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

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 1/8/24 16:37, Kees Cook wrote:
> 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@googlegroups.com>
>> Suggested-by: Vegard Nossum <vegard.nossum@oracle.com>
>> Suggested-by: Gustavo A. R. Silva <gustavoars@kernel.org>
>> Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
> 
> Thanks for getting this fixed!
> 
> Yeah, it's a "false positive" in the sense that the code was expecting

It's a false positive _bug_, and a legitimate _warning_ coming from fortified
memcpy().

> 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.

The warning is triggered because of a write beyond the boundaries of
`dg_info->msg`. It's not directly related to the fact that there is a
flexible-array member following `dg_info->msg`.

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101832 (fixed in GCC 14+)
> 	 (not yet fixed in Clang)

This issue is not related to the compiler bugs mentioned above.

> 
> Reviewed-by: Kees Cook <keescook@chromium.org>
> 

Thanks!
--
Gustavo