From: Jann Horn <jannh@google.com>
Date: Thu, 4 Apr 2019 23:59:25 +0200
Subject: tracing: Fix buffer_ref pipe ops
Git-commit: b987222654f84f7b4ca95b3a55eca784cb30235b
Git-commit: e9e1a2e7b486e3940badb6d743c8841ed94517b6
Patch-mainline: v5.1-rc7
References: bsc#1133698

[ vbabka@suse.cz: fix up applying after backporting 15fab63e1e57b by
  effectively applying the changes from merge commit e9e1a2e7b486e ]

This fixes multiple issues in buffer_pipe_buf_ops:

 - The ->steal() handler must not return zero unless the pipe buffer has
   the only reference to the page. But generic_pipe_buf_steal() assumes
   that every reference to the pipe is tracked by the page's refcount,
   which isn't true for these buffers - buffer_pipe_buf_get(), which
   duplicates a buffer, doesn't touch the page's refcount.
   Fix it by using generic_pipe_buf_nosteal(), which refuses every
   attempted theft. It should be easy to actually support ->steal, but the
   only current users of pipe_buf_steal() are the virtio console and FUSE,
   and they also only use it as an optimization. So it's probably not worth
   the effort.
 - The ->get() and ->release() handlers can be invoked concurrently on pipe
   buffers backed by the same struct buffer_ref. Make them safe against
   concurrency by using refcount_t.
 - The pointers stored in ->private were only zeroed out when the last
   reference to the buffer_ref was dropped. As far as I know, this
   shouldn't be necessary anyway, but if we do it, let's always do it.

Link: http://lkml.kernel.org/r/20190404215925.253531-1-jannh@google.com

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@vger.kernel.org
Fixes: 73a757e63114d ("ring-buffer: Return reader page back into existing ring buffer")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Miroslav Benes <mbenes@suse.cz>
---
 fs/splice.c               |    4 ++--
 include/linux/pipe_fs_i.h |    1 +
 kernel/trace/trace.c      |   30 +++++++++++++++---------------
 3 files changed, 18 insertions(+), 17 deletions(-)

--- a/fs/splice.c
+++ b/fs/splice.c
@@ -332,8 +332,8 @@ const struct pipe_buf_operations default
 	.get = generic_pipe_buf_get,
 };
 
-static int generic_pipe_buf_nosteal(struct pipe_inode_info *pipe,
-				    struct pipe_buffer *buf)
+int generic_pipe_buf_nosteal(struct pipe_inode_info *pipe,
+			     struct pipe_buffer *buf)
 {
 	return 1;
 }
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -183,6 +183,7 @@ void free_pipe_info(struct pipe_inode_in
 bool generic_pipe_buf_get(struct pipe_inode_info *, struct pipe_buffer *);
 int generic_pipe_buf_confirm(struct pipe_inode_info *, struct pipe_buffer *);
 int generic_pipe_buf_steal(struct pipe_inode_info *, struct pipe_buffer *);
+int generic_pipe_buf_nosteal(struct pipe_inode_info *, struct pipe_buffer *);
 void generic_pipe_buf_release(struct pipe_inode_info *, struct pipe_buffer *);
 void pipe_buf_mark_unmergeable(struct pipe_buffer *buf);
 
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6513,19 +6513,23 @@ struct buffer_ref {
 	struct ring_buffer	*buffer;
 	void			*page;
 	int			cpu;
-	int			ref;
+	refcount_t		refcount;
 };
 
+static void buffer_ref_release(struct buffer_ref *ref)
+{
+	if (!refcount_dec_and_test(&ref->refcount))
+		return;
+	ring_buffer_free_read_page(ref->buffer, ref->cpu, ref->page);
+	kfree(ref);
+}
+
 static void buffer_pipe_buf_release(struct pipe_inode_info *pipe,
 				    struct pipe_buffer *buf)
 {
 	struct buffer_ref *ref = (struct buffer_ref *)buf->private;
 
-	if (--ref->ref)
-		return;
-
-	ring_buffer_free_read_page(ref->buffer, ref->cpu, ref->page);
-	kfree(ref);
+	buffer_ref_release(ref);
 	buf->private = 0;
 }
 
@@ -6534,10 +6538,10 @@ static bool buffer_pipe_buf_get(struct p
 {
 	struct buffer_ref *ref = (struct buffer_ref *)buf->private;
 
-	if (ref->ref > INT_MAX/2)
+	if (refcount_read(&ref->refcount) > INT_MAX/2)
 		return false;
 
-	ref->ref++;
+	refcount_inc(&ref->refcount);
 	return true;
 }
 
@@ -6546,7 +6550,7 @@ static const struct pipe_buf_operations
 	.can_merge		= 0,
 	.confirm		= generic_pipe_buf_confirm,
 	.release		= buffer_pipe_buf_release,
-	.steal			= generic_pipe_buf_steal,
+	.steal			= generic_pipe_buf_nosteal,
 	.get			= buffer_pipe_buf_get,
 };
 
@@ -6559,11 +6563,7 @@ static void buffer_spd_release(struct sp
 	struct buffer_ref *ref =
 		(struct buffer_ref *)spd->partial[i].private;
 
-	if (--ref->ref)
-		return;
-
-	ring_buffer_free_read_page(ref->buffer, ref->cpu, ref->page);
-	kfree(ref);
+	buffer_ref_release(ref);
 	spd->partial[i].private = 0;
 }
 
@@ -6618,7 +6618,7 @@ tracing_buffers_splice_read(struct file
 			break;
 		}
 
-		ref->ref = 1;
+		refcount_set(&ref->refcount, 1);
 		ref->buffer = iter->trace_buffer->buffer;
 		ref->page = ring_buffer_alloc_read_page(ref->buffer, iter->cpu_file);
 		if (IS_ERR(ref->page)) {