Re: [PATCH 04/14] nbd/client: Add safety check on chunk payload length

[Vladimir Sementsov-Ogievskiy]

04.12.2021 02:15, Eric Blake wrote:

Our existing use of structured replies either reads into a qiov capped
at 32M (NBD_CMD_READ) or caps allocation to 1000 bytes (see
NBD_MAX_MALLOC_PAYLOAD in block/nbd.c).  But the existing length
checks are rather late; if we encounter a buggy (or malicious) server
that sends a super-large payload length, we should drop the connection
right then rather than assuming the layer on top will be careful.
This becomes more important when we permit 64-bit lengths which are
even more likely to have the potential for attempted denial of service
abuse.

Signed-off-by: Eric Blake 



Reviewed-by: Vladimir Sementsov-Ogievskiy 


---
  nbd/client.c | 12 
  1 file changed, 12 insertions(+)

diff --git a/nbd/client.c b/nbd/client.c
index 30d5383cb195..8f137c2320bb 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -1412,6 +1412,18 @@ static int nbd_receive_structured_reply_chunk(QIOChannel 
*ioc,
  chunk->handle = be64_to_cpu(chunk->handle);
  chunk->length = be32_to_cpu(chunk->length);

+/*
+ * Because we use BLOCK_STATUS with REQ_ONE, and cap READ requests
+ * at 32M, no valid server should send us payload larger than
+ * this.  Even if we stopped using REQ_ONE, sane servers will cap
+ * the number of extents they return for block status.
+ */
+if (chunk->length > NBD_MAX_BUFFER_SIZE + sizeof(NBDStructuredReadData)) {
+error_setg(errp, "server chunk %" PRIu32 " (%s) payload is too long",
+   chunk->type, nbd_rep_lookup(chunk->type));
+return -EINVAL;
+}
+
  return 0;
  }




--
Best regards,
Vladimir

[PATCH 04/14] nbd/client: Add safety check on chunk payload length

[Eric Blake]

Our existing use of structured replies either reads into a qiov capped
at 32M (NBD_CMD_READ) or caps allocation to 1000 bytes (see
NBD_MAX_MALLOC_PAYLOAD in block/nbd.c).  But the existing length
checks are rather late; if we encounter a buggy (or malicious) server
that sends a super-large payload length, we should drop the connection
right then rather than assuming the layer on top will be careful.
This becomes more important when we permit 64-bit lengths which are
even more likely to have the potential for attempted denial of service
abuse.

Signed-off-by: Eric Blake 
---
 nbd/client.c | 12 
 1 file changed, 12 insertions(+)

diff --git a/nbd/client.c b/nbd/client.c
index 30d5383cb195..8f137c2320bb 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -1412,6 +1412,18 @@ static int nbd_receive_structured_reply_chunk(QIOChannel 
*ioc,
 chunk->handle = be64_to_cpu(chunk->handle);
 chunk->length = be32_to_cpu(chunk->length);

+/*
+ * Because we use BLOCK_STATUS with REQ_ONE, and cap READ requests
+ * at 32M, no valid server should send us payload larger than
+ * this.  Even if we stopped using REQ_ONE, sane servers will cap
+ * the number of extents they return for block status.
+ */
+if (chunk->length > NBD_MAX_BUFFER_SIZE + sizeof(NBDStructuredReadData)) {
+error_setg(errp, "server chunk %" PRIu32 " (%s) payload is too long",
+   chunk->type, nbd_rep_lookup(chunk->type));
+return -EINVAL;
+}
+
 return 0;
 }

-- 
2.33.1