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crypto: ecdsa - Avoid signed integer overflow on signature decoding

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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>
This commit is contained in:
Lukas Wunner 2024-09-10 16:30:24 +02:00 committed by Herbert Xu
parent 4df86c6ea5
commit 3b0565c703
1 changed files with 7 additions and 12 deletions
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19
crypto/ecdsa.c
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@ -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);