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 :(
