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, 4 insertions, 0 deletions

diff --git a/fs/splice.c b/fs/splice.c
index de2ede048473..90c29675d573 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1597,6 +1597,8 @@ retry:
 			 */
 			obuf->flags &= ~PIPE_BUF_FLAG_GIFT;
 
+			pipe_buf_mark_unmergeable(obuf);
+
 			obuf->len = len;
 			opipe->nrbufs++;
 			ibuf->offset += obuf->len;
@@ -1671,6 +1673,8 @@ static int link_pipe(struct pipe_inode_info *ipipe,
 		 */
 		obuf->flags &= ~PIPE_BUF_FLAG_GIFT;
 
+		pipe_buf_mark_unmergeable(obuf);
+
 		if (obuf->len > len)
 			obuf->len = len;