From 3b0565c703503f832d6cd7ba805aafa3b330cb9d Mon Sep 17 00:00:00 2001 From: Lukas Wunner Date: Tue, 10 Sep 2024 16:30:24 +0200 Subject: crypto: ecdsa - Avoid signed integer overflow on signature decoding When extracting a signature component r or s from an ASN.1-encoded integer, ecdsa_get_signature_rs() subtracts the expected length "bufsize" from the ASN.1 length "vlen" (both of unsigned type size_t) and stores the result in "diff" (of signed type ssize_t). This results in a signed integer overflow if vlen > SSIZE_MAX + bufsize. The kernel is compiled with -fno-strict-overflow, which implies -fwrapv, meaning signed integer overflow is not undefined behavior. And the function does check for overflow: if (-diff >= bufsize) return -EINVAL; So the code is fine in principle but not very obvious. In the future it might trigger a false-positive with CONFIG_UBSAN_SIGNED_WRAP=y. Avoid by comparing the two unsigned variables directly and erroring out if "vlen" is too large. Signed-off-by: Lukas Wunner Reviewed-by: Stefan Berger Reviewed-by: Jonathan Cameron Signed-off-by: Herbert Xu --- crypto/ecdsa.c | 19 +++++++------------ 1 file changed, 7 insertions(+), 12 deletions(-) (limited to 'crypto') diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c index 3b9873f56b0a..4a0ca93c99ea 100644 --- a/crypto/ecdsa.c +++ b/crypto/ecdsa.c @@ -35,29 +35,24 @@ static int ecdsa_get_signature_rs(u64 *dest, size_t hdrlen, unsigned char tag, const void *value, size_t vlen, unsigned int ndigits) { size_t bufsize = ndigits * sizeof(u64); - ssize_t diff = vlen - bufsize; const char *d = value; - if (!value || !vlen) + if (!value || !vlen || vlen > bufsize + 1) return -EINVAL; - /* diff = 0: 'value' has exacly the right size - * diff > 0: 'value' has too many bytes; one leading zero is allowed that - * makes the value a positive integer; error on more - * diff < 0: 'value' is missing leading zeros + /* + * vlen may be 1 byte larger than bufsize due to a leading zero byte + * (necessary if the most significant bit of the integer is set). */ - if (diff > 0) { + if (vlen > bufsize) { /* skip over leading zeros that make 'value' a positive int */ if (*d == 0) { vlen -= 1; - diff--; d++; - } - if (diff) + } else { return -EINVAL; + } } - if (-diff >= bufsize) - return -EINVAL; ecc_digits_from_bytes(d, vlen, dest, ndigits); -- cgit v1.2.3