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 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 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. - 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);
