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Date: Mon, 7 Nov 2016 14:15:37 +0100
From: Richard Weinberger <richard@....at>
To: Eric Biggers <ebiggers@...gle.com>, linux-fsdevel@...r.kernel.org
Cc: linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
linux-crypto@...r.kernel.org, tytso@....edu, jaegeuk@...nel.org,
luto@...nel.org
Subject: Re: [PATCH 1/2] fscrypto: don't use on-stack buffer for filename
encryption
On 03.11.2016 23:03, Eric Biggers wrote:
> With the new (in 4.9) option to use a virtually-mapped stack
> (CONFIG_VMAP_STACK), stack buffers cannot be used as input/output for
> the scatterlist crypto API because they may not be directly mappable to
> struct page. For short filenames, fname_encrypt() was encrypting a
As Kent and Andy pointed out, they are but here are dragons.
The pages can be non-linear and on platforms with different cache architectures
extra flush operations may be needed.
> stack buffer holding the padded filename. Fix it by encrypting the
> filename in-place in the output buffer, thereby making the temporary
> buffer unnecessary.
>
> This bug could most easily be observed in a CONFIG_DEBUG_SG kernel
> because this allowed the BUG in sg_set_buf() to be triggered.
>
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---
> fs/crypto/fname.c | 53 +++++++++++++++++++++--------------------------------
> 1 file changed, 21 insertions(+), 32 deletions(-)
>
> diff --git a/fs/crypto/fname.c b/fs/crypto/fname.c
> index 9a28133..9b774f4 100644
> --- a/fs/crypto/fname.c
> +++ b/fs/crypto/fname.c
> @@ -39,65 +39,54 @@ static void fname_crypt_complete(struct crypto_async_request *req, int res)
> static int fname_encrypt(struct inode *inode,
> const struct qstr *iname, struct fscrypt_str *oname)
> {
> - u32 ciphertext_len;
> struct skcipher_request *req = NULL;
> DECLARE_FS_COMPLETION_RESULT(ecr);
> struct fscrypt_info *ci = inode->i_crypt_info;
> struct crypto_skcipher *tfm = ci->ci_ctfm;
> int res = 0;
> char iv[FS_CRYPTO_BLOCK_SIZE];
> - struct scatterlist src_sg, dst_sg;
> + struct scatterlist sg;
> int padding = 4 << (ci->ci_flags & FS_POLICY_FLAGS_PAD_MASK);
> - char *workbuf, buf[32], *alloc_buf = NULL;
> - unsigned lim;
> + unsigned int lim;
> + unsigned int cryptlen;
>
> lim = inode->i_sb->s_cop->max_namelen(inode);
> if (iname->len <= 0 || iname->len > lim)
> return -EIO;
>
> - ciphertext_len = max(iname->len, (u32)FS_CRYPTO_BLOCK_SIZE);
> - ciphertext_len = round_up(ciphertext_len, padding);
> - ciphertext_len = min(ciphertext_len, lim);
> + /*
> + * Copy the filename to the output buffer for encrypting in-place and
> + * pad it with the needed number of NUL bytes.
> + */
> + cryptlen = max_t(unsigned int, iname->len, FS_CRYPTO_BLOCK_SIZE);
> + cryptlen = round_up(cryptlen, padding);
> + cryptlen = min(cryptlen, lim);
> + memcpy(oname->name, iname->name, iname->len);
> + memset(oname->name + iname->len, 0, cryptlen - iname->len);
>
> - if (ciphertext_len <= sizeof(buf)) {
> - workbuf = buf;
> - } else {
> - alloc_buf = kmalloc(ciphertext_len, GFP_NOFS);
> - if (!alloc_buf)
> - return -ENOMEM;
> - workbuf = alloc_buf;
> - }
> + /* Initialize the IV */
> + memset(iv, 0, FS_CRYPTO_BLOCK_SIZE);
You can initialize it with iv = {0} at the beginning such that you don't
need the memset here.
> - /* Allocate request */
> + /* Set up the encryption request */
> req = skcipher_request_alloc(tfm, GFP_NOFS);
> if (!req) {
> printk_ratelimited(KERN_ERR
> - "%s: crypto_request_alloc() failed\n", __func__);
> - kfree(alloc_buf);
> + "%s: skcipher_request_alloc() failed\n", __func__);
> return -ENOMEM;
> }
> skcipher_request_set_callback(req,
> CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
> fname_crypt_complete, &ecr);
> + sg_init_one(&sg, oname->name, cryptlen);
> + skcipher_request_set_crypt(req, &sg, &sg, cryptlen, iv);
>
> - /* Copy the input */
> - memcpy(workbuf, iname->name, iname->len);
> - if (iname->len < ciphertext_len)
> - memset(workbuf + iname->len, 0, ciphertext_len - iname->len);
> -
> - /* Initialize IV */
> - memset(iv, 0, FS_CRYPTO_BLOCK_SIZE);
> -
> - /* Create encryption request */
> - sg_init_one(&src_sg, workbuf, ciphertext_len);
> - sg_init_one(&dst_sg, oname->name, ciphertext_len);
> - skcipher_request_set_crypt(req, &src_sg, &dst_sg, ciphertext_len, iv);
> + /* Do the encryption */
> res = crypto_skcipher_encrypt(req);
> if (res == -EINPROGRESS || res == -EBUSY) {
> + /* Request is being completed asynchronously; wait for it */
> wait_for_completion(&ecr.completion);
> res = ecr.res;
> }
> - kfree(alloc_buf);
> skcipher_request_free(req);
> if (res < 0) {
> printk_ratelimited(KERN_ERR
> @@ -105,7 +94,7 @@ static int fname_encrypt(struct inode *inode,
> return res;
> }
>
> - oname->len = ciphertext_len;
> + oname->len = cryptlen;
> return 0;
> }
Reviewed-by: Richard Weinberger <richard@....at>
Thanks,
//richard
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