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 <[email protected]> Suggested-by: Vegard Nossum <[email protected]> Suggested-by: Gustavo A. R. Silva <[email protected]> Signed-off-by: Harshit Mogalapalli <[email protected]>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 <[email protected]>
Thanks! -- Gustavo
