crypto: hash - Fix the pointer voodoo in unaligned ahash

Add documentation for the pointer voodoo that is happening in crypto/ahash.c
in ahash_op_unaligned(). This code is quite confusing, so add a beefy chunk
of documentation.

Moreover, make sure the mangled request is completely restored after finishing
this unaligned operation. This means restoring all of .result, .base.data
and .base.complete .

Also, remove the crypto_completion_t complete = ... line present in the
ahash_op_unaligned_done() function. This type actually declares a function
pointer, which is very confusing.

Finally, yet very important nonetheless, make sure the req->priv is free()'d
only after the original request is restored in ahash_op_unaligned_done().
The req->priv data must not be free()'d before that in ahash_op_unaligned_finish(),
since we would be accessing previously free()'d data in ahash_op_unaligned_done()
and cause corruption.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: David S. Miller <davem@davemloft.net>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 49 insertions, 7 deletions

diff --git a/crypto/ahash.c b/crypto/ahash.c
index a92dc382f781..4fdb4d3fddec 100644
--- a/crypto/ahash.c
+++ b/crypto/ahash.c
@@ -201,22 +201,34 @@ static void ahash_op_unaligned_finish(struct ahash_request *req, int err)
                 memcpy(priv->result, req->result,
                        crypto_ahash_digestsize(crypto_ahash_reqtfm(req)));
 
+        /* Restore the original crypto request. */
+        req->result = priv->result;
+        req->base.complete = priv->complete;
+        req->base.data = priv->data;
+        req->priv = NULL;
+
+        /* Free the req->priv.priv from the ADJUSTED request. */
         kzfree(priv);
 }
 
 static void ahash_op_unaligned_done(struct crypto_async_request *req, int err)
 {
         struct ahash_request *areq = req->data;
-        struct ahash_request_priv *priv = areq->priv;
-        crypto_completion_t complete = priv->complete;
-        void *data = priv->data;
 
-        ahash_op_unaligned_finish(areq, err);
+        /*
+         * Restore the original request, see ahash_op_unaligned() for what
+         * goes where.
+         *
+         * The "struct ahash_request *req" here is in fact the "req.base"
+         * from the ADJUSTED request from ahash_op_unaligned(), thus as it
+         * is a pointer to self, it is also the ADJUSTED "req" .
+         */
 
-        areq->base.complete = complete;
-        areq->base.data = data;
+        /* First copy areq->result into areq->priv.result */
+        ahash_op_unaligned_finish(areq, err);
 
-        complete(&areq->base, err);
+        /* Complete the ORIGINAL request. */
+        areq->base.complete(&areq->base, err);
 }
 
 static int ahash_op_unaligned(struct ahash_request *req,
@@ -234,9 +246,39 @@ static int ahash_op_unaligned(struct ahash_request *req,
         if (!priv)
                 return -ENOMEM;
 
+        /*
+         * WARNING: Voodoo programming below!
+         *
+         * The code below is obscure and hard to understand, thus explanation
+         * is necessary. See include/crypto/hash.h and include/linux/crypto.h
+         * to understand the layout of structures used here!
+         *
+         * The code here will replace portions of the ORIGINAL request with
+         * pointers to new code and buffers so the hashing operation can store
+         * the result in aligned buffer. We will call the modified request
+         * an ADJUSTED request.
+         *
+         * The newly mangled request will look as such:
+         *
+         * req {
+         *   .result        = ADJUSTED[new aligned buffer]
+         *   .base.complete = ADJUSTED[pointer to completion function]
+         *   .base.data     = ADJUSTED[*req (pointer to self)]
+         *   .priv          = ADJUSTED[new priv] {
+         *           .result   = ORIGINAL(result)
+         *           .complete = ORIGINAL(base.complete)
+         *           .data     = ORIGINAL(base.data)
+         *   }
+         */
+
         priv->result = req->result;
         priv->complete = req->base.complete;
         priv->data = req->base.data;
+        /*
+         * WARNING: We do not backup req->priv here! The req->priv
+         *          is for internal use of the Crypto API and the
+         *          user must _NOT_ _EVER_ depend on it's content!
+         */
 
         req->result = PTR_ALIGN((u8 *)priv->ubuf, alignmask + 1);
         req->base.complete = ahash_op_unaligned_done;