Error about "somebody tries to fake us!"

Stefan Kalkowski stefan.kalkowski at ...1...
Tue Oct 14 13:08:17 CEST 2014


On 10/08/2014 02:43 PM, li94575 wrote:
> Hi all,
> Recently,when I ported some linux drivers to genode, occasionally, there
> will be such an error "*somebody tries to fake us!*". To find the cause
> of the error,I analysed the communication between client and server.
> First, client creats the connection to the server, then the server will
> creat a server object with an valid id, also an Ipc_gate object(binding
> to the entry_point thread) is created and the id is set as the
> gate-label. Last, the server returns the cap of the ipc_gate.
> I grab the session capability the client obtained after the connection,
> and get the id(local name)of the session capability. Once I call the
> interface function the server provided, the id will be encapsulated into
> Message Register(mr[0]). When the server receives the requesting
> message, it will compare the id(the server recevied) and label of the
> gate. I tried to illustrate it by making a simple diagram.Is this correct?

Your analysis of the communication behavior is perfectly fine.

> Now I know the value of the id on client side is 0x14dc,the label of the
> gate is 0x14dc, but the id on server side is 0x4. My doubt is:where the
> object id could be tampered?

I assume that your message buffer when sending a message to the server
is corrupt, so that it contains a "0x4" as first word within the
message. Although it is not impossible that we've a general bug in the
base-foc code, we didn't observed such behavior in the past, though we
test it heavily regularly. Therefore I think you've a memory corruption
problem within your client, or did you also touched other code (e.g.:
something in the parent tree of your client). Where is your connection
initiated, within the BSS segment, heap, or stack?

Maybe you can provide some code snippet that helps to reproduce the problem?


>  Thanks in advanced.
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Stefan Kalkowski
Genode Labs ·

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