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