From: Jann Horn jannh@google.com

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 --- fs/splice.c | 4 ++-- include/linux/pipe_fs_i.h | 1 + kernel/trace/trace.c | 28 ++++++++++++++-------------- 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)) {