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). > 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, Dominik --- 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) { +#ifdef CONFIG_RANDOM_TRUST_BOOTLOADER + crng_fast_load(buf, size); +#else + crng_slow_load(buf, size); +#endif /* CONFIG_RANDOM_TRUST_BOOTLOADER */ + } + + spin_lock_irqsave(&input_pool.lock, flags); + _mix_pool_bytes(&input_pool, buf, size); + _mix_pool_bytes(&input_pool, &time, sizeof(time)); + spin_unlock_irqrestore(&input_pool.lock, flags); + +#ifdef CONFIG_RANDOM_TRUST_BOOTLOADER + credit_entropy_bits(&input_pool, size * 8); +#endif } EXPORT_SYMBOL_GPL(add_bootloader_randomness);
