Vulnerability: The __driver_rfc4106_decrypt function in arch/x86/crypto/aesni-intel_glue.c in the Linux kernel before 3.19.3 does not properly determine the memory locations used for encrypted data, which allows context-dependent attackers to cause a denial of service (buffer overflow and system crash) or possibly execute arbitrary code by triggering a crypto API call, as demonstrated by use of a libkcapi test program with an AF_ALG(aead) socket.
Vulnerability ID: CVE-2015-3331
CWE: CWE-119
Severity score: 10
Project: https://github.com/torvalds/linux
Published: 2015-05-27
File: arch/x86/crypto/aesni-intel_glue.c
Commit message: crypto: aesni - fix memory usage in GCM decryption

The kernel crypto API logic requires the caller to provide the
length of (ciphertext || authentication tag) as cryptlen for the
AEAD decryption operation. Thus, the cipher implementation must
calculate the size of the plaintext output itself and cannot simply use
cryptlen.

The RFC4106 GCM decryption operation tries to overwrite cryptlen memory
in req->dst. As the destination buffer for decryption only needs to hold
the plaintext memory but cryptlen references the input buffer holding
(ciphertext || authentication tag), the assumption of the destination
buffer length in RFC4106 GCM operation leads to a too large size. This
patch simply uses the already calculated plaintext size.

In addition, this patch fixes the offset calculation of the AAD buffer
pointer: as mentioned before, cryptlen already includes the size of the
tag. Thus, the tag does not need to be added. With the addition, the AAD
will be written beyond the already allocated buffer.

Note, this fixes a kernel crash that can be triggered from user space
via AF_ALG(aead) -- simply use the libkcapi test application
from [1] and update it to use rfc4106-gcm-aes.

Using [1], the changes were tested using CAVS vectors to demonstrate
that the crypto operation still delivers the right results.

[1] http://www.chronox.de/libkcapi.html

CC: Tadeusz Struk <tadeusz.struk@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Commit author: Stephan Mueller

Vulnerable version (before fix):
 static int __driver_rfc4106_decrypt(struct aead_request *req) 		src = kmalloc(req->cryptlen + req->assoclen, GFP_ATOMIC); 		if (!src) 			return -ENOMEM; 		assoc = (src + req->cryptlen + auth_tag_len); 		scatterwalk_map_and_copy(src, req->src, 0, req->cryptlen, 0); 		scatterwalk_map_and_copy(assoc, req->assoc, 0, 			req->assoclen, 0); static int __driver_rfc4106_decrypt(struct aead_request *req) 		scatterwalk_done(&src_sg_walk, 0, 0); 		scatterwalk_done(&assoc_sg_walk, 0, 0); 	} else { 		scatterwalk_map_and_copy(dst, req->dst, 0, req->cryptlen, 1); 		kfree(src); 	} 	return retval;

Fixed version (after fix):
 static int __driver_rfc4106_decrypt(struct aead_request *req) 		src = kmalloc(req->cryptlen + req->assoclen, GFP_ATOMIC); 		if (!src) 			return -ENOMEM; 		assoc = (src + req->cryptlen); 		scatterwalk_map_and_copy(src, req->src, 0, req->cryptlen, 0); 		scatterwalk_map_and_copy(assoc, req->assoc, 0, 			req->assoclen, 0); static int __driver_rfc4106_decrypt(struct aead_request *req) 		scatterwalk_done(&src_sg_walk, 0, 0); 		scatterwalk_done(&assoc_sg_walk, 0, 0); 	} else { 		scatterwalk_map_and_copy(dst, req->dst, 0, tempCipherLen, 1); 		kfree(src); 	} 	return retval;

Patch:
patch: @@ -1155,7 +1155,7 @@ static int __driver_rfc4106_decrypt(struct aead_request *req)
 		src = kmalloc(req->cryptlen + req->assoclen, GFP_ATOMIC);
 		if (!src)
 			return -ENOMEM;
-		assoc = (src + req->cryptlen + auth_tag_len);
+		assoc = (src + req->cryptlen);
 		scatterwalk_map_and_copy(src, req->src, 0, req->cryptlen, 0);
 		scatterwalk_map_and_copy(assoc, req->assoc, 0,
 			req->assoclen, 0);
@@ -1180,7 +1180,7 @@ static int __driver_rfc4106_decrypt(struct aead_request *req)
 		scatterwalk_done(&src_sg_walk, 0, 0);
 		scatterwalk_done(&assoc_sg_walk, 0, 0);
 	} else {
-		scatterwalk_map_and_copy(dst, req->dst, 0, req->cryptlen, 1);
+		scatterwalk_map_and_copy(dst, req->dst, 0, tempCipherLen, 1);
 		kfree(src);
 	}
 	return retval;
path: arch/x86/crypto/aesni-intel_glue.c