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

[Kees Cook]

On Mon, Jan 08, 2024 at 08:05:38PM -0600, Gustavo A. R. Silva wrote:
> 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@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
> 
> 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.

Oops, yes, thanks for fixing my confusion. Right, this is a direct write
across members into the flex array, not a composite destination. Yay
all the corner cases. :P

-- 
Kees Cook