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[linux] Avoid starting currticks() from zero every time
iPXE uses currticks() (along with the MAC address(es) of any network
devices) to seed the (non-cryptographic) random number generator.  The
current implementation of linux_currticks() ensures that the first
call to currticks() will always return zero; this results in identical
random number sequences on each run of iPXE on a given machine.  This
can cause odd-looking behaviour due to e.g. the reuse of local TCP
port numbers.

Fix by effectively rounding down the start time recorded by
linux_currticks() to the nearest whole second; this makes it unlikely
that consecutive runs of iPXE will use the exact same RNG sequence.

(Note that none of this affects the cryptographic RNG, which uses
/dev/random as a source of entropy.)

Signed-off-by: Michael Brown <mcb30@ipxe.org>
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mcb30 committed Mar 7, 2014
1 parent 859664e commit 08f9170
Showing 1 changed file with 7 additions and 1 deletion.
8 changes: 7 additions & 1 deletion src/interface/linux/linux_timer.c
Expand Up @@ -55,6 +55,12 @@ static unsigned long linux_ticks_per_sec(void)
* linux doesn't provide an easy access to jiffies so implement it by measuring
* the time since the first call to this function.
*
* Since this function is used to seed the (non-cryptographic) random
* number generator, we round the start time down to the nearest whole
* second. This minimises the chances of generating identical RNG
* sequences (and hence identical TCP port numbers, etc) on
* consecutive invocations of iPXE.
*
* @ret ticks Current time, in ticks
*/
static unsigned long linux_currticks(void)
Expand All @@ -71,7 +77,7 @@ static unsigned long linux_currticks(void)
linux_gettimeofday(&now, NULL);

unsigned long ticks = (now.tv_sec - start.tv_sec) * linux_ticks_per_sec();
ticks += (now.tv_usec - start.tv_usec) / (long)(1000000 / linux_ticks_per_sec());
ticks += now.tv_usec / (long)(1000000 / linux_ticks_per_sec());

return ticks;
}
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