On 1/5/24 10:40, Harshit Mogalapalli wrote:
Syzkaller hit 'WARNING in dg_dispatch_as_host' bug.
memcpy: detected field-spanning write (size 56) of single field "&dg_info->msg"
at drivers/misc/vmw_vmci/vmci_datagram.c:237 (size 24)
WARNING: CPU: 0 PID: 1555 at drivers/misc/vmw_vmci/vmci_datagram.c:237
dg_dispatch_as_host+0x88e/0xa60 drivers/misc/vmw_vmci/vmci_datagram.c:237
Some code commentry, based on my understanding:
544 #define VMCI_DG_SIZE(_dg) (VMCI_DG_HEADERSIZE + (size_t)(_dg)->payload_size)
/// This is 24 + payload_size
memcpy(&dg_info->msg, dg, dg_size);
Destination = dg_info->msg ---> this is a 24 byte
structure(struct vmci_datagram)
Source = dg --> this is a 24 byte structure (struct vmci_datagram)
Size = dg_size = 24 + payload_size
{payload_size = 56-24 =32} -- Syzkaller managed to set payload_size to 32.
35 struct delayed_datagram_info {
36 struct datagram_entry *entry;
37 struct work_struct work;
38 bool in_dg_host_queue;
39 /* msg and msg_payload must be together. */
40 struct vmci_datagram msg;
41 u8 msg_payload[];
42 };
So those extra bytes of payload are copied into msg_payload[], a run time
warning is seen while fuzzing with Syzkaller.
One possible way to fix the warning is to split the memcpy() into
two parts -- one -- direct assignment of msg and second taking care of payload.
Gustavo quoted:
"Under FORTIFY_SOURCE we should not copy data across multiple members
in a structure."
Reported-by: syzkaller <[email protected]>
Suggested-by: Vegard Nossum <[email protected]>
Suggested-by: Gustavo A. R. Silva <[email protected]>
Signed-off-by: Harshit Mogalapalli <[email protected]>
Reviewed-by: Gustavo A. R. Silva <[email protected]>
Thanks!
--
Gustavo
---
This patch is only tested with the C reproducer, not any testing
specific to driver is done.
v1->v2: ( Suggestions from Gustavo )
1. Change the commit message false positive --> legitimate
warning.
2. Remove unneeded payload_size variable.
3. Replace first memcpy() with direct assignment.
---
drivers/misc/vmw_vmci/vmci_datagram.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/misc/vmw_vmci/vmci_datagram.c
b/drivers/misc/vmw_vmci/vmci_datagram.c
index ac6cb0c8d99b..ba379cd6d054 100644
--- a/drivers/misc/vmw_vmci/vmci_datagram.c
+++ b/drivers/misc/vmw_vmci/vmci_datagram.c
@@ -234,7 +234,8 @@ static int dg_dispatch_as_host(u32 context_id, struct
vmci_datagram *dg)
dg_info->in_dg_host_queue = true;
dg_info->entry = dst_entry;
- memcpy(&dg_info->msg, dg, dg_size);
+ dg_info->msg = *dg;
+ memcpy(&dg_info->msg_payload, dg + 1, dg->payload_size);
INIT_WORK(&dg_info->work, dg_delayed_dispatch);
schedule_work(&dg_info->work);
