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SubjectRe: [PATCH] (0/4) Entropy accounting fixes
On Sun, Aug 18, 2002 at 02:31:02AM -0400, Robert Love wrote:
> On Sun, 2002-08-18 at 00:01, Linus Torvalds wrote:
>
> > So I think that if we just made the code be much less trusting (say,
> > consider the TSC information per interrupt to give only a single bit of
> > entropy, for example), and coupled that with making network devices always
> > be considered sources of entropy, we'd have a reasonable balance.
>
> I think that sounds good.
>
> I have a patch which I can send - it needs to be rediffed I suspect -
> that has each network device feed the entropy pool. (Actually, it
> creates a new flag, SA_NET_RANDOM, that defines to SA_SAMPLE_RANDOM or 0
> depending on a configure setting. If you want it unconditional, that is
> just as easy though).

I think I have a compromise that'll make everyone happy here. I've
added a tunable called trust_pct in /proc/sys/kernel/random. Set it to
100% and untrusted sources will always add a bit of entropy. Defaults
to zero, which should be fine for everyone who's got a head.

Then we can add SA_SAMPLE_RANDOM for network devices unconditionally.

Untested, applies on top of my previous patches:

diff -ur a/drivers/char/random.c b/drivers/char/random.c
--- a/drivers/char/random.c 2002-08-17 20:54:02.000000000 -0500
+++ b/drivers/char/random.c 2002-08-18 01:38:58.000000000 -0500
@@ -724,6 +724,7 @@
*/
static int benford[16]={0,0,0,1,2,3,4,5,5,6,7,7,8,9,9,10};
static int last_ctxt=0;
+static int trust_break=50, trust_pct=0, trust_min=0, trust_max=100;

void add_timing_entropy(void *src, unsigned datum)
{
@@ -764,6 +765,18 @@
delta>>=es->shift;
bits=benford[int_log2_16bits(delta & 0xffff)];
}
+
+ /* Throw in an untrusted bit as entropy trust_pct% of the time */
+ if(trust_pct && !bits)
+ {
+ trust_break+=trust_pct;
+ if(trust_break>100)
+ {
+ bits=1;
+ trust_break-=100;
+ }
+ }
+
batch_entropy_store(datum^time, bits);
}

@@ -1779,6 +1792,10 @@
{RANDOM_UUID, "uuid",
NULL, 16, 0444, NULL,
&proc_do_uuid, &uuid_strategy},
+ {RANDOM_TRUST_PCT, "trust_pct",
+ &trust_pct, sizeof(int), 0644, NULL,
+ &proc_dointvec_minmax, &sysctl_intvec, 0,
+ &trust_min, &trust_max},
{0}
};

diff -ur a/include/linux/sysctl.h b/include/linux/sysctl.h
--- a/include/linux/sysctl.h 2002-08-17 00:30:00.000000000 -0500
+++ b/include/linux/sysctl.h 2002-08-18 01:37:54.000000000 -0500
@@ -182,7 +182,8 @@
RANDOM_READ_THRESH=3,
RANDOM_WRITE_THRESH=4,
RANDOM_BOOT_ID=5,
- RANDOM_UUID=6
+ RANDOM_UUID=6,
+ RANDOM_TRUST_PCT=7
};

/* /proc/sys/bus/isa */

--
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