linux-kernel - [PATCH] random: use memmove instead of memcpy for remaining 32 bytes

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Message-Id: <20220413235649.97640-1-Jason@zx2c4.com>
Date:   Thu, 14 Apr 2022 01:56:49 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: [PATCH] random: use memmove instead of memcpy for remaining 32 bytes

In order to immediately overwrite the old key on the stack, before
servicing a userspace request for bytes, we use the remaining 32 bytes
of block 0 as the key. This means moving indices 8,9,a,b,c,d,e,f ->
4,5,6,7,8,9,a,b. Since 4 < 8, for the kernel implementations of
memcpy(), this doesn't actually appear to be a problem in practice. But
relying on that characteristic seems a bit brittle. So let's change that
to a proper memmove(), which is the by-the-books way of handling
overlapping memory copies.

Cc: Dominik Brodowski <linux@...inikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 drivers/char/random.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 6b01b2be9dd4..3a293f919af9 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -333,7 +333,7 @@ static void crng_fast_key_erasure(u8 key[CHACHA_KEY_SIZE],
 	chacha20_block(chacha_state, first_block);

 	memcpy(key, first_block, CHACHA_KEY_SIZE);
-	memcpy(random_data, first_block + CHACHA_KEY_SIZE, random_data_len);
+	memmove(random_data, first_block + CHACHA_KEY_SIZE, random_data_len);
 	memzero_explicit(first_block, sizeof(first_block));
 }

-- 
2.35.1