Bug#587665: [Pkg-sysvinit-devel] Bug#587665: Safety of early boot init of /dev/random seed
On Thu, 15 Jul 2010, Matt Mackall wrote: Don't bother fiddling with the pool size. We don't, but local admins often do, probably in an attempt to better handle bursts of entropy drainage. So, we do want to properly support non-standard pool sizes in Debian if we can. Unless they're manually patching their kernel, they probably aren't succeeding. The pool resize ioctl was disabled ages ago. But there's really nothing to support here: even the largest polynomial in the source is only 2048 bits, or 256 bytes. Well, cat /proc/sys/kernel/random/poolsize 4096 And that is stock mainline 2.6.32.16 on amd64, AFAIK... -- One disk to rule them all, One disk to find them. One disk to bring them all and in the darkness grind them. In the Land of Redmond where the shadows lie. -- The Silicon Valley Tarot Henrique Holschuh -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org
Bug#587665: [Pkg-sysvinit-devel] Bug#587665: Safety of early boot init of /dev/random seed
On Mon, 05 Jul 2010, Matt Mackall wrote: Here are our questions: 1. How much data of unknown quality can we feed the random pool at boot, before it causes damage (i.e. what is the threshold where we violate the you are not goint to be any worse than you were before rule) ? There is no limit. The mixing operations are computationally reversible, which guarantees that no unknown degrees of freedom are clobbered when mixing known data. Good. So, whatever we do, we are never worse off than we were before we did it, at least by design. 2. How dangerous it is to feed the pool with stale seed data in the next boot (i.e. in a failure mode where we do not regenerate the seed file) ? Not at all. 3. What is the optimal size of the seed data based on the pool size ? 1:1. We shall try to keep it at 1:1, then. 4. How dangerous it is to have functions that need randomness (like encripted network and partitions, possibly encripted swap with an ephemeral key), BEFORE initializing the random seed ? Depends on the platform. For instance, if you've got an unattended boot off a Live CD on a machine with a predictable clock, you may get duplicate outputs. I.e. it is somewhat dangerous, and we should try to avoid it by design, so we should try to init it as early as possible. Very well. 5. Is there an optimal size for the pool? Does the quality of the randomness one extracts from the pool increase or decrease with pool size? Don't bother fiddling with the pool size. We don't, but local admins often do, probably in an attempt to better handle bursts of entropy drainage. So, we do want to properly support non-standard pool sizes in Debian if we can. Basically, we need these answers to find our way regarding the following decisions: a) Is it better to seed the pool as early as possible and risk a larger time window for problem (2) above, instead of the current behaviour where we have a large time window where (4) above happens? Earlier is better. b) Is it worth the effort to base the seed file on the size of the pool, instead of just using a constant size? If a constant size is better, which size would that be? 512 bytes? 4096 bytes? 16384 bytes? 512 bytes is plenty. c) What is the maximum seed file size we can allow (maybe based on size of the pool) to try to avoid problem (1) above ? Anything larger than a sector is simply wasting CPU time, but is otherwise harmless. Well, a filesystem block is usually 1024 bytes, and a sector is 4096 bytes nowadays... :-) -- One disk to rule them all, One disk to find them. One disk to bring them all and in the darkness grind them. In the Land of Redmond where the shadows lie. -- The Silicon Valley Tarot Henrique Holschuh -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org
Bug#587665: [Pkg-sysvinit-devel] Bug#587665: Safety of early boot init of /dev/random seed
On Thu, 2010-07-15 at 20:33 -0300, Henrique de Moraes Holschuh wrote: On Mon, 05 Jul 2010, Matt Mackall wrote: Here are our questions: 1. How much data of unknown quality can we feed the random pool at boot, before it causes damage (i.e. what is the threshold where we violate the you are not goint to be any worse than you were before rule) ? There is no limit. The mixing operations are computationally reversible, which guarantees that no unknown degrees of freedom are clobbered when mixing known data. Good. So, whatever we do, we are never worse off than we were before we did it, at least by design. 2. How dangerous it is to feed the pool with stale seed data in the next boot (i.e. in a failure mode where we do not regenerate the seed file) ? Not at all. 3. What is the optimal size of the seed data based on the pool size ? 1:1. We shall try to keep it at 1:1, then. 4. How dangerous it is to have functions that need randomness (like encripted network and partitions, possibly encripted swap with an ephemeral key), BEFORE initializing the random seed ? Depends on the platform. For instance, if you've got an unattended boot off a Live CD on a machine with a predictable clock, you may get duplicate outputs. I.e. it is somewhat dangerous, and we should try to avoid it by design, so we should try to init it as early as possible. Very well. 5. Is there an optimal size for the pool? Does the quality of the randomness one extracts from the pool increase or decrease with pool size? Don't bother fiddling with the pool size. We don't, but local admins often do, probably in an attempt to better handle bursts of entropy drainage. So, we do want to properly support non-standard pool sizes in Debian if we can. Unless they're manually patching their kernel, they probably aren't succeeding. The pool resize ioctl was disabled ages ago. But there's really nothing to support here: even the largest polynomial in the source is only 2048 bits, or 256 bytes. Basically, we need these answers to find our way regarding the following decisions: a) Is it better to seed the pool as early as possible and risk a larger time window for problem (2) above, instead of the current behaviour where we have a large time window where (4) above happens? Earlier is better. b) Is it worth the effort to base the seed file on the size of the pool, instead of just using a constant size? If a constant size is better, which size would that be? 512 bytes? 4096 bytes? 16384 bytes? 512 bytes is plenty. c) What is the maximum seed file size we can allow (maybe based on size of the pool) to try to avoid problem (1) above ? Anything larger than a sector is simply wasting CPU time, but is otherwise harmless. Well, a filesystem block is usually 1024 bytes, and a sector is 4096 bytes nowadays... :-) -- Mathematics is the supreme nostalgia of our time. -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org
Bug#587665: [Pkg-sysvinit-devel] Bug#587665: Safety of early boot init of /dev/random seed
On Sat, 2010-07-03 at 13:08 -0300, Henrique de Moraes Holschuh wrote: (adding Petter Reinholdtsen to CC, stupid MUA...) On Sat, 03 Jul 2010, Henrique de Moraes Holschuh wrote: Hello, We are trying to enhance the Debian support for /dev/random seeding at early boot, and we need some expert help to do it right. Maybe some of you could give us some enlightenment on a few issues? Apologies in advance if I got the list of Linux kernel maintainers wrong. I have also copied LKML just in case. A bit of context: Debian tries to initialize /dev/random, by restoring the pool size and giving it some seed material (through a write to /dev/random) from saved state stored in /var. Since we store the seed data in /var, that means we only feed it to /dev/random relatively late in the boot sequence, after remote filesystems are available. Thus, anything that needs random numbers earlier than that point will run with whatever the kernel managed to harness without any sort of userspace help (which is probably not much, especially on platforms that clear RAM contents at reboot, or after a cold boot). We take care of regenerating the stored seed data as soon as we use it, in order to avoid as much as possible the possibility of reuse of seed data. This means that we write the old seed data to /dev/random, and immediately copy poolsize bytes from /dev/urandom to the seed data file. The seed data file is also regenerated prior to shutdown. We would like to clarify some points, so as to know how safe they are on face of certain error modes, and also whether some of what we do is necessary at all. Unfortunately, real answers require more intimate knowledge of the theory behind Linux' random pools than we have in the Debian initscripts team. Here are our questions: 1. How much data of unknown quality can we feed the random pool at boot, before it causes damage (i.e. what is the threshold where we violate the you are not goint to be any worse than you were before rule) ? There is no limit. The mixing operations are computationally reversible, which guarantees that no unknown degrees of freedom are clobbered when mixing known data. 2. How dangerous it is to feed the pool with stale seed data in the next boot (i.e. in a failure mode where we do not regenerate the seed file) ? Not at all. 3. What is the optimal size of the seed data based on the pool size ? 1:1. 4. How dangerous it is to have functions that need randomness (like encripted network and partitions, possibly encripted swap with an ephemeral key), BEFORE initializing the random seed ? Depends on the platform. For instance, if you've got an unattended boot off a Live CD on a machine with a predictable clock, you may get duplicate outputs. 5. Is there an optimal size for the pool? Does the quality of the randomness one extracts from the pool increase or decrease with pool size? Don't bother fiddling with the pool size. Basically, we need these answers to find our way regarding the following decisions: a) Is it better to seed the pool as early as possible and risk a larger time window for problem (2) above, instead of the current behaviour where we have a large time window where (4) above happens? Earlier is better. b) Is it worth the effort to base the seed file on the size of the pool, instead of just using a constant size? If a constant size is better, which size would that be? 512 bytes? 4096 bytes? 16384 bytes? 512 bytes is plenty. c) What is the maximum seed file size we can allow (maybe based on size of the pool) to try to avoid problem (1) above ? Anything larger than a sector is simply wasting CPU time, but is otherwise harmless. -- Mathematics is the supreme nostalgia of our time. -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org
Bug#587665: [Pkg-sysvinit-devel] Bug#587665: Safety of early boot init of /dev/random seed
(adding Petter Reinholdtsen to CC, stupid MUA...) On Sat, 03 Jul 2010, Henrique de Moraes Holschuh wrote: Hello, We are trying to enhance the Debian support for /dev/random seeding at early boot, and we need some expert help to do it right. Maybe some of you could give us some enlightenment on a few issues? Apologies in advance if I got the list of Linux kernel maintainers wrong. I have also copied LKML just in case. A bit of context: Debian tries to initialize /dev/random, by restoring the pool size and giving it some seed material (through a write to /dev/random) from saved state stored in /var. Since we store the seed data in /var, that means we only feed it to /dev/random relatively late in the boot sequence, after remote filesystems are available. Thus, anything that needs random numbers earlier than that point will run with whatever the kernel managed to harness without any sort of userspace help (which is probably not much, especially on platforms that clear RAM contents at reboot, or after a cold boot). We take care of regenerating the stored seed data as soon as we use it, in order to avoid as much as possible the possibility of reuse of seed data. This means that we write the old seed data to /dev/random, and immediately copy poolsize bytes from /dev/urandom to the seed data file. The seed data file is also regenerated prior to shutdown. We would like to clarify some points, so as to know how safe they are on face of certain error modes, and also whether some of what we do is necessary at all. Unfortunately, real answers require more intimate knowledge of the theory behind Linux' random pools than we have in the Debian initscripts team. Here are our questions: 1. How much data of unknown quality can we feed the random pool at boot, before it causes damage (i.e. what is the threshold where we violate the you are not goint to be any worse than you were before rule) ? 2. How dangerous it is to feed the pool with stale seed data in the next boot (i.e. in a failure mode where we do not regenerate the seed file) ? 3. What is the optimal size of the seed data based on the pool size ? 4. How dangerous it is to have functions that need randomness (like encripted network and partitions, possibly encripted swap with an ephemeral key), BEFORE initializing the random seed ? 5. Is there an optimal size for the pool? Does the quality of the randomness one extracts from the pool increase or decrease with pool size? Basically, we need these answers to find our way regarding the following decisions: a) Is it better to seed the pool as early as possible and risk a larger time window for problem (2) above, instead of the current behaviour where we have a large time window where (4) above happens? b) Is it worth the effort to base the seed file on the size of the pool, instead of just using a constant size? If a constant size is better, which size would that be? 512 bytes? 4096 bytes? 16384 bytes? c) What is the maximum seed file size we can allow (maybe based on size of the pool) to try to avoid problem (1) above ? We would be very grateful if you could help us find good answers to the questions above. -- One disk to rule them all, One disk to find them. One disk to bring them all and in the darkness grind them. In the Land of Redmond where the shadows lie. -- The Silicon Valley Tarot Henrique Holschuh -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org
