CVE-2013-7026 (published 2013-12-09T18:55Z)

Multiple race conditions in ipc/shm.c in the Linux kernel before 3.12.2 allow local users to cause a denial of service (use-after-free and system crash) or possibly have unspecified other impact via a crafted application that uses shmctl IPC_RMID operations in conjunction with other shm system calls.

Severity: MEDIUM
CVSS v2 base score: 4.7 (AV:L/AC:M/Au:N/C:N/I:N/A:C)

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: Greg Thelen
author_date: 2013-11-21 14:32:00-08:00
msg: ipc,shm: fix shm_file deletion races

When IPC_RMID races with other shm operations there's potential for
use-after-free of the shm object's associated file (shm_file).

Here's the race before this patch:

  TASK 1                     TASK 2
  ------                     ------
  shm_rmid()
    ipc_lock_object()
                             shmctl()
                             shp = shm_obtain_object_check()

    shm_destroy()
      shum_unlock()
      fput(shp->shm_file)
                             ipc_lock_object()
                             shmem_lock(shp->shm_file)
                             <OOPS>

The oops is caused because shm_destroy() calls fput() after dropping the
ipc_lock.  fput() clears the file's f_inode, f_path.dentry, and
f_path.mnt, which causes various NULL pointer references in task 2.  I
reliably see the oops in task 2 if with shmlock, shmu

This patch fixes the races by:
1) set shm_file=NULL in shm_destroy() while holding ipc_object_lock().
2) modify at risk operations to check shm_file while holding
   ipc_object_lock().

Example workloads, which each trigger oops...

Workload 1:
  while true; do
    id=$(shmget 1 4096)
    shm_rmid $id &
    shmlock $id &
    wait
  done

  The oops stack shows accessing NULL f_inode due to racing fput:
    _raw_spin_lock
    shmem_lock
    SyS_shmctl

Workload 2:
  while true; do
    id=$(shmget 1 4096)
    shmat $id 4096 &
    shm_rmid $id &
    wait
  done

  The oops stack is similar to workload 1 due to NULL f_inode:
    touch_atime
    shmem_mmap
    shm_mmap
    mmap_region
    do_mmap_pgoff
    do_shmat
    SyS_shmat

Workload 3:
  while true; do
    id=$(shmget 1 4096)
    shmlock $id
    shm_rmid $id &
    shmunlock $id &
    wait
  done

  The oops stack shows second fput tripping on an NULL f_inode.  The
  first fput() completed via from shm_destroy(), but a racing thread did
  a get_file() and queued this fput():
    locks_remove_flock
    __fput
    ____fput
    task_work_run
    do_notify_resume
    int_signal

Fixes: c2c737a0461e ("ipc,shm: shorten critical region for shmat")
Fixes: 2caacaa82a51 ("ipc,shm: shorten critical region for shmctl")
Signed-off-by: Greg Thelen <gthelen@google.com>
Cc: Davidlohr Bueso <davidlohr@hp.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: <stable@vger.kernel.org>  # 3.10.17+ 3.11.6+
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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: shm.c
change_type: ModificationType.MODIFY
diff: @@ -208,15 +208,18 @@ static void shm_open(struct vm_area_struct *vma)
  */
 static void shm_destroy(struct ipc_namespace *ns, struct shmid_kernel *shp)
 {
+	struct file *shm_file;
+
+	shm_file = shp->shm_file;
+	shp->shm_file = NULL;
 	ns->shm_tot -= (shp->shm_segsz + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	shm_rmid(ns, shp);
 	shm_unlock(shp);
-	if (!is_file_hugepages(shp->shm_file))
-		shmem_lock(shp->shm_file, 0, shp->mlock_user);
+	if (!is_file_hugepages(shm_file))
+		shmem_lock(shm_file, 0, shp->mlock_user);
 	else if (shp->mlock_user)
-		user_shm_unlock(file_inode(shp->shm_file)->i_size,
-						shp->mlock_user);
-	fput (shp->shm_file);
+		user_shm_unlock(file_inode(shm_file)->i_size, shp->mlock_user);
+	fput(shm_file);
 	ipc_rcu_putref(shp, shm_rcu_free);
 }
 
@@ -983,6 +986,13 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int, cmd, struct shmid_ds __user *, buf)
 		}
 
 		shm_file = shp->shm_file;
+
+		/* check if shm_destroy() is tearing down shp */
+		if (shm_file == NULL) {
+			err = -EIDRM;
+			goto out_unlock0;
+		}
+
 		if (is_file_hugepages(shm_file))
 			goto out_unlock0;
 
@@ -1101,6 +1111,14 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr,
 		goto out_unlock;
 
 	ipc_lock_object(&shp->shm_perm);
+
+	/* check if shm_destroy() is tearing down shp */
+	if (shp->shm_file == NULL) {
+		ipc_unlock_object(&shp->shm_perm);
+		err = -EIDRM;
+		goto out_unlock;
+	}
+
 	path = shp->shm_file->f_path;
 	path_get(&path);
 	shp->shm_nattch++;

programming_language: C