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Ivanov" To: Theodore Ts'o Cc: Dominik Brodowski , Ard Biesheuvel , linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] random: fix crash on multiple early calls to add_bootloader_randomness() Message-ID: <20211124123228.tftk2x3uxoxetfhx@suse> References: <20211012082708.121931-1-iivanov@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-efi@vger.kernel.org On 11-05 07:04, Dominik Brodowski wrote: > Date: Fri, 5 Nov 2021 07:04:36 +0100 > From: Dominik Brodowski > To: tytso@mit.edu > Cc: "Ivan T. Ivanov" , Ard Biesheuvel , > linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org > Subject: [PATCH v3] random: fix crash on multiple early calls to > add_bootloader_randomness() > Message-ID: Tags: all dt linux me watch > Hi, > If add_bootloader_randomness() or add_hwgenerator_randomness() is > called for the first time during early boot, crng_init equals 0. Then, > crng_fast_load() gets called -- which is safe to do even if the input > pool is not yet properly set up. > > 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. This is not only a hypothetical problem, as qemu > on arm64 may provide bootloader entropy via EFI and via devicetree. > > As crng_global_init_time is set to != 0 once the input pool is properly > set up, check (also) for this condition to determine which branch to take. > > Calls to crng_fast_load() do not modify the input pool; therefore, the > entropy_count for the input pool must not be modified at that early > stage. > > Reported-by: Ivan T. Ivanov > Fixes: 18b915ac6b0a ("efi/random: Treat EFI_RNG_PROTOCOL output as bootloader randomness") > Tested-by: Ivan T. Ivanov > Signed-off-by: Dominik Brodowski What is the plan for this fix? Regards, Ivan > --- > v2->v3: onle one unlikely (Ard Biesheuvel) > v1->v2: fix commit message; unmerge Reported-and-tested-by-tag (Ard Biesheuvel) > > drivers/char/random.c | 11 ++++++++--- > 1 file changed, 8 insertions(+), 3 deletions(-) > > diff --git a/drivers/char/random.c b/drivers/char/random.c > index 605969ed0f96..18fe804c1bf8 100644 > --- a/drivers/char/random.c > +++ b/drivers/char/random.c > @@ -1763,8 +1763,8 @@ 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 > @@ -2274,7 +2274,12 @@ void add_hwgenerator_randomness(const char *buffer, size_t count, > { > struct entropy_store *poolp = &input_pool; > > - if (unlikely(crng_init == 0)) { > + /* We cannot do much with the input pool until it is set up in > + * rand_initalize(); therefore just mix into the crng state. > + * As this does not affect the input pool, we cannot credit > + * entropy for this. > + */ > + if (unlikely(crng_init == 0 || crng_global_init_time == 0)) { > crng_fast_load(buffer, count); > return; > }