From f02f0218be412cff1c844addf58e002071be298b Mon Sep 17 00:00:00 2001 From: Lukas Wunner Date: Sun, 2 Feb 2025 20:00:52 +0100 Subject: [PATCH] crypto: ecdsa - Harden against integer overflows in DIV_ROUND_UP() commit b16510a530d1e6ab9683f04f8fb34f2e0f538275 upstream. 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 Signed-off-by: Lukas Wunner Signed-off-by: Herbert Xu Signed-off-by: Greg Kroah-Hartman --- crypto/ecc.c | 2 +- include/linux/math.h | 12 ++++++++++++ 2 files changed, 13 insertions(+), 1 deletion(-) diff --git a/crypto/ecc.c b/crypto/ecc.c index 21504280aca2..27fcecf3a61f 100644 --- a/crypto/ecc.c +++ b/crypto/ecc.c @@ -69,7 +69,7 @@ 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; diff --git a/include/linux/math.h b/include/linux/math.h index dd4152711de7..ee754ec3dc92 100644 --- a/include/linux/math.h +++ b/include/linux/math.h @@ -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) \