Patch "fix a fencepost error in pipe_advance()" has been added to the 4.9-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    fix a fencepost error in pipe_advance()

to the 4.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     fix-a-fencepost-error-in-pipe_advance.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From b9dc6f65bc5e232d1c05fe34b5daadc7e8bbf1fb Mon Sep 17 00:00:00 2001
From: Al Viro <viro@zeniv.linux.org.uk>
Date: Sat, 14 Jan 2017 19:33:08 -0500
Subject: fix a fencepost error in pipe_advance()

From: Al Viro <viro@zeniv.linux.org.uk>

commit b9dc6f65bc5e232d1c05fe34b5daadc7e8bbf1fb upstream.

The logics in pipe_advance() used to release all buffers past the new
position failed in cases when the number of buffers to release was equal
to pipe->buffers.  If that happened, none of them had been released,
leaving pipe full.  Worse, it was trivial to trigger and we end up with
pipe full of uninitialized pages.  IOW, it's an infoleak.

Reported-by: "Alan J. Wylie" <alan@wylie.me.uk>
Tested-by: "Alan J. Wylie" <alan@wylie.me.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 lib/iov_iter.c |   54 +++++++++++++++++++++++++++++++-----------------------
 1 file changed, 31 insertions(+), 23 deletions(-)

--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -678,43 +678,50 @@ size_t iov_iter_copy_from_user_atomic(st
 }
 EXPORT_SYMBOL(iov_iter_copy_from_user_atomic);
 
+static inline void pipe_truncate(struct iov_iter *i)
+{
+	struct pipe_inode_info *pipe = i->pipe;
+	if (pipe->nrbufs) {
+		size_t off = i->iov_offset;
+		int idx = i->idx;
+		int nrbufs = (idx - pipe->curbuf) & (pipe->buffers - 1);
+		if (off) {
+			pipe->bufs[idx].len = off - pipe->bufs[idx].offset;
+			idx = next_idx(idx, pipe);
+			nrbufs++;
+		}
+		while (pipe->nrbufs > nrbufs) {
+			pipe_buf_release(pipe, &pipe->bufs[idx]);
+			idx = next_idx(idx, pipe);
+			pipe->nrbufs--;
+		}
+	}
+}
+
 static void pipe_advance(struct iov_iter *i, size_t size)
 {
 	struct pipe_inode_info *pipe = i->pipe;
-	struct pipe_buffer *buf;
-	int idx = i->idx;
-	size_t off = i->iov_offset, orig_sz;
-	
 	if (unlikely(i->count < size))
 		size = i->count;
-	orig_sz = size;
-
 	if (size) {
+		struct pipe_buffer *buf;
+		size_t off = i->iov_offset, left = size;
+		int idx = i->idx;
 		if (off) /* make it relative to the beginning of buffer */
-			size += off - pipe->bufs[idx].offset;
+			left += off - pipe->bufs[idx].offset;
 		while (1) {
 			buf = &pipe->bufs[idx];
-			if (size <= buf->len)
+			if (left <= buf->len)
 				break;
-			size -= buf->len;
+			left -= buf->len;
 			idx = next_idx(idx, pipe);
 		}
-		buf->len = size;
 		i->idx = idx;
-		off = i->iov_offset = buf->offset + size;
-	}
-	if (off)
-		idx = next_idx(idx, pipe);
-	if (pipe->nrbufs) {
-		int unused = (pipe->curbuf + pipe->nrbufs) & (pipe->buffers - 1);
-		/* [curbuf,unused) is in use.  Free [idx,unused) */
-		while (idx != unused) {
-			pipe_buf_release(pipe, &pipe->bufs[idx]);
-			idx = next_idx(idx, pipe);
-			pipe->nrbufs--;
-		}
+		i->iov_offset = buf->offset + left;
 	}
-	i->count -= orig_sz;
+	i->count -= size;
+	/* ... and discard everything past that point */
+	pipe_truncate(i);
 }
 
 void iov_iter_advance(struct iov_iter *i, size_t size)
@@ -774,6 +781,7 @@ void iov_iter_pipe(struct iov_iter *i, i
 			size_t count)
 {
 	BUG_ON(direction != ITER_PIPE);
+	WARN_ON(pipe->nrbufs == pipe->buffers);
 	i->type = direction;
 	i->pipe = pipe;
 	i->idx = (pipe->curbuf + pipe->nrbufs) & (pipe->buffers - 1);


Patches currently in stable-queue which might be from viro@zeniv.linux.org.uk are

queue-4.9/fix-a-fencepost-error-in-pipe_advance.patch