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crypto: ecdsa - Harden against integer overflows in DIV_ROUND_UP()

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Herbert notes that DIV_ROUND_UP() may overflow unnecessarily if an ecdsa
implementation's ->key_size() callback returns an unusually large value.
Herbert instead suggests (for a division by 8):

  X / 8 + !!(X & 7)

Based on this formula, introduce a generic DIV_ROUND_UP_POW2() macro and
use it in lieu of DIV_ROUND_UP() for ->key_size() return values.

Additionally, use the macro in ecc_digits_from_bytes(), whose "nbytes"
parameter is a ->key_size() return value in some instances, or a
user-specified ASN.1 length in the case of ecdsa_get_signature_rs().

Link: https://lore.kernel.org/r/Z3iElsILmoSu6FuC@gondor.apana.org.au/
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
This commit is contained in:
Lukas Wunner 2025-02-02 20:00:52 +01:00 • committed by Herbert Xu
parent f4144b6bb7
commit b16510a530
4 changed files with 16 additions and 4 deletions
Download patch file Download diff file Expand all files Collapse all files

2
crypto/ecc.c
View file

@ -71,7 +71,7 @@ EXPORT_SYMBOL(ecc_get_curve);
void ecc_digits_from_bytes(const u8 *in, unsigned int nbytes,
u64 *out, unsigned int ndigits)
{
int diff = ndigits - DIV_ROUND_UP(nbytes, sizeof(u64));
int diff = ndigits - DIV_ROUND_UP_POW2(nbytes, sizeof(u64));
unsigned int o = nbytes & 7;
__be64 msd = 0;

2
crypto/ecdsa-p1363.c
View file

@ -22,7 +22,7 @@ static int ecdsa_p1363_verify(struct crypto_sig *tfm,
{
struct ecdsa_p1363_ctx *ctx = crypto_sig_ctx(tfm);
unsigned int keylen = crypto_sig_keysize(ctx->child);
unsigned int ndigits = DIV_ROUND_UP(keylen, sizeof(u64));
unsigned int ndigits = DIV_ROUND_UP_POW2(keylen, sizeof(u64));
struct ecdsa_raw_sig sig;
if (slen != 2 * keylen)

4
crypto/ecdsa-x962.c
View file

@ -81,8 +81,8 @@ static int ecdsa_x962_verify(struct crypto_sig *tfm,
struct ecdsa_x962_signature_ctx sig_ctx;
int err;
sig_ctx.ndigits = DIV_ROUND_UP(crypto_sig_keysize(ctx->child),
sizeof(u64));
sig_ctx.ndigits = DIV_ROUND_UP_POW2(crypto_sig_keysize(ctx->child),
sizeof(u64));
err = asn1_ber_decoder(&ecdsasignature_decoder, &sig_ctx, src, slen);
if (err < 0)

12
include/linux/math.h
View file

@ -34,6 +34,18 @@
*/
#define round_down(x, y) ((x) & ~__round_mask(x, y))
/**
* DIV_ROUND_UP_POW2 - divide and round up
* @n: numerator
* @d: denominator (must be a power of 2)
*
* Divides @n by @d and rounds up to next multiple of @d (which must be a power
* of 2). Avoids integer overflows that may occur with __KERNEL_DIV_ROUND_UP().
* Performance is roughly equivalent to __KERNEL_DIV_ROUND_UP().
*/
#define DIV_ROUND_UP_POW2(n, d) \
((n) / (d) + !!((n) & ((d) - 1)))
#define DIV_ROUND_UP __KERNEL_DIV_ROUND_UP
#define DIV_ROUND_DOWN_ULL(ll, d) \