Re: [PATCH] crypto: stdrng - Mitigate FIPS RNG failures

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, Oct 26, 2026 at 11:33:32AM -0700, Eric Biggers wrote:
> If the kernel is booted with fips=1 and jitterentropy fails its health
> check, crypto_stdrng_get_bytes() can return an error and leave its
> output buffer uninitialized.
> 
> While the fallibility of the FIPS RNG is by design, leaving the output
> buffer uninitialized lacks practical defense-in-depth, as callers can
> forget to check the error code.  For example, the TPM2 driver didn't
> check the error code from ECDH private key generation
> (https://lore.kernel.org/r/20261025211119.138277-1-ebiggers@kernel.org/).
> Thus, fips=1 mode (ironically) caused a vulnerability.
> 
> Mitigate such bugs by filling the buffer with get_random_bytes_wait() in
> case of failure.  This helps in practice.  (For FIPS certifiability it
> makes no difference, as no "approved" entropy is claimed on failure.)
> 
> Signed-off-by: Eric Biggers <ebiggers@kernel.org>

Agree with the "practical DoD" part.

> ---
> 
> This applies on top of
> https://lore.kernel.org/linux-crypto/20261018204618.104876-1-ebiggers@kernel.org/
> 
>  crypto/drbg.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/crypto/drbg.c b/crypto/drbg.c
> index faa39045bdc6b..f3fed6ded5b5f 100644
> --- a/crypto/drbg.c
> +++ b/crypto/drbg.c
> @@ -561,10 +561,20 @@ int __crypto_stdrng_get_bytes(void *buf, size_t len)
>  
>  	err = crypto_get_default_rng();
>  	if (err)
> -		return err;
> +		goto out;
>  
>  	err = crypto_drbg_get_bytes(crypto_default_rng, buf, len, NULL, 0);
>  	crypto_put_default_rng();
> +
> +out:
> +	/*
> +	 * On failure, fill the buffer using the normal Linux RNG rather than
> +	 * leave it uninitialized.  This provides practical defense-in-depth for
> +	 * callers that forget to check the error code.  (It makes no difference
> +	 * for FIPS certifications; no entropy is claimed on failure anyway.)
> +	 */
> +	if (err)
> +		get_random_bytes_wait(buf, len);
>  	return err;
>  }
>  EXPORT_SYMBOL_GPL(__crypto_stdrng_get_bytes);
> 
> base-commit: 666d66df25253070cb297cb920b0bcfcfe5eb871
> prerequisite-patch-id: 4c318ab271fa6443e9fe0ac47537515cc2dc6999
> prerequisite-patch-id: 5992995f4dea89d9332b0e33ca1248e3dd672bd8
> prerequisite-patch-id: c38f29a49c34d0fdfdc8466fd64c3d69fb4f0626
> prerequisite-patch-id: 424590a4282a04c94a095d4f059ac0d3a8af44b1
> prerequisite-patch-id: b5e762e41f7f6a68e70f6540ab07ff2a23ae6880
> prerequisite-patch-id: bc08a9f0be6e4f1e25c0e6ab0881f39df60aa632
> prerequisite-patch-id: 487bd5a23a597d190e2794e178266fcb6fc92849
> prerequisite-patch-id: 3c6f92ab77fee4702702f8760514a1bb55a409cb
> prerequisite-patch-id: 1536028517d2aa0bcbc4ec601c18cd8cf5f7d0ad
> prerequisite-patch-id: cb214555f3c44cf2a2e610adfbf60311a3647aa5
> prerequisite-patch-id: cc0998aae8d75e7b09562f8623c132bc21e77a2b
> prerequisite-patch-id: 351a1bcfe1ac87d42c5aa46b867f291aba49caa0
> prerequisite-patch-id: f71bbeee6d6644ca1b464e22f4df891bab45680f
> prerequisite-patch-id: 2f726ebd0df8d5a420cf4a870e0b6973248bf6be
> -- 
> 2.56.0
> 


Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

Br, Jarkko