CVE-2023-3108 (published 2023-07-11T16:15Z)

A flaw was found in the subsequent get_user_pages_fast in the Linux kernel’s interface for symmetric key cipher algorithms in the skcipher_recvmsg of crypto/algif_skcipher.c function. This flaw allows a local user to crash the system.

CVSS v3 base score: 4.7, MEDIUM (CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H)

Weakness information:
cwe_id: CWE-362
cwe_name: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
description: The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.
extended_description: 
            This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider.
            A race condition occurs within concurrent environments, and is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc.
            A race condition violates these properties, which are closely related:
               
                  Exclusivity - the code sequence is given exclusive access to the shared resource, i.e., no other code sequence can modify properties of the shared resource before the original sequence has completed execution.
                  Atomicity - the code sequence is behaviorally atomic, i.e., no other thread or process can concurrently execute the same sequence of instructions (or a subset) against the same resource.
               
            A race condition exists when an "interfering code sequence" can still access the shared resource, violating exclusivity. Programmers may assume that certain code sequences execute too quickly to be affected by an interfering code sequence; when they are not, this violates atomicity. For example, the single "x++" statement may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read (the original value of x), followed by a computation (x+1), followed by a write (save the result to x).
            The interfering code sequence could be "trusted" or "untrusted." A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.
         
         

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Linus Torvalds
author_date: 2015-02-10 19:55:45-08:00
msg: crypto: fix af_alg_make_sg() conversion to iov_iter

Commit 1d10eb2f156f ("crypto: switch af_alg_make_sg() to iov_iter")
broke af_alg_make_sg() and skcipher_recvmsg() in the process of moving
them to the iov_iter interfaces.  The 'npages' calculation in the formar
calculated the number of *bytes* in the pages, and in the latter case
the conversion didn't re-read the value of 'ctx->used' after waiting for
it to become non-zero.

This reverts to the original code for both these cases.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
repository: repo_url: https://github.com/torvalds/linux
repo_name: torvalds/linux
description: Linux kernel source tree
repo_language: C
owner: torvalds
file_changes: filename: af_alg.c
change_type: ModificationType.MODIFY
diff: @@ -348,7 +348,7 @@ int af_alg_make_sg(struct af_alg_sgl *sgl, struct iov_iter *iter, int len)
 	if (n < 0)
 		return n;
 
-	npages = PAGE_ALIGN(off + n);
+	npages = (off + n + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	if (WARN_ON(npages == 0))
 		return -EINVAL;
 

programming_language: C
filename: algif_skcipher.c
change_type: ModificationType.MODIFY
diff: @@ -439,14 +439,13 @@ static int skcipher_recvmsg(struct kiocb *unused, struct socket *sock,
 		while (!sg->length)
 			sg++;
 
-		used = ctx->used;
-		if (!used) {
+		if (!ctx->used) {
 			err = skcipher_wait_for_data(sk, flags);
 			if (err)
 				goto unlock;
 		}
 
-		used = min_t(unsigned long, used, iov_iter_count(&msg->msg_iter));
+		used = min_t(unsigned long, ctx->used, iov_iter_count(&msg->msg_iter));
 
 		used = af_alg_make_sg(&ctx->rsgl, &msg->msg_iter, used);
 		err = used;

programming_language: C