On 4 Jul 2018, at 7:17, Michal Hocko wrote: > 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. A strange thing is that __mm_populate() is only called by do_mlock() from mm/mlock.c, which makes len PAGE_ALIGN already. That VM_BUG_ON should not be triggered. — Best Regards, Yan Zi
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