tracing: Fix buffer_ref pipe ops

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>

3 files changed, 17 insertions, 16 deletions

diff --git a/fs/splice.c b/fs/splice.c
index 3ee7e82df48f..e75807380caa 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -330,8 +330,8 @@ const struct pipe_buf_operations default_pipe_buf_ops = {
         .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;
 }
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index 787d224ff43e..a830e9a00eb9 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -174,6 +174,7 @@ void free_pipe_info(struct pipe_inode_info *);
 void 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);
 
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 21153e64bf1c..0cfa13a60086 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7025,19 +7025,23 @@ 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;
 }
 
@@ -7046,14 +7050,14 @@ static void buffer_pipe_buf_get(struct pipe_inode_info *pipe,
 {
         struct buffer_ref *ref = (struct buffer_ref *)buf->private;
 
-        ref->ref++;
+        refcount_inc(&ref->refcount);
 }
 
 /* Pipe buffer operations for a buffer. */
 static const struct pipe_buf_operations buffer_pipe_buf_ops = {
         .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,
 };
 
@@ -7066,11 +7070,7 @@ static void buffer_spd_release(struct splice_pipe_desc *spd, unsigned int i)
         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;
 }
 
@@ -7125,7 +7125,7 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
                         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)) {