Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> On Sat, Jul 6, 2019 at 12:55 PM Salvatore Mesoraca
> <s.mesoraca16@xxxxxxxxx> wrote:
> > Prevent a task from opening, in "write" mode, any /proc/*/mem
> > file that operates on the task's mm.
> > A process could use it to overwrite read-only memory, bypassing
> > S.A.R.A. restrictions.
> [...]
> > +static void sara_task_to_inode(struct task_struct *t, struct inode *i)
> > +{
> > + get_sara_inode_task(i) = t;
>
> This looks bogus. Nothing is actually holding a reference to `t` here, right?
I think you are right, I should probably store the PID here.
> > +}
> > +
> > static struct security_hook_list data_hooks[] __lsm_ro_after_init = {
> > LSM_HOOK_INIT(cred_prepare, sara_cred_prepare),
> > LSM_HOOK_INIT(cred_transfer, sara_cred_transfer),
> > LSM_HOOK_INIT(shm_alloc_security, sara_shm_alloc_security),
> > + LSM_HOOK_INIT(task_to_inode, sara_task_to_inode),
> > };
> [...]
> > +static int sara_file_open(struct file *file)
> > +{
> > + struct task_struct *t;
> > + struct mm_struct *mm;
> > + u16 sara_wxp_flags = get_current_sara_wxp_flags();
> > +
> > + /*
> > + * Prevent write access to /proc/.../mem
> > + * if it operates on the mm_struct of the
> > + * current process: it could be used to
> > + * bypass W^X.
> > + */
> > +
> > + if (!sara_enabled ||
> > + !wxprot_enabled ||
> > + !(sara_wxp_flags & SARA_WXP_WXORX) ||
> > + !(file->f_mode & FMODE_WRITE))
> > + return 0;
> > +
> > + t = get_sara_inode_task(file_inode(file));
> > + if (unlikely(t != NULL &&
> > + strcmp(file->f_path.dentry->d_name.name,
> > + "mem") == 0)) {
>
> This should probably at least have a READ_ONCE() somewhere in case the
> file concurrently gets renamed?
My understanding here is that /proc/$pid/mem files cannot be renamed.
t != NULL implies we are in procfs.
Under these assumptions I think that that code is fine.
Am I wrong?
> > + get_task_struct(t);
> > + mm = get_task_mm(t);
> > + put_task_struct(t);
>
> Getting and dropping a reference to the task_struct here is completely
> useless. Either you have a reference, in which case you don't need to
> take another one, or you don't have a reference, in which case you
> also can't take one.
Absolutely agree.
> > + if (unlikely(mm == current->mm))
> > + sara_warn_or_goto(error,
> > + "write access to /proc/*/mem");
>
> Why is the current process so special that it must be protected more
> than other processes? Is the idea here to rely on other protections to
> protect all other tasks? This should probably come with a comment that
> explains this choice.
Yes, I should have spent some more words here.
Access to /proc/$pid/mem from other processes is already regulated by
security_ptrace_access_check (i.e. Yama).
Unfortunately, that hook is ignored when "mm == current->mm".
It seems that there is some user-space software that relies on /proc/self/mem
being writable (cfr. commit f511c0b17b08).
Thank you for your suggestions.
