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 01:33, Dan Carpenter 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>
>> ---
>> 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.
> 
> The commit message is fine.
> 
> Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
> 
> But, I mean, it's not really "legitimate".  It meets the fortify source
> heuristic, but it's still a false positive.  Fortify source is
> *supposed* to find memory corruption bugs and this is not a memory
> corruption bug.  It's just that these days we have to treat foritify
> false positives as crashing bugs because people enable it and we have to
> fix it.
> 
> Let's not pretend that fortify has helped us in this situation, it has
> caused us a problem.  It has taken valid code and created a crashing
> bug.  I'm not saying that the cost isn't worth it, but let's not pretend.
> 

It's a "legitimate warning" (which differs from a "legitimate memory
corruption bug") in the sense that the feature is doing what it's
supposed to do: reporting a write beyond the boundaries of a field/member
in a structure.

Is that simple.  I don't see the "pretense" here.

BTW, is this _warning_ really causing a crash?

Thanks
--
Gustavo