splice: don't merge into linked buffers

Before this patch, it was possible for two pipes to affect each other after
data had been transferred between them with tee():

============
$ cat tee_test.c

int main(void) {
  int pipe_a[2];
  if (pipe(pipe_a)) err(1, "pipe");
  int pipe_b[2];
  if (pipe(pipe_b)) err(1, "pipe");
  if (write(pipe_a[1], "abcd", 4) != 4) err(1, "write");
  if (tee(pipe_a[0], pipe_b[1], 2, 0) != 2) err(1, "tee");
  if (write(pipe_b[1], "xx", 2) != 2) err(1, "write");

  char buf[5];
  if (read(pipe_a[0], buf, 4) != 4) err(1, "read");
  buf[4] = 0;
  printf("got back: '%s'\n", buf);
}
$ gcc -o tee_test tee_test.c
$ ./tee_test
got back: 'abxx'
$
============

As suggested by Al Viro, fix it by creating a separate type for
non-mergeable pipe buffers, then changing the types of buffers in
splice_pipe_to_pipe() and link_pipe().

Cc: <stable@vger.kernel.org>
Fixes: 7c77f0b3f920 ("splice: implement pipe to pipe splicing")
Fixes: 70524490ee2e ("[PATCH] splice: add support for sys_tee()")
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 14 insertions, 0 deletions

diff --git a/fs/pipe.c b/fs/pipe.c
index bdc5d3c0977d..c51750ed4011 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -234,6 +234,14 @@ static const struct pipe_buf_operations anon_pipe_buf_ops = {
         .get = generic_pipe_buf_get,
 };
 
+static const struct pipe_buf_operations anon_pipe_buf_nomerge_ops = {
+        .can_merge = 0,
+        .confirm = generic_pipe_buf_confirm,
+        .release = anon_pipe_buf_release,
+        .steal = anon_pipe_buf_steal,
+        .get = generic_pipe_buf_get,
+};
+
 static const struct pipe_buf_operations packet_pipe_buf_ops = {
         .can_merge = 0,
         .confirm = generic_pipe_buf_confirm,
@@ -242,6 +250,12 @@ static const struct pipe_buf_operations packet_pipe_buf_ops = {
         .get = generic_pipe_buf_get,
 };
 
+void pipe_buf_mark_unmergeable(struct pipe_buffer *buf)
+{
+        if (buf->ops == &anon_pipe_buf_ops)
+                buf->ops = &anon_pipe_buf_nomerge_ops;
+}
+
 static ssize_t
 pipe_read(struct kiocb *iocb, struct iov_iter *to)
 {