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From: Adhemerval Zanella <adhemerval.zanella@linaro.org>
To: Paul Eggert <eggert@cs.ucla.edu>
Cc: libc-alpha@sourceware.org, bug-gnulib@gnu.org
Subject: Re: [PATCH 2/2] posix: Improve randomness on try_tempname_len
Date: Mon, 11 Jan 2021 09:29:46 -0300	[thread overview]
Message-ID: <b1688e82-8c8f-482c-7c9e-2e36e30b1e5a@linaro.org> (raw)
In-Reply-To: <c5a34863-2a7f-740d-1ac8-cafc4207ef76@cs.ucla.edu>



On 08/01/2021 23:20, Paul Eggert wrote:
> On 1/4/21 9:03 AM, Adhemerval Zanella wrote:
>> For __GT_NOCREATE (mktemp, tempnam, tmpnam) getrandom is also used
>> on first try, otherwise randomness is obtained using the clock plus
>> a linear congruential generator.
> 
> Why not use getrandom in the first try also for __GT_DIR (mkdtemp) and __GT_FILE (mkostemp, mkostemps, mkstemp, mkstemps, tmpfile)? That is what Gnulib tempname.c is doing now. This not only simplifies the code, it improves resistance to some (admittedly less-likely) attacks.

The idea is to always issue getrandom for __GT_DIR or __GT_FILE on first try,
as you suggested initially [1].  I followed your idea [2]:

  Here's an idea: use getrandom in the first try only for the __GT_NOCREATE case. 
  Although a bit more complicated, I expect this would address both your entropy 
  and my security concerns.

The current code should address Jakub concerns of using getrandom without 
GRND_NONBLOCK and not using on on first try (to avoid deplete the random 
entropy pool) and use getrandom only when a collision if found. I will merge
the code, close the bug, and we can work whether use getrandom only for
__GT_DIR/__GT_FILE is an improvement or not.

> 
>> Also for getrandom GRND_NONBLOCK is used to avoid blocking indefinitely
>> on some older kernels.
> 
> Thanks, I installed that part of the proposal into Gnulib by installing the attached. The idea is for tempname.c to be identical after we get the abovementioned issue worked out.

[1] https://sourceware.org/pipermail/libc-alpha/2020-September/117535.html
[2] https://sourceware.org/pipermail/libc-alpha/2020-September/117539.html


  reply	other threads:[~2021-01-11 12:29 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-04 17:03 [PATCH 1/2] posix: Sync tempname with gnulib [BZ #26648] Adhemerval Zanella
2021-01-04 17:03 ` [PATCH 2/2] posix: Improve randomness on try_tempname_len Adhemerval Zanella
2021-01-09  2:20   ` Paul Eggert
2021-01-11 12:29     ` Adhemerval Zanella [this message]
2021-01-12  1:06       ` Paul Eggert
2021-01-09  1:58 ` [PATCH 1/2] posix: Sync tempname with gnulib [BZ #26648] Paul Eggert
2021-01-11 12:30   ` Adhemerval Zanella

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