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    SubjectRe: [PATCH] /dev/mem: Bail out upon SIGKILL when reading memory.
    On Thu, Aug 22, 2019 at 9:42 AM Greg Kroah-Hartman
    <gregkh@linuxfoundation.org> wrote:
    >
    > 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 :(

    Agree. It wasn't supposed to reach it, but it figured out how to mount
    devfs and then open "./mem" bypassing all checks. Fortunately there
    is a config to disable /dev/mem, so we are going to turn it off.

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