Re: [PATCH v2 2/2] VMCI: Fix memcpy() run-time warning in dg_dispatch_as_host()

[Kees Cook]

On Fri, Jan 05, 2024 at 08:40:00AM -0800, 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 <syzkaller@xxxxxxxxxxxxxxxx>
> Suggested-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
> Suggested-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@xxxxxxxxxx>

Thanks for getting this fixed!

Yeah, it's a "false positive" in the sense that the code was expecting
to write into msg_payload. The warning is triggered because of the write
across the flex array boundary, which trips a bug in GCC and Clang,
which we're forced to work around.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101832 (fixed in GCC 14+)
https://github.com/llvm/llvm-project/issues/72032 (not yet fixed in Clang)

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook