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Date:      Sat, 29 Jun 2019 16:05:52 +0000 (UTC)
From:      Mark Johnston <markj@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r349546 - head/sys/kern
Message-ID:  <201906291605.x5TG5qBA019555@repo.freebsd.org>
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Author: markj
Date: Sat Jun 29 16:05:52 2019
New Revision: 349546
URL: https://svnweb.freebsd.org/changeset/base/349546

Log:
  Fix mutual exclusion in pipe_direct_write().
  
  We use PIPE_DIRECTW as a semaphore for direct writes to a pipe, where
  the reader copies data directly from pages mapped into the writer.
  However, when a reader finishes such a copy, it previously cleared
  PIPE_DIRECTW, allowing multiple writers to race and corrupt the state
  used to track wired pages belonging to the writer.
  
  Fix this by having the writer clear PIPE_DIRECTW and instead use the
  count of unread bytes to determine whether a write is finished.
  
  Reported by:	syzbot+21811cc0a89b2a87a9e7@syzkaller.appspotmail.com
  Reviewed by:	kib, mjg
  Tested by:	pho
  MFC after:	1 week
  Sponsored by:	The FreeBSD Foundation
  Differential Revision:	https://reviews.freebsd.org/D20784

Modified:
  head/sys/kern/sys_pipe.c

Modified: head/sys/kern/sys_pipe.c
==============================================================================
--- head/sys/kern/sys_pipe.c	Sat Jun 29 15:28:36 2019	(r349545)
+++ head/sys/kern/sys_pipe.c	Sat Jun 29 16:05:52 2019	(r349546)
@@ -724,7 +724,7 @@ pipe_read(struct file *fp, struct uio *uio, struct ucr
 			rpipe->pipe_map.pos += size;
 			rpipe->pipe_map.cnt -= size;
 			if (rpipe->pipe_map.cnt == 0) {
-				rpipe->pipe_state &= ~(PIPE_DIRECTW|PIPE_WANTW);
+				rpipe->pipe_state &= ~PIPE_WANTW;
 				wakeup(rpipe);
 			}
 #endif
@@ -855,13 +855,17 @@ pipe_build_write_buffer(struct pipe *wpipe, struct uio
 }
 
 /*
- * unmap and unwire the process buffer
+ * Unwire the process buffer.
  */
 static void
 pipe_destroy_write_buffer(struct pipe *wpipe)
 {
 
 	PIPE_LOCK_ASSERT(wpipe, MA_OWNED);
+	KASSERT((wpipe->pipe_state & PIPE_DIRECTW) != 0,
+	    ("%s: PIPE_DIRECTW not set on %p", __func__, wpipe));
+
+	wpipe->pipe_state &= ~PIPE_DIRECTW;
 	vm_page_unhold_pages(wpipe->pipe_map.ms, wpipe->pipe_map.npages);
 	wpipe->pipe_map.npages = 0;
 }
@@ -880,13 +884,15 @@ pipe_clone_write_buffer(struct pipe *wpipe)
 	int pos;
 
 	PIPE_LOCK_ASSERT(wpipe, MA_OWNED);
+	KASSERT((wpipe->pipe_state & PIPE_DIRECTW) != 0,
+	    ("%s: PIPE_DIRECTW not set on %p", __func__, wpipe));
+
 	size = wpipe->pipe_map.cnt;
 	pos = wpipe->pipe_map.pos;
 
 	wpipe->pipe_buffer.in = size;
 	wpipe->pipe_buffer.out = 0;
 	wpipe->pipe_buffer.cnt = size;
-	wpipe->pipe_state &= ~PIPE_DIRECTW;
 
 	PIPE_UNLOCK(wpipe);
 	iov.iov_base = wpipe->pipe_buffer.buffer;
@@ -925,7 +931,7 @@ retry:
 		pipeunlock(wpipe);
 		goto error1;
 	}
-	while (wpipe->pipe_state & PIPE_DIRECTW) {
+	if (wpipe->pipe_state & PIPE_DIRECTW) {
 		if (wpipe->pipe_state & PIPE_WANTR) {
 			wpipe->pipe_state &= ~PIPE_WANTR;
 			wakeup(wpipe);
@@ -968,8 +974,7 @@ retry:
 		goto error1;
 	}
 
-	error = 0;
-	while (!error && (wpipe->pipe_state & PIPE_DIRECTW)) {
+	while (wpipe->pipe_map.cnt != 0) {
 		if (wpipe->pipe_state & PIPE_EOF) {
 			pipe_destroy_write_buffer(wpipe);
 			pipeselwakeup(wpipe);
@@ -987,20 +992,19 @@ retry:
 		error = msleep(wpipe, PIPE_MTX(wpipe), PRIBIO | PCATCH,
 		    "pipdwt", 0);
 		pipelock(wpipe, 0);
+		if (error != 0)
+			break;
 	}
 
 	if (wpipe->pipe_state & PIPE_EOF)
 		error = EPIPE;
-	if (wpipe->pipe_state & PIPE_DIRECTW) {
-		/*
-		 * this bit of trickery substitutes a kernel buffer for
-		 * the process that might be going away.
-		 */
+	if (error == EINTR || error == ERESTART)
 		pipe_clone_write_buffer(wpipe);
-	} else {
+	else
 		pipe_destroy_write_buffer(wpipe);
-	}
 	pipeunlock(wpipe);
+	KASSERT((wpipe->pipe_state & PIPE_DIRECTW) == 0,
+	    ("pipe %p leaked PIPE_DIRECTW", wpipe));
 	return (error);
 
 error1:

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