Doubt: core not dumped when binary give up root privileges.

Doubt: core not dumped when binary give up root privileges.

[Alexandre Pereira Nunes]

Hi,

I wrote a program which runs with uid 0, but later give up root privs by 
calling setreuid(x, x) where x is an unprivileged user.
Before doing that, it chdirs to a directory owned by that unprivileged 
user, with mode 700.

The program explicitly sets RLIMIT_CORE to RLIM_INFINITY when still 
running with uid 0.

If instead of calling the program as root, I call it from the non-priv 
uid in question, if it crashes, it dumps core on the mentioned dir. 
That's the desired behaviour, since I can then take the core and debug. 
But if I run it as root (in fact, I would have to), and it crashes (or 
is forced to ,by means of kill -SEGV), after it gives up root 
credentials, it won't leave a core dump file, which in turn means I 
cannot debug it later.

Any ideas?


Please CC-me since I'm not subscribed.

Thanks,

Alexandre

Re: Doubt: core not dumped when binary give up root privileges.

[Alexandre Pereira Nunes]

Ok, now I took the time to read the kernel sources and found why it 
works like that, and also the "prctl" ioctl.

I guess this should do the trick.

Thanks for the people who gave me some suggestions,

Alexandre

Re: Doubt: core not dumped when binary give up root privileges.

[Alan Cox]

On Gwe, 2003-08-22 at 20:25, Alexandre Pereira Nunes wrote:
> The program explicitly sets RLIMIT_CORE to RLIM_INFINITY when still 
> running with uid 0.

The kernel assumes a core image from something that was priviledged may
be unsafe.

> If instead of calling the program as root, I call it from the non-priv 
> uid in question, if it crashes, it dumps core on the mentioned dir. 
> That's the desired behaviour, since I can then take the core and debug. 
> But if I run it as root (in fact, I would have to), and it crashes (or 
> is forced to ,by means of kill -SEGV), after it gives up root 
> credentials, it won't leave a core dump file, which in turn means I 
> cannot debug it later.
> 
> Any ideas?

2.4-ac has support for enabling setuid core dumps and setting the dump
path, so you can write such dumps to /root/dumps and the kernel will
make them root accessible only.

The 2.6 test tree and Marcelo 2.4 don't currently support this

Re: Doubt: core not dumped when binary give up root privileges.

[Alexandre Pereira Nunes]

Alan Cox wrote:

>On Gwe, 2003-08-22 at 20:25, Alexandre Pereira Nunes wrote:
>  
>
>>The program explicitly sets RLIMIT_CORE to RLIM_INFINITY when still 
>>running with uid 0.
>>    
>>
>
>The kernel assumes a core image from something that was priviledged may
>be unsafe.
>  
>
That makes sense... By that time, I took a look at the sources and found 
the prctl syscall, and with a little tweaking it was possible to do what 
I wish (keep reading)...

>  
>
>>If instead of calling the program as root, I call it from the non-priv 
>>uid in question, if it crashes, it dumps core on the mentioned dir. 
>>That's the desired behaviour, since I can then take the core and debug. 
>>But if I run it as root (in fact, I would have to), and it crashes (or 
>>is forced to ,by means of kill -SEGV), after it gives up root 
>>credentials, it won't leave a core dump file, which in turn means I 
>>cannot debug it later.
>>
>>Any ideas?
>>    
>>
>
>2.4-ac has support for enabling setuid core dumps and setting the dump
>path, so you can write such dumps to /root/dumps and the kernel will
>make them root accessible only.
>
>The 2.6 test tree and Marcelo 2.4 don't currently support this
>
>  
>

That's an interesting feature and is almost exactly what I needed: I 
wrote a daemon, which is started by root at boot time. In some point at 
startup, it chroot to an application-specific directory and gives up 
privileges. If it would dump core, it had to be on that directory, which 
has somewhat restrict permissions, so that I (and only I) can get it 
later for further analysis. By means of chroot + prctl it was possible 
to achieve this, but it would be simpler with this approach of yours, 
which could also help in debugging in case something else crashes. I'm 
inclined to adopting it, even though the daemon will ship to run on 
third-parties machines, and I'll have no much control of what kernel is 
running there (probably distribution default ones). I'll see what is 
possible here... Thank you!

Alexandre