Commit f6581f5b authored by Jann Horn's avatar Jann Horn Committed by Eric W. Biederman
Browse files

ptrace: restore smp_rmb() in __ptrace_may_access() 



Restore the read memory barrier in __ptrace_may_access() that was deleted
a couple years ago. Also add comments on this barrier and the one it pairs
with to explain why they're there (as far as I understand).

Fixes: bfedb589 ("mm: Add a user_ns owner to mm_struct and fix ptrace permission checks")
Cc: stable@vger.kernel.org
Acked-by: default avatarKees Cook <keescook@chromium.org>
Acked-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarJann Horn <jannh@google.com>
Signed-off-by: default avatarEric W. Biederman <ebiederm@xmission.com>
parent f6e2aa91

kernel/cred.c

+9 −0
Original line number Diff line number Diff line
@@ -450,6 +450,15 @@ int commit_creds(struct cred *new) 
		if (task->mm)

			set_dumpable(task->mm, suid_dumpable);

		task->pdeath_signal = 0;

		/*

		 * If a task drops privileges and becomes nondumpable,

		 * the dumpability change must become visible before

		 * the credential change; otherwise, a __ptrace_may_access()

		 * racing with this change may be able to attach to a task it

		 * shouldn't be able to attach to (as if the task had dropped

		 * privileges without becoming nondumpable).

		 * Pairs with a read barrier in __ptrace_may_access().

		 */

		smp_wmb();

	}

kernel/ptrace.c

+10 −0
Original line number Diff line number Diff line
@@ -323,6 +323,16 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode) 
	return -EPERM;

ok:

	rcu_read_unlock();

	/*

	 * If a task drops privileges and becomes nondumpable (through a syscall

	 * like setresuid()) while we are trying to access it, we must ensure

	 * that the dumpability is read after the credentials; otherwise,

	 * we may be able to attach to a task that we shouldn't be able to

	 * attach to (as if the task had dropped privileges without becoming

	 * nondumpable).

	 * Pairs with a write barrier in commit_creds().

	 */

	smp_rmb();

	mm = task->mm;

	if (mm &&

	    ((get_dumpable(mm) != SUID_DUMP_USER) &&

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