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

[Greg Kroah-Hartman]

On Thu, Aug 22, 2019 at 11:00:59PM +0900, Tetsuo Handa wrote:
> On 2019/08/22 22:35, Greg Kroah-Hartman wrote:
> > 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?
> 
> Yes. But before sending a patch, I'm trying to dump values using debug printk().
> 
> > 
> > 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?
> 
> Probably similar idea: "lockdown: Restrict /dev/{mem,kmem,port} when the kernel is locked down"
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/drivers/char/mem.c?h=next-20190822&id=9b9d8dda1ed72e9bd560ab0ca93d322a9440510e
> 
> Then, syzbot might want to blacklist writing to /dev/mem .

syzbot should probably blacklist that now, you can do a lot of bad
things writing to that device node :(