Re: [PATCH 3/3] alfred: properly initialize stack buffer before sending over unix socket

[Marek Lindner <mareklindner@neomailbox.ch>]

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On Saturday, 22 January 2022 09:03:12 CET Sven Eckelmann wrote:
> > The struct alfred_change_interface_v0 -> ifaces[IFNAMSIZ * 16] may be
> > written to but not fully initialized. The interface name may be much
> > shorter than the buffer holding it. Same applies struct
> > alfred_change_bat_iface_v0 -> bat_iface[IFNAMSIZ] but to a lesser extent
> > because the buffer is smaller.
>
> But strncpy writes n bytes (second parameter of n). So the name + n-
> strlen(name) 0-bytes. I thought I've corrected my earlier statement about
> strncpy but maybe I forgot it. So strlcpy will take care of always writing a
> single 0-byte at the end of a non-0-length buffer - but is not writing more
> than 1x 0-byte (so half of the buffer might be uninitialised). strncpy will
> fill the remaining bytes with 0 but might end up writing NO 0-bytes when
> the buffer was already full.

Thanks for highlighting this difference between strncpy() and strlcpy(). I see 
your point.


> But in your status patch, not all 16 name entries were written. So it leaks
> things from the stack and the receiver might parse things which are not
> actually written by the sender. And your code was not explicitly making sure
> that the buffer ends with a 0-byte.

The server status patch iterates over the list of interfaces and performs 
individual strncpy() calls, so that strncpy() can't initialize the entire 
buffer unless there are 16 interfaces to begin with. Ok!

Then drop this patch.

Kind regards,
Marek Lindner

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