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    • Eric W. Biederman's avatar
      (upstream) mm: Add a user_ns owner to mm_struct and fix ptrace permission checks · de8a1b67
      Eric W. Biederman authored 
      BugLink: https://bugs.launchpad.net/bugs/1639345

During exec dumpable is cleared if the file that is being executed is
not readable by the user executing the file.  A bug in
ptrace_may_access allows reading the file if the executable happens to
enter into a subordinate user namespace (aka clone(CLONE_NEWUSER),
unshare(CLONE_NEWUSER), or setns(fd, CLONE_NEWUSER).

This problem is fixed with only necessary userspace breakage by adding
a user namespace owner to mm_struct, captured at the time of exec, so
it is clear in which user namespace CAP_SYS_PTRACE must be present in
to be able to safely give read permission to the executable.

The function ptrace_may_access is modified to verify that the ptracer
has CAP_SYS_ADMIN in task->mm->user_ns instead of task->cred->user_ns.
This ensures that if the task changes it's cred into a subordinate
user namespace it does not become ptraceable.

The function ptrace_attach is modified to only set PT_PTRACE_CAP when
CAP_SYS_PTRACE is held over task->mm->user_ns.  The intent of
PT_PTRACE_CAP is to be a flag to note that whatever permission changes
the task might go through the tracer has sufficient permissions for
it not to be an issue.  task->cred->user_ns is always the same
as or descendent of mm->user_ns.  Which guarantees that having
CAP_SYS_PTRACE over mm->user_ns is the worst case for the tasks
credentials.

To prevent regressions mm->dumpable and mm->user_ns are not considered
when a task has no mm.  As simply failing ptrace_may_attach causes
regressions in privileged applications attempting to read things
such as /proc/<pid>/stat

Cc: stable@vger.kernel.org
Acked-by: default avatarKees Cook <keescook@chromium.org>
Tested-by: default avatarCyrill Gorcunov <gorcunov@openvz.org>
Fixes: 8409cca7 ("userns: allow ptrace from non-init user namespaces")
Signed-off-by: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
(cherry picked from commit 2e41414828bb0b066bde2f156cfa848c38531edf linux-next)
CVE-2015-8709
Signed-off-by: default avatarSeth Forshee <seth.forshee@canonical.com>
Acked-by: default avatarBrad Figg <brad.figg@canonical.com>
Acked-by: default avatarTim Gardner <tim.gardner@canonical.com>
Signed-off-by: default avatarLuis Henriques <luis.henriques@canonical.com>
      de8a1b67
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