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], 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. 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(). > > - For add_bootloader_randomness(), it makes no sense at all to call > wait_event_interruptible(). > > Therefore, it might make more sense to > > - modify add_bootloader_randomness() to always call > add_device_randomness(), and if CONFIG_RANDOM_TRUST_BOOTLOADER is > enabled, to call credit_entropy_bits(). > > - update credit_entropy_bits() to not call credit_entropy_bits() > if crng_global_init_time==0, as workqueues (and possibly other > infrastructure) might not be available at that time. > > What do you think? Draft patch below. @Ivan: Could you re-test on your > system, please? > > 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, unconditionally call add_device_randomness() but not > add_hwgenerator_randomness() in add_bootloader_randomness(). This has the > additional advantage that the seed provided by the first call to > add_bootloader_randomness() is not only used by crng_{fast,slow}_load(), > but also mixed into the input pool. If CONFIG_RANDOM_TRUST_BOOTLOADER is > set, explicitly credit the entropy. 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> > > --- > v3->v4: complete rewrite > v2->v3: only one unlikely (Ard Biesheuvel) > v1->v2: fix commit message; unmerge Reported-and-tested-by-tag (Ard Biesheuvel) > > > diff --git a/drivers/char/random.c b/drivers/char/random.c > index 605969ed0f96..d8614b426dfb 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 +2292,13 @@ 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) > { > + add_device_randomness(buf, size); > if (IS_ENABLED(CONFIG_RANDOM_TRUST_BOOTLOADER)) > - add_hwgenerator_randomness(buf, size, size * 8); > - else > - add_device_randomness(buf, size); > + credit_entropy_bits(&input_pool, size * 8); > } > EXPORT_SYMBOL_GPL(add_bootloader_randomness);
