RE: [PATCH 2/6] Drivers: hv: vmbus: Avoid double fetch of msgtype in vmbus_on_msg_dpc()
From: Michael Kelley <hidden>
Date: 2020-12-06 17:11:32
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From: Andrea Parri (Microsoft) <parri.andrea@gmail.com> Sent: Wednesday, November 18, 2020 6:37 AM
vmbus_on_msg_dpc() double fetches from msgtype. The double fetch can lead to an out-of-bound access when accessing the channel_message_table array. In turn, the use of the out-of-bound entry could lead to code execution primitive (entry->message_handler()). Avoid the double fetch by saving the value of msgtype into a local variable.
The commit message is missing some context. Why is a double fetch a problem? The comments below in the code explain why, but the why should also be briefly explained in the commit message.
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Reported-by: Juan Vazquez <redacted> Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com> --- drivers/hv/vmbus_drv.c | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-)diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c index 0a2711aa63a15..82b23baa446d7 100644 --- a/drivers/hv/vmbus_drv.c +++ b/drivers/hv/vmbus_drv.c@@ -1057,6 +1057,7 @@ void vmbus_on_msg_dpc(unsigned long data) struct hv_message *msg = (struct hv_message *)page_addr + VMBUS_MESSAGE_SINT; struct vmbus_channel_message_header *hdr; + enum vmbus_channel_message_type msgtype; const struct vmbus_channel_message_table_entry *entry; struct onmessage_work_context *ctx; u32 message_type = msg->header.message_type;@@ -1072,12 +1073,19 @@ void vmbus_on_msg_dpc(unsigned long data) /* no msg */ return; + /* + * The hv_message object is in memory shared with the host. The host + * could erroneously or maliciously modify such object. Make sure to + * validate its fields and avoid double fetches whenever feasible.
The "when feasible" phrase sounds like not doing double fetches is optional in some circumstances. But I think we always have to protect against modification of memory shared with the host. So perhaps the comment should be more precise.
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+ */ + hdr = (struct vmbus_channel_message_header *)msg->u.payload; + msgtype = hdr->msgtype; trace_vmbus_on_msg_dpc(hdr); - if (hdr->msgtype >= CHANNELMSG_COUNT) { - WARN_ONCE(1, "unknown msgtype=%d\n", hdr->msgtype); + if (msgtype >= CHANNELMSG_COUNT) { + WARN_ONCE(1, "unknown msgtype=%d\n", msgtype); goto msg_handled; }@@ -1087,14 +1095,14 @@ void vmbus_on_msg_dpc(unsigned long data) goto msg_handled; } - entry = &channel_message_table[hdr->msgtype]; + entry = &channel_message_table[msgtype]; if (!entry->message_handler) goto msg_handled; if (msg->header.payload_size < entry->min_payload_len) { WARN_ONCE(1, "message too short: msgtype=%d len=%d\n", - hdr->msgtype, msg->header.payload_size); + msgtype, msg->header.payload_size); goto msg_handled; }@@ -1115,7 +1123,7 @@ void vmbus_on_msg_dpc(unsigned long data) * by offer_in_progress and by channel_mutex. See also the * inline comments in vmbus_onoffer_rescind(). */ - switch (hdr->msgtype) { + switch (msgtype) { case CHANNELMSG_RESCIND_CHANNELOFFER: /* * If we are handling the rescind message; --2.25.1