Re: [PATCH] /dev/mem: Bail out upon SIGKILL when reading memory.

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Aug 22, 2019 at 06:59:25PM +0900, Tetsuo Handa wrote:
> Tetsuo Handa wrote:
> > Greg Kroah-Hartman wrote:
> > > Oh, nice!  This shouldn't break anything that is assuming that the read
> > > will complete before a signal is delivered, right?
> > >
> > > I know userspace handling of "short" reads is almost always not there...
> > 
> > Since this check will give up upon SIGKILL, userspace won't be able to see
> > the return value from read(). Thus, returning 0 upon SIGKILL will be safe. ;-)
> > Maybe we also want to add cond_resched()...
> > 
> > By the way, do we want similar check on write_mem() side?
> > If aborting "write to /dev/mem" upon SIGKILL (results in partial write) is
> > unexpected, we might want to ignore SIGKILL for write_mem() case.
> > But copying data from killed threads (especially when killed by OOM killer
> > and userspace memory is reclaimed by OOM reaper before write_mem() returns)
> > would be after all unexpected. Then, it might be preferable to check SIGKILL
> > on write_mem() side...
> > 
> 
> Ha, ha. syzbot reported the same problem using write_mem().
> https://syzkaller.appspot.com/text?tag=CrashLog&x=1018055a600000
> We want fatal_signal_pending() check on both sides.

Ok, want to send a patch for that?

And does anything use /dev/mem anymore?  I think X stopped using it a
long time ago.

> By the way, write_mem() worries me whether there is possibility of replacing
> kernel code/data with user-defined memory data supplied from userspace.
> If write_mem() were by chance replaced with code that does
> 
>    while (1);
> 
> we won't be able to return from write_mem() even if we added fatal_signal_pending() check.
> Ditto for replacing local variables with unexpected values...

I'm sorry, I don't really understand what you mean here, but I haven't
had my morning coffee...  Any hints as to an example?

thanks,

greg k-h