On Tue, Nov 19, 2024 at 2:36 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> On Tue, Nov 19, 2024 at 11:30 AM Pasha Tatashin
> <pasha.tatashin@xxxxxxxxxx> wrote:
> >
> > On Tue, Nov 19, 2024 at 1:23 PM Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote:
> > >
> > > On Tue, Nov 19, 2024 at 10:08:36AM -0500, Pasha Tatashin wrote:
> > > > On Mon, Nov 18, 2024 at 8:09 PM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > > > >
> > > > > On Mon, Nov 18, 2024 at 05:08:42PM -0500, Pasha Tatashin wrote:
> > > > > > Additionally, using crash/drgn is not feasible for us at this time, it
> > > > > > requires keeping external tools on our hosts, also it requires
> > > > > > approval and a security review for each script before deployment in
> > > > > > our fleet.
> > > > >
> > > > > So it's ok to add a totally insecure kernel feature to your fleet
> > > > > instead? You might want to reconsider that policy decision :)
> > > >
> > > > Hi Greg,
> > > >
> > > > While some risk is inherent, we believe the potential for abuse here
> > > > is limited, especially given the existing CAP_SYS_ADMIN requirement.
> > > > But, even with root access compromised, this tool presents a smaller
> > > > attack surface than alternatives like crash/drgn. It exposes less
> > > > sensitive information, unlike crash/drgn, which could potentially
> > > > allow reading all of kernel memory.
> > >
> > > The problem here is with using dmesg for output. No security-sensitive
> > > information should go there. Even exposing raw kernel pointers is not
> > > considered safe.
> >
> > I am OK in writing the output to a debugfs file in the next version,
> > the only concern I have is that implies that dump_page() would need to
> > be basically duplicated, as it now outputs everything via printk's.
>
> Perhaps you can refactor the code in dump_page() to use a seq_buf,
> then have dump_page() printk that seq_buf using seq_buf_do_printk(),
> and have page detective output that seq_buf to the debugfs file?
Good idea, I will look into modifying it this way.
> We do something very similar with memory_stat_format(). We use the
void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
{
/* Use static buffer, for the caller is holding oom_lock. */
static char buf[PAGE_SIZE];
....
seq_buf_init(&s, buf, sizeof(buf));
memory_stat_format(memcg, &s);
seq_buf_do_printk(&s, KERN_INFO);
}
This is a callosal stack allocation, given that our fleet only has 8K
stacks. :-)
> same function to generate the memcg stats in a seq_buf, then we use
> that seq_buf to output the stats to memory.stat as well as the OOM
> log.
