Messages in this thread | ![/](/images/icornerl.gif) | | Date | Mon, 8 Jan 2024 14:37:21 -0800 | From | Kees Cook <> | Subject | Re: [PATCH v2 2/2] VMCI: Fix memcpy() run-time warning in dg_dispatch_as_host() |
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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 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@chromium.org>
-- Kees Cook
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