fscrypto: don't use on-stack buffer for filename encryption

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
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.

Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 21 insertions, 32 deletions

diff --git a/fs/crypto/fname.c b/fs/crypto/fname.c
index 9a28133ac3b8..9b774f4b50c8 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);
 
-        /* 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;
 }