[PATCH 1/2] splice: don't merge into linked buffers

[Jann Horn <jannh@xxxxxxxxxx>]

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'
$
============

Fix it by explicitly marking buffers as mergeable and clearing that flag in
splice_pipe_to_pipe() and link_pipe().

Cc: <stable@xxxxxxxxxxxxxxx>
Fixes: 7c77f0b3f920 ("splice: implement pipe to pipe splicing")
Fixes: 70524490ee2e ("[PATCH] splice: add support for sys_tee()")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
Cleanup in the next patch, to simplify backporting.

 fs/pipe.c                 | 5 +++--
 fs/splice.c               | 6 ++++++
 include/linux/pipe_fs_i.h | 8 ++++++++
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index bdc5d3c0977d..4e2eee77f855 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -379,7 +379,8 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 		struct pipe_buffer *buf = pipe->bufs + lastbuf;
 		int offset = buf->offset + buf->len;
 
-		if (buf->ops->can_merge && offset + chars <= PAGE_SIZE) {
+		if (buf->ops->can_merge && offset + chars <= PAGE_SIZE &&
+		    (buf->flags & PIPE_BUF_FLAG_MAYMERGE)) {
 			ret = pipe_buf_confirm(pipe, buf);
 			if (ret)
 				goto out;
@@ -439,7 +440,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 			buf->ops = &anon_pipe_buf_ops;
 			buf->offset = 0;
 			buf->len = copied;
-			buf->flags = 0;
+			buf->flags = PIPE_BUF_FLAG_MAYMERGE;
 			if (is_packetized(filp)) {
 				buf->ops = &packet_pipe_buf_ops;
 				buf->flags = PIPE_BUF_FLAG_PACKET;
diff --git a/fs/splice.c b/fs/splice.c
index b3daa971f597..111977c80dfd 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1593,6 +1593,9 @@ static int splice_pipe_to_pipe(struct pipe_inode_info *ipipe,
 			 */
 			obuf->flags &= ~PIPE_BUF_FLAG_GIFT;
 
+			/* We can't merge data into a buffer we don't own. */
+			obuf->flags &= ~PIPE_BUF_FLAG_MAYMERGE;
+
 			obuf->len = len;
 			opipe->nrbufs++;
 			ibuf->offset += obuf->len;
@@ -1667,6 +1670,9 @@ static int link_pipe(struct pipe_inode_info *ipipe,
 		 */
 		obuf->flags &= ~PIPE_BUF_FLAG_GIFT;
 
+		/* We can't merge data into a buffer we don't own. */
+		obuf->flags &= ~PIPE_BUF_FLAG_MAYMERGE;
+
 		if (obuf->len > len)
 			obuf->len = len;
 
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index 5a3bb3b7c9ad..8893711f9171 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -8,6 +8,14 @@
 #define PIPE_BUF_FLAG_ATOMIC	0x02	/* was atomically mapped */
 #define PIPE_BUF_FLAG_GIFT	0x04	/* page is a gift */
 #define PIPE_BUF_FLAG_PACKET	0x08	/* read() as a packet */
+/*
+ * Set this flag if the generic pipe read/write may coalesce data into an
+ * existing buffer. If this is not set, a new pipe page segment is always used
+ * for new data.
+ * When pipe data is copied by reference (as in the tee() syscall), this flag
+ * must be cleared on the copy.
+ */
+#define PIPE_BUF_FLAG_MAYMERGE	0x10
 
 /**
  *	struct pipe_buffer - a linux kernel pipe buffer
-- 
2.19.0.605.g01d371f741-goog