On Wed 04-07-18 19:01:51, Tetsuo Handa wrote: > +Michal Hocko > > On 2018/07/04 13:19, syzbot wrote: > > Hello, > > > > syzbot found the following crash on: > > > > HEAD commit: d3bc0e67f852 Merge tag 'for-4.18-rc2-tag' of git://git.ker.. > > git tree: upstream > > console output: https://syzkaller.appspot.com/x/log.txt?x=1000077c400000 > > kernel config: https://syzkaller.appspot.com/x/.config?x=a63be0c83e84d370 > > dashboard link: https://syzkaller.appspot.com/bug?extid=5dcb560fe12aa5091c06 > > compiler: gcc (GCC) 8.0.1 20180413 (experimental) > > userspace arch: i386 > > syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=158577a2400000 > > Here is C reproducer made from syz reproducer. mlockall(MCL_FUTURE) is involved. > > This problem is triggerable by an unprivileged user. > Shows different result on x86_64 (crash) and x86_32 (stall). > > ------------------------------------------------------------ > /* Need to compile using "-m32" option if host is 64bit. */ > #include <sys/types.h> > #include <sys/stat.h> > #include <fcntl.h> > #include <unistd.h> > #include <sys/mman.h> > int uselib(const char *library); > > int main(int argc, char *argv[]) > { > int fd = open("file", O_WRONLY | O_CREAT, 0644); > write(fd, "\x7f\x45\x4c\x46\x00\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02" > "\x00\x06\x00\xca\x3f\x8b\xca\x00\x00\x00\x00\x38\x00\x00\x00\x00\x00" > "\x00\xf7\xff\xff\xff\xff\xff\xff\x1f\x00\x02\x00\x00\x00\x00\x00\x00" > "\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\xf8\x7b" > "\x66\xff\x00\x00\x05\x00\x00\x00\x76\x86\x00\x00\x00\x00\x00\x00\x00" > "\x00\x00\x00\x31\x0f\xf3\xee\xc1\xb0\x00\x0c\x08\x53\x55\xbe\x88\x47" > "\xc2\x2e\x30\xf5\x62\x82\xc6\x2c\x95\x72\x3f\x06\x8f\xe4\x2d\x27\x96" > "\xcc", 120); > fchmod(fd, 0755); > close(fd); > mlockall(MCL_FUTURE); /* Removing this line avoids the bug. */ > uselib("file"); > return 0; > } > ------------------------------------------------------------ > > ------------------------------------------------------------ > CentOS Linux 7 (Core) > Kernel 4.18.0-rc3 on an x86_64 > > localhost login: [ 81.210241] emacs (9634) used greatest stack depth: 10416 bytes left > [ 140.099935] ------------[ cut here ]------------ > [ 140.101904] kernel BUG at mm/gup.c:1242! Is this VM_BUG_ON(len != PAGE_ALIGN(len)); in __mm_populate? I do not really get why we should VM_BUG_ON when the len is not page aligned to be honest. The library is probably containing some funky setup but if we simply cannot round up to the next PAGE_SIZE boundary then we should probably just error out and fail. This is an area I am really familiar with so I cannot really judge. -- Michal Hocko SUSE Labs
