Am Tue, Dec 07, 2021 at 03:09:21PM +0800 schrieb Hsin-Yi Wang: > On Tue, Dec 7, 2021 at 4:58 AM Dominik Brodowski > <linux@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > Am Mon, Dec 06, 2021 at 01:42:01PM +0800 schrieb Hsin-Yi Wang: > > > On Fri, Dec 3, 2021 at 3:59 PM Dominik Brodowski > > > <linux@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > Hi Jason, > > > > > > > > Am Thu, Dec 02, 2021 at 11:55:10AM -0500 schrieb Jason A. Donenfeld: > > > > > Thanks for the patch. One trivial nit and one question: > > > > > > > > Thanks for your review! > > > > > > > > > On Thu, Dec 2, 2021 at 6:35 AM Dominik Brodowski > > > > > <linux@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > + /* We cannot do much with the input pool until it is set up in > > > > > > + * rand_initalize(); therefore just mix into the crng state. > > > > > > > > > > I think you meant "rand_initialize()" here (missing 'i'). > > > > > > > > Indeed, sorry about that. > > > > > > > > > > If the added entropy suffices to increase crng_init to 1, future calls > > > > > > to add_bootloader_randomness() or add_hwgenerator_randomness() used to > > > > > > progress to credit_entropy_bits(). However, if the input pool is not yet > > > > > > properly set up, the cmpxchg call within that function can lead to an > > > > > > infinite recursion. > > > > > > > > > > I see what this patch does with crng_global_init_time, and that seems > > > > > probably sensible, but I didn't understand this part of the reasoning > > > > > in the commit message; I might just be a bit slow here. Where's the > > > > > recursion exactly? Or even an infinite loop? > > > > > > > > On arm64, it was actually a NULL pointer dereference reported by Ivan T. > > > > Ivanov; see > > > > > > > > https://lore.kernel.org/lkml/20211012082708.121931-1-iivanov@xxxxxxx/ > > > > > > > > Trying to reproduce this rather bluntly on x86/qemu by multiple manual calls > > > > to add_bootloader_randomness(), I mis-interpreted the symptoms to point to an > > > > infinite recursion. The real problem seems to be that crng_reseed() isn't > > > > ready to be called too early in the boot process, in particular before > > > > workqueues are ready (see the call to numa_crng_init()). > > > > > > > > However, there seem be additional issues with add_bootloader_randomness() > > > > not yet addressed (or worsened) by my patch: > > > > > > > > - If CONFIG_RANDOM_TRUST_BOOTLOADER is enabled and crng_init==0, > > > > add_hwgenerator_randomness() calls crng_fast_load() and returns > > > > immediately. If it is disabled and crng_init==0, > > > > add_device_randnomness() calls crng_slow_load() but still > > > > continues to call _mix_pool_bytes(). That means the seed is > > > > used more extensively if CONFIG_RANDOM_TRUST_BOOTLOADER is not > > > > set! > > > If called by the crng_slow_load(), it's mixed into the pool but we're > > > not trusting it. But in crng_fast_load() we're using it to init crng. > > > > > > > > > > > - If CONFIG_RANDOM_TRUST_BOOTLOADER is enabled and crng_init==0, > > > > the entropy is not credited -- same as if > > > > CONFIG_RANDOM_TRUST_BOOTLOADER is not set. Only subsequent calls > > > > > > In crng_fast_load(), the seed would be mixed to primary_crng.state[4], > > > > Actually, that is also the case for crng_slow_load() (see dest_buf there). > > > Right, but the difference is if we want to credit(trust) that for crng init. ... which is, unfortunately, not the only difference between slow and fast... > > > and then crng_init will be 1 if the added seed is enough. > > > rng-seed in dt (called in early_init_dt_scan_chosen()) also needs to > > > use this function to init crng. > > > > Indeed, crng_init should be set to 1 in that case. > > > > > With the patch, we're seeing > > > [ 0.000000] random: get_random_u64 called from > > > __kmem_cache_create+0x34/0x270 with crng_init=0 > > > > > > While before it should be > > > [ 0.000000] random: get_random_u64 called from > > > __kmem_cache_create+0x34/0x280 with crng_init=1 > > > > > > > to add_bootloader_randomness() would credit entropy, but that > > > > causes the issue NULL pointer dereference or the hang... > > > > > > > > - As crng_fast_load() returns early, that actually means that my > > > > patch causes the additional entropy submitted to > > > > add_hwgenerator_randomness() by subsequent calls to be completely > > > > lost. > > > Only when crng_init==0, if crng is initialized, it would continue with > > > credit_entropy_bits(). > > > > However, if workqueues are not up and running (yet), it will fail. > > > > New draft below! > > Thanks, the new draft now takes care of the crng init. > [ 0.000000] random: get_random_u64 called from > __kmem_cache_create+0x34/0x270 with crng_init=1 Thanks for testing! > > --- > > > > Currently, if CONFIG_RANDOM_TRUST_BOOTLOADER is enabled, mutliple calls > > to add_bootloader_randomness() are broken and can cause a NULL pointer > > dereference, as noted by Ivan T. Ivanov. This is not only a hypothetical > > problem, as qemu on arm64 may provide bootloader entropy via EFI and via > > devicetree. > > > > On the first call to add_hwgenerator_randomness(), crng_fast_load() is > > executed, and if the seed is long enough, crng_init will be set to 1. > > However, no entropy is currently credited for that, even though the > > name and description of CONFIG_RANDOM_TRUST_BOOTLOADER states otherwise. > > > > On subsequent calls to add_bootloader_randomness() and then to > > add_hwgenerator_randomness(), crng_fast_load() will be skipped. Instead, > > wait_event_interruptible() (which makes no sense for the init process) > > and then credit_entropy_bits() will be called. If the entropy count for > > that second seed is large enough, that proceeds to crng_reseed(). > > However, crng_reseed() may depend on workqueues being available, which > > is not the case early during boot. > > > > To fix these issues, explicitly call crng_fast_load() or crng_slow_load() > > depending on whether the bootloader is trusted -- only in the first > > instance, crng_init may progress to 1. Also, mix the seed into the > > input pool unconditionally, and credit the entropy for that iff > > CONFIG_RANDOM_TRUST_BOOTLOADER is set. However, avoid a call to > > crng_reseed() too early during boot. It is safe to be called after > > rand_initialize(), so use crng_global_init_time (which is set to != 0 > > in that function) to determine which branch to take. > > > > Reported-by: Ivan T. Ivanov <iivanov@xxxxxxx> > > Fixes: 18b915ac6b0a ("efi/random: Treat EFI_RNG_PROTOCOL output as bootloader randomness") > > Signed-off-by: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx> > > > > diff --git a/drivers/char/random.c b/drivers/char/random.c > > index 605969ed0f96..abe4571fd2c0 100644 > > --- a/drivers/char/random.c > > +++ b/drivers/char/random.c > > @@ -722,7 +722,8 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits) > > if (r == &input_pool) { > > int entropy_bits = entropy_count >> ENTROPY_SHIFT; > > > > - if (crng_init < 2 && entropy_bits >= 128) > > + if (crng_init < 2 && entropy_bits >= 128 && > > + crng_global_init_time > 0) > > crng_reseed(&primary_crng, r); > > } > > } > > @@ -1763,8 +1764,8 @@ static void __init init_std_data(struct entropy_store *r) > > } > > > > /* > > - * Note that setup_arch() may call add_device_randomness() > > - * long before we get here. This allows seeding of the pools > > + * add_device_randomness() or add_bootloader_randomness() may be > > + * called long before we get here. This allows seeding of the pools > > * with some platform dependent data very early in the boot > > * process. But it limits our options here. We must use > > * statically allocated structures that already have all > > @@ -2291,15 +2292,29 @@ void add_hwgenerator_randomness(const char *buffer, size_t count, > > EXPORT_SYMBOL_GPL(add_hwgenerator_randomness); > > > > /* Handle random seed passed by bootloader. > > - * If the seed is trustworthy, it would be regarded as hardware RNGs. Otherwise > > - * it would be regarded as device data. > > + * If the seed is trustworthy, its entropy will be credited. > > * The decision is controlled by CONFIG_RANDOM_TRUST_BOOTLOADER. > > */ > > void add_bootloader_randomness(const void *buf, unsigned int size) > > { > > - if (IS_ENABLED(CONFIG_RANDOM_TRUST_BOOTLOADER)) > > - add_hwgenerator_randomness(buf, size, size * 8); > > - else > > - add_device_randomness(buf, size); > > + unsigned long time = random_get_entropy() ^ jiffies; > > + unsigned long flags; > > + > > + if (!crng_ready() && size) { > size is checked here but not below? credit_entropy_bits() returns early if bits==0. Thanks, Dominik
