set_fs(KERNEL_DS);
-		ret = file->f_op->read(file, (void __user *)buf, count, pos);
-		set_fs(old_fs);
 	} else if (file->f_op->read_iter) {
 		struct kvec iov = { .iov_base = buf, .iov_len = count };
 		struct kiocb kiocb;
@@ -450,6 +456,8 @@ ssize_t __kernel_read(struct file *file, void *buf, size_t count, loff_t *pos)
 		ret = file->f_op->read_iter(&kiocb, &iter);
 		*pos = kiocb.ki_pos;
 	} else {
+		if (file->f_op->read)
+			warn_unsupported(file, "read");
 		ret = -EINVAL;
 	}
 	if (ret > 0) {
@@ -539,13 +547,6 @@ ssize_t __kernel_write(struct file *file, const void *buf, size_t count,
 	if (file->f_op->write_uptr) {
 		ret = file->f_op->write_uptr(file, KERNEL_UPTR((void *)buf),
 				count, pos);
-	} else if (file->f_op->write) {
-		mm_segment_t old_fs = get_fs();
-
-		set_fs(KERNEL_DS);
-		ret = file->f_op->write(file, (__force const char __user *)buf,
-				count, pos);
-		set_fs(old_fs);
 	} else if (file->f_op->write_iter) {
 		struct kvec iov = { .iov_base = (void *)buf, .iov_len = count };
 		struct kiocb kiocb;
@@ -558,6 +559,8 @@ ssize_t __kernel_write(struct file *file, const void *buf, size_t count,
 		if (ret > 0)
 			*pos = kiocb.ki_pos;
 	} else {
+		if (file->f_op->write)
+			warn_unsupported(file, "write");
 		ret = -EINVAL;
 	}
 	if (ret > 0) {
-- 
2.26.2