Patch "crypto: ecdsa - Avoid signed integer overflow on signature decoding" has been added to the 6.6-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    crypto: ecdsa - Avoid signed integer overflow on signature decoding

to the 6.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     crypto-ecdsa-avoid-signed-integer-overflow-on-signat.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit bff91a5e31a75df1b810958068278ff04294b171
Author: Lukas Wunner <lukas@xxxxxxxxx>
Date:   Tue Sep 10 16:30:24 2024 +0200

    crypto: ecdsa - Avoid signed integer overflow on signature decoding
    
    [ 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@xxxxxxxxx>
    Reviewed-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
    Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
    Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c
index 28441311af36..da04df3c8ecf 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);