Fix condition where we could exit with a fatal error when an input

buffer became too large and the remote end had advertised a big window.
The problem was a mismatch in the backoff math between the channels code
and the buffer code, so make a buffer_check_alloc() function that the
channels code can use to propsectivly check whether an incremental
allocation will succeed.  bz #1131, debugged with the assistance of
cove AT wildpackets.com; ok dtucker@ deraadt@

3 files changed, 54 insertions, 22 deletions

diff --git a/usr.bin/ssh/buffer.c b/usr.bin/ssh/buffer.c
index de404e60245..ba718daf221 100644
--- a/usr.bin/ssh/buffer.c
+++ b/usr.bin/ssh/buffer.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: buffer.c,v 1.26 2006/03/25 13:17:01 djm Exp $ */
+/* $OpenBSD: buffer.c,v 1.27 2006/04/16 00:48:52 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -18,6 +18,10 @@
 #include "buffer.h"
 #include "log.h"
 
+#define	BUFFER_MAX_CHUNK	0x100000
+#define	BUFFER_MAX_LEN		0xa00000
+#define	BUFFER_ALLOCSZ		0x008000
+
 /* Initializes the buffer structure. */
 
 void
@@ -66,6 +70,23 @@ buffer_append(Buffer *buffer, const void *data, u_int len)
 	memcpy(p, data, len);
 }
 
+static int
+buffer_compact(Buffer *buffer)
+{
+	/*
+	 * If the buffer is quite empty, but all data is at the end, move the
+	 * data to the beginning.
+	 */
+	if (buffer->offset > MIN(buffer->alloc, BUFFER_MAX_CHUNK)) {
+		memmove(buffer->buf, buffer->buf + buffer->offset,
+			buffer->end - buffer->offset);
+		buffer->end -= buffer->offset;
+		buffer->offset = 0;
+		return (1);
+	}
+	return (0);
+}
+
 /*
  * Appends space to the buffer, expanding the buffer if necessary. This does
  * not actually copy the data into the buffer, but instead returns a pointer
@@ -93,20 +114,13 @@ restart:
 		buffer->end += len;
 		return p;
 	}
-	/*
-	 * If the buffer is quite empty, but all data is at the end, move the
-	 * data to the beginning and retry.
-	 */
-	if (buffer->offset > MIN(buffer->alloc, BUFFER_MAX_CHUNK)) {
-		memmove(buffer->buf, buffer->buf + buffer->offset,
-			buffer->end - buffer->offset);
-		buffer->end -= buffer->offset;
-		buffer->offset = 0;
+
+	/* Compact data back to the start of the buffer if necessary */
+	if (buffer_compact(buffer))
 		goto restart;
-	}
-	/* Increase the size of the buffer and retry. */
 
-	newlen = buffer->alloc + len + 32768;
+	/* Increase the size of the buffer and retry. */
+	newlen = roundup(buffer->alloc + len, BUFFER_ALLOCSZ);
 	if (newlen > BUFFER_MAX_LEN)
 		fatal("buffer_append_space: alloc %u not supported",
 		    newlen);
@@ -116,6 +130,27 @@ restart:
 	/* NOTREACHED */
 }
 
+/*
+ * Check whether an allocation of 'len' will fit in the buffer
+ * This must follow the same math as buffer_append_space
+ */
+int
+buffer_check_alloc(Buffer *buffer, u_int len)
+{
+	if (buffer->offset == buffer->end) {
+		buffer->offset = 0;
+		buffer->end = 0;
+	}
+ restart:
+	if (buffer->end + len < buffer->alloc)
+		return (1);
+	if (buffer_compact(buffer))
+		goto restart;
+	if (roundup(buffer->alloc + len, BUFFER_ALLOCSZ) <= BUFFER_MAX_LEN)
+		return (1);
+	return (0);
+}
+
 /* Returns the number of bytes of data in the buffer. */
 
 u_int
diff --git a/usr.bin/ssh/buffer.h b/usr.bin/ssh/buffer.h
index abdaea3493d..43414ae99e5 100644
--- a/usr.bin/ssh/buffer.h
+++ b/usr.bin/ssh/buffer.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: buffer.h,v 1.14 2006/03/25 22:22:42 djm Exp $ */
+/* $OpenBSD: buffer.h,v 1.15 2006/04/16 00:48:52 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -23,9 +23,6 @@ typedef struct {
 	u_int	 end;		/* Offset of last byte containing data. */
 }       Buffer;
 
-#define	BUFFER_MAX_CHUNK	0x100000
-#define	BUFFER_MAX_LEN		0xa00000
-
 void	 buffer_init(Buffer *);
 void	 buffer_clear(Buffer *);
 void	 buffer_free(Buffer *);
@@ -36,6 +33,8 @@ void	*buffer_ptr(Buffer *);
 void	 buffer_append(Buffer *, const void *, u_int);
 void	*buffer_append_space(Buffer *, u_int);
 
+int	 buffer_check_alloc(Buffer *, u_int);
+
 void	 buffer_get(Buffer *, void *, u_int);
 
 void	 buffer_consume(Buffer *, u_int);
diff --git a/usr.bin/ssh/channels.c b/usr.bin/ssh/channels.c
index 854bc2fd226..1d4a24a473f 100644
--- a/usr.bin/ssh/channels.c
+++ b/usr.bin/ssh/channels.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: channels.c,v 1.249 2006/03/30 09:41:25 djm Exp $ */
+/* $OpenBSD: channels.c,v 1.250 2006/04/16 00:48:52 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -746,12 +746,10 @@ channel_pre_open(Channel *c, fd_set *readset, fd_set *writeset)
 {
 	u_int limit = compat20 ? c->remote_window : packet_get_maxsize();
 
-	/* check buffer limits */
-	limit = MIN(limit, (BUFFER_MAX_LEN - BUFFER_MAX_CHUNK - CHAN_RBUF));
-
 	if (c->istate == CHAN_INPUT_OPEN &&
 	    limit > 0 &&
-	    buffer_len(&c->input) < limit)
+	    buffer_len(&c->input) < limit &&
+	    buffer_check_alloc(&c->input, CHAN_RBUF))
 		FD_SET(c->rfd, readset);
 	if (c->ostate == CHAN_OUTPUT_OPEN ||
 	    c->ostate == CHAN_OUTPUT_WAIT_DRAIN) {