Code: static int encrypt(struct blkcipher_desc *desc,
		   struct scatterlist *dst, struct scatterlist *src,
		   unsigned int nbytes)
{
	struct blkcipher_walk walk;
	struct crypto_blkcipher *tfm = desc->tfm;
	struct salsa20_ctx *ctx = crypto_blkcipher_ctx(tfm);
	int err;

	blkcipher_walk_init(&walk, dst, src, nbytes);
	err = blkcipher_walk_virt_block(desc, &walk, 64);
 
 	salsa20_ivsetup(ctx, walk.iv);
 
	if (likely(walk.nbytes == nbytes))
	{
		salsa20_encrypt_bytes(ctx, walk.dst.virt.addr,
				      walk.src.virt.addr, nbytes);
		return blkcipher_walk_done(desc, &walk, 0);
	}
 	while (walk.nbytes >= 64) {
 		salsa20_encrypt_bytes(ctx, walk.dst.virt.addr,
 				      walk.src.virt.addr,
				      walk.nbytes - (walk.nbytes % 64));
		err = blkcipher_walk_done(desc, &walk, walk.nbytes % 64);
	}

	if (walk.nbytes) {
		salsa20_encrypt_bytes(ctx, walk.dst.virt.addr,
				      walk.src.virt.addr, walk.nbytes);
		err = blkcipher_walk_done(desc, &walk, 0);
	}

	return err;
}

Vulnerability Type: DoS 
CWE ID: CWE-20
Summary: The Salsa20 encryption algorithm in the Linux kernel before 4.14.8 does not correctly handle zero-length inputs, allowing a local attacker able to use the AF_ALG-based skcipher interface (CONFIG_CRYPTO_USER_API_SKCIPHER) to cause a denial of service (uninitialized-memory free and kernel crash) or have unspecified other impact by executing a crafted sequence of system calls that use the blkcipher_walk API. Both the generic implementation (crypto/salsa20_generic.c) and x86 implementation (arch/x86/crypto/salsa20_glue.c) of Salsa20 were vulnerable.
Commit Message: crypto: salsa20 - fix blkcipher_walk API usage

When asked to encrypt or decrypt 0 bytes, both the generic and x86
implementations of Salsa20 crash in blkcipher_walk_done(), either when
doing 'kfree(walk->buffer)' or 'free_page((unsigned long)walk->page)',
because walk->buffer and walk->page have not been initialized.

The bug is that Salsa20 is calling blkcipher_walk_done() even when
nothing is in 'walk.nbytes'.  But blkcipher_walk_done() is only meant to
be called when a nonzero number of bytes have been provided.

The broken code is part of an optimization that tries to make only one
call to salsa20_encrypt_bytes() to process inputs that are not evenly
divisible by 64 bytes.  To fix the bug, just remove this "optimization"
and use the blkcipher_walk API the same way all the other users do.

Reproducer:

    #include <linux/if_alg.h>
    #include <sys/socket.h>
    #include <unistd.h>

    int main()
    {
            int algfd, reqfd;
            struct sockaddr_alg addr = {
                    .salg_type = "skcipher",
                    .salg_name = "salsa20",
            };
            char key[16] = { 0 };

            algfd = socket(AF_ALG, SOCK_SEQPACKET, 0);
            bind(algfd, (void *)&addr, sizeof(addr));
            reqfd = accept(algfd, 0, 0);
            setsockopt(algfd, SOL_ALG, ALG_SET_KEY, key, sizeof(key));
            read(reqfd, key, sizeof(key));
    }

Reported-by: syzbot <syzkaller@googlegroups.com>
Fixes: eb6f13eb9f81 ("[CRYPTO] salsa20_generic: Fix multi-page processing")
Cc: <stable@vger.kernel.org> # v2.6.25+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>