[PATCH AUTOSEL 6.12 1/6] crypto: ecdsa - Avoid signed integer overflow on signature decoding

Sasha Levin
2024-11-24T12:45:18Z

From: Lukas Wunner <lukas@wunner.de>

[ Upstream commit 3b0565c703503f832d6cd7ba805aafa3b330cb9d ]

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 <lukas@wunner.de>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
crypto/ecdsa.c | 19 +++++++------------
1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c
index d5a10959ec281..80ef16ae6a40b 100644
--- a/crypto/ecdsa.c
+++ b/crypto/ecdsa.c
@@ -36,29 +36,24 @@ 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);

--
2.43.0