| Message ID | 20220211210757.612595-1-Jason@zx2c4.com |
|---|---|
| State | New |
| Headers | show |
| Series | [RFC,v0] random: block in /dev/urandom | expand |
On Fri, Feb 11, 2022 at 1:08 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote: > > Maybe. And this is why this is a request for grumbles patch: the Linus > Jitter Dance relies on random_get_entropy() returning a cycle counter > value. Yeah. I think this patch is fine for architectures that do have that cycle counter value. Considering that the jitter thing has been there for 2.5 years by now, and nobody has really complained about it (*), I think we can call that thing a success. And on those architectures where try_to_generate_entropy() works, removing the code that then does that GRND_INSECURE makes sense. We just don't have any such case any more. BUT. When try_to_generate_entropy() doesn't work, I think you now removed the possible fallback for user space to say "yeah, just give me best effort". And you might re-introduce a deadlock as a result. Those systems are arguably broken from a randomness standpoint - what the h*ll are you supposed to do if there's nothing generating entropy - but broken or not, I suspect they still exists. Those horrendous MIPS things were quite common in embedded networking (routers, access points - places that *should* care) Do I have a constructive suggestion for those broken platforms? No, I don't. That arguably is the reason for GRND_INSECURE existing, and the reason to keep it around. Long story short: I like your patch, but I worry that it would cause problems on broken platforms. And almost nobody tests those broken platforms: even people who build new kernels for those embedded networking things probably end up using said kernels with an existing user space setup - where people have some existing saved source of pseudo-entropy. So they might not ever even trigger the "first boot problem" that tends to be the worst case. I'd be willing to apply such a thing anyway - at some point "worry about broken platforms" ends up being too weak an excuse not to just apply it - but I'd like to hear more of a reason for this simplification. If it's just "slight cleanup", maybe we should just keep the stupid stuff around as a "doesn't hurt good platforms, might help broken ones". Linus (*) Honestly, I think all the complaints would have been from the theoretical posers that don't have any practical suggestions anyway
On Fri, 11 Feb 2022, Jason A. Donenfeld wrote: > + * Reading from /dev/random and /dev/urandom both the same effect as > + * calling getrandom(2) with flags=0. In earlier versions, however, > + * they each had vastly different semantics and should therefore be > + * avoided to prevent backwards compatibility issues. If the end result "should be avoided", then why bother? IOW, how does this improve the ABI? I know you said it's a "panacea" but I'm afraid that's not clear to me and the patch description doesn't explain it.
On Sat, 12 Feb 2022, Joshua Kinard wrote: > # grep -r "PRID_IMP_R6000" * > arch/mips/include/asm/cpu.h:70:#define PRID_IMP_R6000 0x0300 > /* Same as R3000A */ > arch/mips/include/asm/cpu.h:72:#define PRID_IMP_R6000A 0x0600 > arch/mips/include/asm/timex.h:94: else if (likely(imp != > PRID_IMP_R6000 && imp != PRID_IMP_R6000A)) > > I'd say it's better to remove the check and simplify the conditional to > eliminate this corner case. Maybe keep the #defines around for > documentation, but even that may not be necessary for CPUs that likely don't > exist anymore. IIRC Ralf used to have a working R6k machine, but I have no idea what has happened to it. No port of Linux has been made for that system though, that's for sure. Maciej
Hi Jason, On Mon, Feb 14, 2022 at 3:05 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote: > On Sun, Feb 13, 2022 at 12:06 AM Joshua Kinard <kumba@gentoo.org> wrote: > > The R6000/R6000A CPU only ever existed in systems in the late 1980's that > > were fairly large, and I don't think there is a complete, working unit out > > there that can actually boot up, let alone boot a Linux kernel. > > So from what you've written, it sounds like MIPS is actually not a problem here. > > So the only systems we're actually talking about without a good cycle > counter are non-Amiga m68k? If so, that'd be a pretty terrific > finding. It'd mean that this idea can move forward, and we only need > to worry about some m68k museum pieces with misconfigured > userspaces... I'm afraid you missed one important detail. You wrote: > On every platform, random_get_entropy() is connected to get_cycles(), > except for three: m68k, MIPS, and RISC-V. The default implementation in include/asm-generic/timex.h is: static inline cycles_t get_cycles(void) { return 0; } Several architectures do not implement get_cycles(), or implement it with a variant that's very similar or identical to the generic version. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
On Mo, 14.02.22 15:13, Jason A. Donenfeld (Jason@zx2c4.com) wrote: > Hi Lennart, > > On Mon, Feb 14, 2022 at 9:53 AM Lennart Poettering <mzxreary@0pointer.de> wrote: > > So, systemd uses (potentially half-initialized) /dev/urandom for > > seeding its hash tables. For that its kinda OK if the random values > > have low entropy initially, as we'll automatically reseed when too > > many hash collisions happen, and then use a newer (and thus hopefully > > better) seed, again acquired through /dev/urandom. i.e. if the seeds > > are initially not good enough to thwart hash collision attacks, once > > the hash table are actually attacked we'll replace the seeds with > > someting better. For that all we need is that the random pool > > eventually gets better, that's all. > > > > So for that usecase /dev/urandom behaving the way it so far does is > > kinda nice. > > Oh that's an interesting point. But that sounds to me like the problem > with this patch is not that it makes /dev/urandom block (its primary > purpose) but that it also removes GRND_INSECURE (a distraction). So > perhaps an improved patch would be something like the below, which > changes /dev/urandom for new kernels but doesn't remove GRND_INSECURE. > Then your hash tables could continue to use GRND_INSECURE and all would > be well. (And for kernels without getrandom(), they'd just fall back to > /dev/urandom like normal which would have old semantics, so works.) In fact, systemd already uses getrandom(GRND_INSECURE) for this, if it is supported, and falls back to /dev/urandom only if it is not. So as long as GRND_INSECURE remains available we are good. Lennart -- Lennart Poettering, Berlin
On Mon, 14 Feb 2022, Jason A. Donenfeld wrote: > > So the only systems we're actually talking about without a good cycle > counter are non-Amiga m68k? If so, that'd be a pretty terrific finding. > It'd mean that this idea can move forward, and we only need to worry > about some m68k museum pieces with misconfigured userspaces... > A processor cycle counter is helpful when mounting a timing attack but my museum pieces don't suffer from that problem. Also, they are and always were immune from spectre, meltdown etc. You misrepresent those secure hardware designs as being problematic, just because of some bad advice on some random blogs about RNG API usage. Do you have a phone that no longer gets updates from its vendor? Have you tried patching it? Your insecure museum pieces are the real problem, not my secure ones.
diff --git a/drivers/char/mem.c b/drivers/char/mem.c index cc296f0823bd..9f586025dbe6 100644 --- a/drivers/char/mem.c +++ b/drivers/char/mem.c @@ -707,7 +707,7 @@ static const struct memdev { [5] = { "zero", 0666, &zero_fops, FMODE_NOWAIT }, [7] = { "full", 0666, &full_fops, 0 }, [8] = { "random", 0666, &random_fops, 0 }, - [9] = { "urandom", 0666, &urandom_fops, 0 }, + [9] = { "urandom", 0666, &random_fops, 0 }, #ifdef CONFIG_PRINTK [11] = { "kmsg", 0644, &kmsg_fops, 0 }, #endif diff --git a/drivers/char/random.c b/drivers/char/random.c index c564f795f68c..868334ea0ce3 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -88,8 +88,6 @@ static LIST_HEAD(random_ready_list); /* Control how we warn userspace. */ static struct ratelimit_state unseeded_warning = RATELIMIT_STATE_INIT("warn_unseeded_randomness", HZ, 3); -static struct ratelimit_state urandom_warning = - RATELIMIT_STATE_INIT("warn_urandom_randomness", HZ, 3); static int ratelimit_disable __read_mostly; module_param_named(ratelimit_disable, ratelimit_disable, int, 0644); MODULE_PARM_DESC(ratelimit_disable, "Disable random ratelimit suppression"); @@ -321,11 +319,6 @@ static void crng_reseed(void) unseeded_warning.missed); unseeded_warning.missed = 0; } - if (urandom_warning.missed) { - pr_notice("%d urandom warning(s) missed due to ratelimiting\n", - urandom_warning.missed); - urandom_warning.missed = 0; - } } } @@ -978,10 +971,8 @@ int __init rand_initialize(void) pr_notice("crng init done (trusting CPU's manufacturer)\n"); } - if (ratelimit_disable) { - urandom_warning.interval = 0; + if (ratelimit_disable) unseeded_warning.interval = 0; - } return 0; } @@ -1363,20 +1354,17 @@ static void try_to_generate_entropy(void) * getrandom(2) is the primary modern interface into the RNG and should * be used in preference to anything else. * - * Reading from /dev/random has the same functionality as calling - * getrandom(2) with flags=0. In earlier versions, however, it had - * vastly different semantics and should therefore be avoided, to - * prevent backwards compatibility issues. - * - * Reading from /dev/urandom has the same functionality as calling - * getrandom(2) with flags=GRND_INSECURE. Because it does not block - * waiting for the RNG to be ready, it should not be used. + * Reading from /dev/random and /dev/urandom both the same effect as + * calling getrandom(2) with flags=0. In earlier versions, however, + * they each had vastly different semantics and should therefore be + * avoided to prevent backwards compatibility issues. * * Writing to either /dev/random or /dev/urandom adds entropy to * the input pool but does not credit it. * - * Polling on /dev/random indicates when the RNG is initialized, on - * the read side, and when it wants new entropy, on the write side. + * Polling on /dev/random or /dev/urandom indicates when the RNG + * is initialized, on the read side, and when it wants new entropy, + * on the write side. * * Both /dev/random and /dev/urandom have the same set of ioctls for * adding entropy, getting the entropy count, zeroing the count, and @@ -1387,6 +1375,8 @@ static void try_to_generate_entropy(void) SYSCALL_DEFINE3(getrandom, char __user *, buf, size_t, count, unsigned int, flags) { + int ret; + if (flags & ~(GRND_NONBLOCK | GRND_RANDOM | GRND_INSECURE)) return -EINVAL; @@ -1400,15 +1390,13 @@ SYSCALL_DEFINE3(getrandom, char __user *, buf, size_t, count, unsigned int, if (count > INT_MAX) count = INT_MAX; - if (!(flags & GRND_INSECURE) && !crng_ready()) { - int ret; + if ((flags & GRND_NONBLOCK) && !crng_ready()) + return -EAGAIN; + + ret = wait_for_random_bytes(); + if (ret != 0) + return ret; - if (flags & GRND_NONBLOCK) - return -EAGAIN; - ret = wait_for_random_bytes(); - if (unlikely(ret)) - return ret; - } return get_random_bytes_user(buf, count); } @@ -1461,21 +1449,6 @@ static ssize_t random_write(struct file *file, const char __user *buffer, return (ssize_t)count; } -static ssize_t urandom_read(struct file *file, char __user *buf, size_t nbytes, - loff_t *ppos) -{ - static int maxwarn = 10; - - if (!crng_ready() && maxwarn > 0) { - maxwarn--; - if (__ratelimit(&urandom_warning)) - pr_notice("%s: uninitialized urandom read (%zd bytes read)\n", - current->comm, nbytes); - } - - return get_random_bytes_user(buf, nbytes); -} - static ssize_t random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos) { @@ -1562,15 +1535,6 @@ const struct file_operations random_fops = { .llseek = noop_llseek, }; -const struct file_operations urandom_fops = { - .read = urandom_read, - .write = random_write, - .unlocked_ioctl = random_ioctl, - .compat_ioctl = compat_ptr_ioctl, - .fasync = random_fasync, - .llseek = noop_llseek, -}; - /******************************************************************** * diff --git a/include/uapi/linux/random.h b/include/uapi/linux/random.h index dcc1b3e6106f..9ec1703f45ad 100644 --- a/include/uapi/linux/random.h +++ b/include/uapi/linux/random.h @@ -49,7 +49,7 @@ struct rand_pool_info { * * GRND_NONBLOCK Don't block and return EAGAIN instead * GRND_RANDOM No effect - * GRND_INSECURE Return non-cryptographic random bytes + * GRND_INSECURE No effect */ #define GRND_NONBLOCK 0x0001 #define GRND_RANDOM 0x0002
This is very much an RFC patch, or maybe even an RFG -- request for grumbles. This topic has come up a million times, and usually doesn't go anywhere. This time I thought I'd bring it up with a slightly narrower focus. Before you read further, realize that I do not intend to merge this without there being an appropriate amount of consensus for it and discussion about it. Ever since Linus' 50ee7529ec45 ("random: try to actively add entropy rather than passively wait for it"), the RNG does a haveged-style jitter dance around the scheduler, in order to produce entropy (and credit it) for the case when we're stuck in wait_for_random_bytes(). How ever you feel about the Linus Jitter Dance is beside the point: it's been there for three years and usually gets the RNG initialized in a second or so. As a matter of fact, this is what happens currently when people use getrandom(2). So, given that the kernel has grown this mechanism for seeding itself from nothing, and that this procedure happens pretty fast, maybe there's no point any longer in having /dev/urandom give insecure bytes. In the past we didn't want the boot process to deadlock, which was understandable. But now, in the worst case, a second goes by, and the problem is resolved. It seems like maybe we're finally at a point when we can get rid of the infamous "urandom read hole". Maybe. And this is why this is a request for grumbles patch: the Linus Jitter Dance relies on random_get_entropy() returning a cycle counter value. The first lines of try_to_generate_entropy() are: stack.now = random_get_entropy(); /* Slow counter - or none. Don't even bother */ if (stack.now == random_get_entropy()) return; So it would appear that what seemed initially like a panacea does not in fact work everywhere. Where doesn't it work? On every platform, random_get_entropy() is connected to get_cycles(), except for three: m68k, MIPS, and RISC-V. On m68k, it looks like this: if (mach_random_get_entropy) return mach_random_get_entropy(); return 0; And mach_random_get_entropy seems to be set in amiga/config.c only. On MIPS, it looks like this: if (can_use_mips_counter(prid)) return read_c0_count(); else if (likely(imp != PRID_IMP_R6000 && imp != PRID_IMP_R6000A)) return read_c0_random(); else return 0; So it seems like we're okay except for R6000 and R6000A. Finally on RISC-V, it looks like this: if (unlikely(clint_time_val == NULL)) return 0; return get_cycles(); Where clint_time_val is eventually filled in later in boot with clint_timer_init_dt(). So I assume that's a case where it _eventually_ works, which is probably good enough for our purposes. I think what this adds up to is that this change would positively affect everybody, except for _possibly_ negatively affecting poorly configured non-Amiga m68k systems and the MIPS R6000 and R6000A. Does that analysis seem correct to folks reading, or did I miss something? Are there other cases where the cycle counter does exist but is simply too slow? Perhaps some computer historians can chime in here. If my general analysis is correct, are these ancient platforms really worth holding this back? I halfway expect to receive a few thrown tomatoes, an angry fist, and a "get off my lawn!", and if that's _all_ I hear, I'll take a hint and we can forget I ever proposed this. As mentioned, I do not intend to merge this unless there's broad consensus about it. But on the off chance that people feel differently, perhaps the Linus Jitter Dance is finally the solution to years of /dev/urandom kvetching. Cc: Paul Walmsley <paul.walmsley@sifive.com> Cc: Palmer Dabbelt <palmer@dabbelt.com> Cc: Albert Ou <aou@eecs.berkeley.edu> Cc: linux-riscv@lists.infradead.org Cc: Geert Uytterhoeven <geert@linux-m68k.org> Cc: linux-m68k@lists.linux-m68k.org Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de> Cc: linux-mips@vger.kernel.org Cc: Dominik Brodowski <linux@dominikbrodowski.net> Cc: Eric Biggers <ebiggers@google.com> Cc: Ard Biesheuvel <ardb@kernel.org> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Andy Lutomirski <luto@kernel.org> Cc: Kees Cook <keescook@chromium.org> Cc: Lennart Poettering <mzxreary@0pointer.de> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Theodore Ts'o <tytso@mit.edu> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> --- drivers/char/mem.c | 2 +- drivers/char/random.c | 68 +++++++++---------------------------- include/uapi/linux/random.h | 2 +- 3 files changed, 18 insertions(+), 54 deletions(-)