WTF: patch "[PATCH] fscrypto: don't use on-stack buffer for key derivation" was seriously submitted to be applied to the 4.8-stable tree?

The patch below was submitted to be applied to the 4.8-stable tree.

I fail to see how this patch meets the stable kernel rules as found at Documentation/stable_kernel_rules.txt.

I could be totally wrong, and if so, please respond to <[email protected]> and let me know why this patch should be applied. Otherwise, it is now dropped from my patch queues, never to be seen again.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From: Eric Biggers <[email protected]>
Date: Sun, 13 Nov 2016 20:41:09 -0500
Subject: [PATCH] fscrypto: don't use on-stack buffer for key derivation

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. get_crypt_info() was using a stack buffer to hold the output from the encryption operation used to derive the per-file key. Fix it by using a heap buffer.

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.

index 82f0285f5d08..67fb6d8876d0 100644
@@ -185,7 +185,7 @@ int get_crypt_info(struct inode *inode)
 	struct crypto_skcipher *ctfm;
 	const char *cipher_str;
 	int keysize;
-	u8 raw_key[FS_MAX_KEY_SIZE];
+	u8 *raw_key = NULL;
 	int res;
 
 	res = fscrypt_initialize();

@@ -238,6 +238,15 @@ int get_crypt_info(struct inode *inode)
 	if (res)
 		goto out;
 
+	/*
+	 * This cannot be a stack buffer because it is passed to the scatterlist
+	 * crypto API as part of key derivation.
+	 */
+	res = -ENOMEM;
+	raw_key = kmalloc(FS_MAX_KEY_SIZE, GFP_NOFS);
+	if (!raw_key)
+		goto out;
+
 	if (fscrypt_dummy_context_enabled(inode)) {
 		memset(raw_key, 0x42, FS_AES_256_XTS_KEY_SIZE);
 		goto got_key;

@@ -276,7 +285,8 @@ int get_crypt_info(struct inode *inode)
 	if (res)
 		goto out;
 
-	memzero_explicit(raw_key, sizeof(raw_key));
+	kzfree(raw_key);
+	raw_key = NULL;
 	if (cmpxchg(&inode->i_crypt_info, NULL, crypt_info) != NULL) {
 		put_crypt_info(crypt_info);
 		goto retry;

@@ -287,7 +297,7 @@ int get_crypt_info(struct inode *inode)
 	if (res == -ENOKEY)
 		res = 0;
 	put_crypt_info(crypt_info);
-	memzero_explicit(raw_key, sizeof(raw_key));
+	kzfree(raw_key);
 	return res;
 }
 

Hi Greg,

Indeed, as far as I'm concerned you should drop these two patches. They fix problems with virtually-mapped stacks which were added in 4.9, so there's no need to apply them to older kernels. I think Cc stable was left on by accident because the patches were originally going to wait until 4.10.

Thanks!

Eric

Ok, thanks, both now dropped.

greg k-h