On 09/28/2016 06:54 PM, Jann Horn wrote: > This is a breaking change for SELinux users that restrict EXECMEM: It might > break gdb if gdb is executed in a domain that does not have EXECMEM > privilege over the debuggee domain.
Since this would break compatibility with existing SELinux policies, you have to wrap it with a conditional on a policy capability that you can then enable in newer policies. See commit 2be4d74f2fd45460d70d4fe65cc1972ef45bf849 for an example. This requires a corresponding update to libsepol, and then adding the new policy capability to your policy (in the policy_capabilities file). > > Unlike most other SELinux hooks, this one takes the subject credentials as > an argument instead of looking up current_cred(). This is done because the > security_forced_write() LSM hook can be invoked from within the write > handler of /proc/$pid/mem, where current_cred() is pretty useless. > > Signed-off-by: Jann Horn <[email protected]> > Reviewed-by: Janis Danisevskis <[email protected]> > --- > security/selinux/hooks.c | 15 +++++++++++++++ > 1 file changed, 15 insertions(+) > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > index 13185a6..e36682a 100644 > --- a/security/selinux/hooks.c > +++ b/security/selinux/hooks.c > @@ -2149,6 +2149,20 @@ static int selinux_ptrace_traceme(struct task_struct > *parent) > return task_has_perm(parent, current, PROCESS__PTRACE); > } > > +static int selinux_forced_write(struct vm_area_struct *vma, > + const struct cred *subject_cred, > + const struct cred *object_cred) > +{ > + /* Permitting a write to readonly memory is fine - making the readonly > + * memory executable afterwards would require EXECMOD permission because > + * anon_vma would be non-NULL. > + */ > + if ((vma->vm_flags & VM_EXEC) == 0) > + return 0; > + > + return cred_has_perm(subject_cred, object_cred, PROCESS__EXECMEM); > +} > + > static int selinux_capget(struct task_struct *target, kernel_cap_t > *effective, > kernel_cap_t *inheritable, kernel_cap_t *permitted) > { > @@ -6033,6 +6047,7 @@ static struct security_hook_list selinux_hooks[] = { > > LSM_HOOK_INIT(ptrace_access_check, selinux_ptrace_access_check), > LSM_HOOK_INIT(ptrace_traceme, selinux_ptrace_traceme), > + LSM_HOOK_INIT(forced_write, selinux_forced_write), > LSM_HOOK_INIT(capget, selinux_capget), > LSM_HOOK_INIT(capset, selinux_capset), > LSM_HOOK_INIT(capable, selinux_capable), >
