Messages in this thread | | | Subject | Re: [PATCH] /dev/mem: Bail out upon SIGKILL when reading memory. | From | Tetsuo Handa <> | Date | Thu, 22 Aug 2019 23:00:59 +0900 |
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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 .
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