| Message ID | cover.1673539719.git.ydroneaud@opteya.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | random: a simple vDSO mechanism for reseeding userspace CSPRNGs | expand |
Sorry Yann, but I'm not interested in this approach, and I don't think reviewing the details of it are a good allocation of time. I don't want to lock the kernel into having specific reseeding semantics that are a contract with userspace, which is what this approach does. Please just let me iterate on my original patchset for a little bit, without adding more junk to the already overly large conversation.
Hi 12 janvier 2023 à 18:07 "Jason A. Donenfeld" <Jason@zx2c4.com> a écrit: > Sorry Yann, but I'm not interested in this approach, and I don't think > reviewing the details of it are a good allocation of time. I don't > want to lock the kernel into having specific reseeding semantics that > are a contract with userspace, which is what this approach does. This patch adds a mean for the kernel to tell userspace: between the last time you call us with getrandom(timestamp,, GRND_TIMESTAMP), something happened that trigger an update to the opaque cookie given to getrandom(timestamp, GRND_TIMESTAMP). When such update happen, userspace is advised to discard buffered random data and retry. The meaning of the timestamp cookie is up to the kernel, and can be changed anytime. Userspace is not expected to read the content of this blob. Userspace only acts on the length returned by getrandom(,, GRND_TIMESTAMP): -1 : not supported 0 : cookie not updated, no need to discard buffered data >0 : cookie updated, userspace should discard buffered data For the cookie, I've used a single u64, but two u64 could be a better start, providing room for implementing improved behavior in future kernel versions. > Please just let me iterate on my original patchset for a little bit, > without adding more junk to the already overly large conversation. I like the simplicity of my so called "junk". It's streamlined, doesn't require a new syscall, doesn't require a new copy of ChaCha20 code. I'm sorry it doesn't fit your expectations. Regards.
On 1/12/23 11:55, Yann Droneaud wrote: > Hi > > 12 janvier 2023 à 18:07 "Jason A. Donenfeld" <Jason@zx2c4.com> a écrit: > >> Sorry Yann, but I'm not interested in this approach, and I don't think >> reviewing the details of it are a good allocation of time. I don't >> want to lock the kernel into having specific reseeding semantics that >> are a contract with userspace, which is what this approach does. > > This patch adds a mean for the kernel to tell userspace: between the > last time you call us with getrandom(timestamp,, GRND_TIMESTAMP), > something happened that trigger an update to the opaque cookie given > to getrandom(timestamp, GRND_TIMESTAMP). When such update happen, > userspace is advised to discard buffered random data and retry. > > The meaning of the timestamp cookie is up to the kernel, and can be > changed anytime. Userspace is not expected to read the content of this > blob. Userspace only acts on the length returned by getrandom(,, GRND_TIMESTAMP): > -1 : not supported > 0 : cookie not updated, no need to discard buffered data > >0 : cookie updated, userspace should discard buffered data > > For the cookie, I've used a single u64, but two u64 could be a better start, > providing room for implementing improved behavior in future kernel versions. > >> Please just let me iterate on my original patchset for a little bit, >> without adding more junk to the already overly large conversation. > > I like the simplicity of my so called "junk". It's streamlined, doesn't > require a new syscall, doesn't require a new copy of ChaCha20 code. > > I'm sorry it doesn't fit your expectations. > Why would anything more than a 64-bit counter be ever necessary? It only needs to be incremented. Let user space manage keeping track of the cookie matching its own buffers. You do NOT want this to be stateful, because that's just begging for multiple libraries to step on each other. Export the cookie from the vdso and volià, a very cheap check around any user space randomness buffer will work: static clone_cookie_t last_cookie; clone_cookie_t this_cookie; this_cookie = get_clone_cookie(); do { while (this_cookie != last_cookie) { last_cookie = this_cookie; reinit_randomness(); this_cookie = get_clone_cookie(); } extract_randomness_from_buffer(); this_cookie = get_clone_cookie(); } while (this_cookie != last_cookie); last_cookie = this_cookie; -hpa
> On Jan 13, 2023, at 7:16 PM, H. Peter Anvin <hpa@zytor.com> wrote: > > On 1/12/23 11:55, Yann Droneaud wrote: >> Hi >> 12 janvier 2023 à 18:07 "Jason A. Donenfeld" <Jason@zx2c4.com> a écrit: >> >>> Sorry Yann, but I'm not interested in this approach, and I don't think >>> reviewing the details of it are a good allocation of time. I don't >>> want to lock the kernel into having specific reseeding semantics that >>> are a contract with userspace, which is what this approach does. >> This patch adds a mean for the kernel to tell userspace: between the >> last time you call us with getrandom(timestamp,, GRND_TIMESTAMP), >> something happened that trigger an update to the opaque cookie given >> to getrandom(timestamp, GRND_TIMESTAMP). When such update happen, >> userspace is advised to discard buffered random data and retry. >> The meaning of the timestamp cookie is up to the kernel, and can be >> changed anytime. Userspace is not expected to read the content of this >> blob. Userspace only acts on the length returned by getrandom(,, GRND_TIMESTAMP): >> -1 : not supported >> 0 : cookie not updated, no need to discard buffered data >> >0 : cookie updated, userspace should discard buffered data >> For the cookie, I've used a single u64, but two u64 could be a better start, >> providing room for implementing improved behavior in future kernel versions. >>> Please just let me iterate on my original patchset for a little bit, >>> without adding more junk to the already overly large conversation. >> I like the simplicity of my so called "junk". It's streamlined, doesn't >> require a new syscall, doesn't require a new copy of ChaCha20 code. >> I'm sorry it doesn't fit your expectations. > > Why would anything more than a 64-bit counter be ever necessary? It only needs to be incremented. This is completely broken with CRIU or, for that matter, with VM forking. > > Let user space manage keeping track of the cookie matching its own buffers. You do NOT want this to be stateful, because that's just begging for multiple libraries to step on each other. > > Export the cookie from the vdso and volià, a very cheap check around any user space randomness buffer will work: > > static clone_cookie_t last_cookie; > clone_cookie_t this_cookie; > > this_cookie = get_clone_cookie(); > do { > while (this_cookie != last_cookie) { > last_cookie = this_cookie; > reinit_randomness(); > this_cookie = get_clone_cookie(); > } > > extract_randomness_from_buffer(); > this_cookie = get_clone_cookie(); > } while (this_cookie != last_cookie); > > last_cookie = this_cookie; > > -hpa
On January 16, 2023 11:49:42 AM PST, Andy Lutomirski <luto@amacapital.net> wrote: > > >> On Jan 13, 2023, at 7:16 PM, H. Peter Anvin <hpa@zytor.com> wrote: >> >> On 1/12/23 11:55, Yann Droneaud wrote: >>> Hi >>> 12 janvier 2023 à 18:07 "Jason A. Donenfeld" <Jason@zx2c4.com> a écrit: >>> >>>> Sorry Yann, but I'm not interested in this approach, and I don't think >>>> reviewing the details of it are a good allocation of time. I don't >>>> want to lock the kernel into having specific reseeding semantics that >>>> are a contract with userspace, which is what this approach does. >>> This patch adds a mean for the kernel to tell userspace: between the >>> last time you call us with getrandom(timestamp,, GRND_TIMESTAMP), >>> something happened that trigger an update to the opaque cookie given >>> to getrandom(timestamp, GRND_TIMESTAMP). When such update happen, >>> userspace is advised to discard buffered random data and retry. >>> The meaning of the timestamp cookie is up to the kernel, and can be >>> changed anytime. Userspace is not expected to read the content of this >>> blob. Userspace only acts on the length returned by getrandom(,, GRND_TIMESTAMP): >>> -1 : not supported >>> 0 : cookie not updated, no need to discard buffered data >>> >0 : cookie updated, userspace should discard buffered data >>> For the cookie, I've used a single u64, but two u64 could be a better start, >>> providing room for implementing improved behavior in future kernel versions. >>>> Please just let me iterate on my original patchset for a little bit, >>>> without adding more junk to the already overly large conversation. >>> I like the simplicity of my so called "junk". It's streamlined, doesn't >>> require a new syscall, doesn't require a new copy of ChaCha20 code. >>> I'm sorry it doesn't fit your expectations. >> >> Why would anything more than a 64-bit counter be ever necessary? It only needs to be incremented. > >This is completely broken with CRIU or, for that matter, with VM forking. > >> >> Let user space manage keeping track of the cookie matching its own buffers. You do NOT want this to be stateful, because that's just begging for multiple libraries to step on each other. >> >> Export the cookie from the vdso and volià, a very cheap check around any user space randomness buffer will work: >> >> static clone_cookie_t last_cookie; >> clone_cookie_t this_cookie; >> >> this_cookie = get_clone_cookie(); >> do { >> while (this_cookie != last_cookie) { >> last_cookie = this_cookie; >> reinit_randomness(); >> this_cookie = get_clone_cookie(); >> } >> >> extract_randomness_from_buffer(); >> this_cookie = get_clone_cookie(); >> } while (this_cookie != last_cookie); >> >> last_cookie = this_cookie; >> >> -hpa > For those you would randomize the counter.
Hi, 16 janvier 2023 à 20:50 "Andy Lutomirski" <luto@amacapital.net> a écrit: > > On Jan 13, 2023, at 7:16 PM, H. Peter Anvin <hpa@zytor.com> wrote: > > On 1/12/23 11:55, Yann Droneaud wrote: > > > 12 janvier 2023 à 18:07 "Jason A. Donenfeld" <Jason@zx2c4.com> a écrit: > > > > > > > Sorry Yann, but I'm not interested in this approach, and I don't think > > reviewing the details of it are a good allocation of time. I don't > > want to lock the kernel into having specific reseeding semantics that > > are a contract with userspace, which is what this approach does. > > > > > > > > This patch adds a mean for the kernel to tell userspace: between the > > > last time you call us with getrandom(timestamp,, GRND_TIMESTAMP), > > > something happened that trigger an update to the opaque cookie given > > > to getrandom(timestamp, GRND_TIMESTAMP). When such update happen, > > > userspace is advised to discard buffered random data and retry. > > > The meaning of the timestamp cookie is up to the kernel, and can be > > > changed anytime. Userspace is not expected to read the content of this > > > blob. Userspace only acts on the length returned by getrandom(,, GRND_TIMESTAMP): > > > -1 : not supported > > > 0 : cookie not updated, no need to discard buffered data > > > >0 : cookie updated, userspace should discard buffered data > > > For the cookie, I've used a single u64, but two u64 could be a better start, > > > providing room for implementing improved behavior in future kernel versions. > > > > > > > Please just let me iterate on my original patchset for a little bit, > > without adding more junk to the already overly large conversation. > > > > > > > > I like the simplicity of my so called "junk". It's streamlined, doesn't > > > require a new syscall, doesn't require a new copy of ChaCha20 code. > > > I'm sorry it doesn't fit your expectations. > > > > > > > > > Why would anything more than a 64-bit counter be ever necessary? It only needs to be incremented. > > > > This is completely broken with CRIU or, for that matter, with VM forking. > Which raise the question of the support of CRIU with Jason's vDSO proposal. AFAIK CRIU handle vDSO[1] by interposing symbols so that, on restore, the process will call the interposed functions, which will resolve the new vDSO's functions. vgetrandom_alloc() would have been called before the checkpoint, allocating one opaque state of size x. After the restore, the vDSO's getrandom() would be given this opaque state, expecting it having size y. As the content of the opaque state should have been cleared per MADV_WIPEONFORK, there's nothing in the state that could help vDSO's getrandom() to achieve backward compatibility. I think backward compatibility can be achieved by adding an opaque state size argument to vDSO's getrandom(). What to think Jason ? [1] https://criu.org/Vdso Regards.
Hi, Here's my humble hack at improving kernel for a faster secure arc4random() userspace implementation, by allowing userspace to buffer getrandom() generated entropy, discarding it as the kernel's own CSPRNG is reseeded. It's largely built upon the vDSO work of Jason A. Donenfeld, as part of its latest patchset "[PATCH v14 0/7] implement getrandom() in vDSO" [1] but it's made simpler by making available only one of the missing tools for the userspace to properly buffer the output of getrandom(). Using MADV_WIPEONFORK and mlock(), userspace can reasonably offer forward secrecy*, until something like VM_DROPPABLE[2] is provided by the kernel, to allow for the buffer memory to never, ever be written to the disk before its used, being inherited accross fork(), and isn't limited by RLIMIT_MEMLOCK. * provided userspace can mlock() the memory and calls mlock() on buffer after fork, as memory locks are not inherited accross fork(). As it's a hack, it's far from perfect. The main drawback I see is the case where fresh entropy has to be discarded as the kernel's CSPRNG generation is updated as the result of calling getrandom() to generate the mentionned entropy. The workaround, is to limit the amount of fresh entropy fetched when a kernel's CSPRNG generation change is detected, and to increase the amount the data retrieved with getrandom() when generation doesn't change between calls. Performance wise, the improvements are here, as one can check with the test program provided: getrandom(,,GRND_TIMESTAMP) test getrandom() support GRND_TIMESTAMP found getrandom() in vDSO at 0x7ffc3efccc60 == direct syscall getrandom(), 16777216 u32, 2.866324020 s, 5.853 M u32/s, 170.846 ns/u32 == direct vDSO getrandom(), 16777216 u32, 2.883473280 s, 5.818 M u32/s, 171.868 ns/u32 == pooled syscall getrandom(), 16777216 u32, 1.152421219 s, 14.558 M u32/s, 68.690 ns/u32, (0 bytes discarded) == pooled vDSO getrandom(), 16777216 u32, 0.162477863 s, 103.258 M u32/s, 9.684 ns/u32, (0 bytes discarded) With the requirement to mlock() the memory page(s) used to buffer getrandom() output, I'm not sure userspace could afford to allocate 4KBytes per thread, before being hit by RLIMIT_MEMLOCK (or worse, OOM killer). Thus, some form of sharing between threads would be needed, which would require locking, reducing the performances shown above. Also I haven't studied the security impact of making the kernel base CSPRNG seed generation available to userspace. It can be made more opaque if needed. Regards. [1] https://lore.kernel.org/all/20230101162910.710293-1-Jason@zx2c4.com/ [2] https://lore.kernel.org/all/20230101162910.710293-3-Jason@zx2c4.com/ Jason A. Donenfeld (2): random: introduce generic vDSO getrandom(,, GRND_TIMESTAMP) fast path x86: vdso: Wire up getrandom() vDSO implementation. Yann Droneaud (2): random: introduce getrandom() GRND_TIMESTAMP testing: add a getrandom() GRND_TIMESTAMP vDSO demonstration/benchmark MAINTAINERS | 1 + arch/x86/Kconfig | 1 + arch/x86/entry/vdso/Makefile | 3 +- arch/x86/entry/vdso/vdso.lds.S | 2 + arch/x86/entry/vdso/vgetrandom.c | 17 + arch/x86/include/asm/vdso/getrandom.h | 42 +++ arch/x86/include/asm/vdso/vsyscall.h | 2 + arch/x86/include/asm/vvar.h | 16 + drivers/char/random.c | 52 ++- include/linux/random.h | 31 ++ include/uapi/linux/random.h | 2 + include/vdso/datapage.h | 9 + lib/vdso/Kconfig | 5 + lib/vdso/getrandom.c | 51 +++ tools/testing/crypto/getrandom/Makefile | 4 + .../testing/crypto/getrandom/test-getrandom.c | 307 ++++++++++++++++++ 16 files changed, 543 insertions(+), 2 deletions(-) create mode 100644 arch/x86/entry/vdso/vgetrandom.c create mode 100644 arch/x86/include/asm/vdso/getrandom.h create mode 100644 lib/vdso/getrandom.c create mode 100644 tools/testing/crypto/getrandom/Makefile create mode 100644 tools/testing/crypto/getrandom/test-getrandom.c