[PATCH] crypto: ecdh - avoid buffer overflow in ecdh_set_secret()

[Ard Biesheuvel]

Pavel reports that commit 17858b140bf4 ("crypto: ecdh - avoid unaligned
accesses in ecdh_set_secret()") fixes one problem but introduces another:
the unconditional memcpy() introduced by that commit may overflow the
target buffer if the source data is invalid, which could be the result of
intentional tampering.

So check params.key_size explicitly against the size of the target buffer
before validating the key further.

Fixes: 17858b140bf4 ("crypto: ecdh - avoid unaligned accesses in 
ecdh_set_secret()")
Reported-by: Pavel Machek <pa...@denx.de>
Cc: <sta...@vger.kernel.org>
Signed-off-by: Ard Biesheuvel <a...@kernel.org>
---
 crypto/ecdh.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/crypto/ecdh.c b/crypto/ecdh.c
index d56b8603dec9..96f80c8f8e30 100644
--- a/crypto/ecdh.c
+++ b/crypto/ecdh.c
@@ -39,7 +39,8 @@ static int ecdh_set_secret(struct crypto_kpp *tfm, const void 
*buf,
        struct ecdh params;
        unsigned int ndigits;
 
-       if (crypto_ecdh_decode_key(buf, len, &params) < 0)
+       if (crypto_ecdh_decode_key(buf, len, &params) < 0 ||
+           params.key_size > sizeof(ctx->private_key))
                return -EINVAL;
 
        ndigits = ecdh_supported_curve(params.curve_id);
-- 
2.17.1