linux-kernel - Re: random: /dev/random often returns short reads

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Message-ID: <09f2ce2d-3c84-bb12-560c-3208691d2c55@redhat.com>
Date:   Tue, 17 Jan 2017 18:34:21 +0100
From:   Denys Vlasenko <dvlasenk@...hat.com>
To:     Theodore Ts'o <tytso@....edu>,
        Denys Vlasenko <vda.linux@...glemail.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...ux.intel.com>
Subject: Re: random: /dev/random often returns short reads



On 01/17/2017 06:15 PM, Theodore Ts'o wrote:
> On Tue, Jan 17, 2017 at 09:21:31AM +0100, Denys Vlasenko wrote:
>>> If someone wants to send me a patch, I'll happily take a look at it,
>>
>> Will something along these lines be accepted?
>
> The problem is that this won't work.  In the cases that we're talking
> about, the entropy counter in the secondary pool is not zero, but
> close to zero, we'll still have short reads.  And that's going to
> happen a fair amount of the time.
>
> Perhaps the best *hacky* solution would be to say, ok if the entropy
> count is less than some threshold, don't use the correct entropy
> calculation, but rather assume that all of the new bits won't land on
> top of existing entropy bits.

IOW, something like this:

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -653,6 +653,9 @@ static void credit_entropy_bits(struct
entropy_store *r, int nbits)
         if (nfrac < 0) {
                 /* Debit */
                 entropy_count += nfrac;
+       } else if (entropy_count < ((8 * 8) << ENTROPY_SHIFT)) {
+               /* Credit, and the pool is almost empty */
+               entropy_count += nfrac;
         } else {
                 /*
                  * Credit: we have to account for the possibility of
                  * overwriting already present entropy.  Even in the

Want the patch? If yes, what name of the constant you prefer? How about

/* Has less than 8 bytes */
#define ALMOST_EMPTY_POOL_frac   ((8 * 8) << ENTROPY_SHIFT)