Re: [PATCH stable v4.9.y] gcc-plugins: latent_entropy: use /dev/urandom

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Apr 18, 2022 at 11:42:41AM +0200, Jason A. Donenfeld wrote:
> commit c40160f2998c897231f8454bf797558d30a20375 upstream.
> 
> While the latent entropy plugin mostly doesn't derive entropy from
> get_random_const() for measuring the call graph, when __latent_entropy is
> applied to a constant, then it's initialized statically to output from
> get_random_const(). In that case, this data is derived from a 64-bit
> seed, which means a buffer of 512 bits doesn't really have that amount
> of compile-time entropy.
> 
> This patch fixes that shortcoming by just buffering chunks of
> /dev/urandom output and doling it out as requested.
> 
> At the same time, it's important that we don't break the use of
> -frandom-seed, for people who want the runtime benefits of the latent
> entropy plugin, while still having compile-time determinism. In that
> case, we detect whether gcc's set_random_seed() has been called by
> making a call to get_random_seed(noinit=true) in the plugin init
> function, which is called after set_random_seed() is called but before
> anything that calls get_random_seed(noinit=false), and seeing if it's
> zero or not. If it's not zero, we're in deterministic mode, and so we
> just generate numbers with a basic xorshift prng.
> 
> Note that we don't detect if -frandom-seed is being used using the
> documented local_tick variable, because it's assigned via:
>    local_tick = (unsigned) tv.tv_sec * 1000 + tv.tv_usec / 1000;
> which may well overflow and become -1 on its own, and so isn't
> reliable: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105171
> 
> [kees: The 256 byte rnd_buf size was chosen based on average (250),
>  median (64), and std deviation (575) bytes of used entropy for a
>  defconfig x86_64 build]
> 
> Fixes: 38addce8b600 ("gcc-plugins: Add latent_entropy plugin")
> Cc: stable@vger.kernel.org
> Cc: PaX Team <pageexec@freemail.hu>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Link: https://lore.kernel.org/r/20220405222815.21155-1-Jason@zx2c4.com
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  scripts/gcc-plugins/latent_entropy_plugin.c | 44 +++++++++++++--------
>  1 file changed, 27 insertions(+), 17 deletions(-)

Queued this one up now instead, thanks for the backport!

greg k-h