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Message-ID: <20130130191257.GH6181@port70.net>
Date: Wed, 30 Jan 2013 20:12:58 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] Add support for mkostemp, mkstemps and mkostemps
* Rich Felker <dalias@...ifal.cx> [2013-01-30 11:51:27 -0500]:
> current time. Better use of the stack address in generating the
> filenames could prevent knowing the set of output filenames for a
> range of times without knowing the stack address in the program being
> attacked. In fact, I'm a little bit worried that the current approach
> discloses too much information about the stack address to an attacker.
> If nothing else, I think some shuffling should be done so that the
> (typically more valuable) high bits of the stack address are matched
> with the low (least predictable) bits of the clock.
void __randname(char *p)
{
struct timespec ts;
unsigned long r;
int i;
clock_gettime(CLOCK_REALTIME, &ts);
r = ts.tv_nsec*65537 ^ (uintptr_t)&ts / 16 + (uintptr_t)p;
for (i=0; i<6; i++, r>>=5)
p[i] = 'A'+(r&15)+(r&16)*2;
}
this uses 30bits of r and mixes the random low bits of nsec
into the high bits
using ^ as i guess that way it's harder to do useful arithmetics
with known r values
> > more significant improvement can be done by larger
> > set of names and better entropy source
>
> Other implementations probably use 36 bits or slightly less (base64
> perhaps modified base64).
>
> I could see it being feasible to increase this slightly and maybe even
<= 36bits is probably ok
> > the entropy source is mostly problematic on embedded
> > systems with bad clock, but there is probably no
> > good source at all there
>
> Are you sure this is an issue? IMO it's the kernel's responsibility to
it was just a guess, iirc there are devices with
low resolution clock (lower than nanoseconds) which
can mean short period of the last few bits of nsec
but i dont know how this works
> give a good clock value however it can. IIRC even mips has a cpu
> counter or something that could be used to compensate for bad clock
> hardware, so it seems like a kernel failing if clock_gettime has bad
> resolution.
>
> Rich
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