On Mon, Jul 09, 2018 at 12:52:21PM +0200, Dmitry Vyukov wrote: > On Mon, Jul 9, 2018 at 12:48 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: > > On Mon, Jul 9, 2018 at 12:15 PM, Kirill A. Shutemov > > <kirill@xxxxxxxxxxxxx> wrote: > >> On Sun, Jul 08, 2018 at 10:51:03PM -0700, syzbot wrote: > >>> Hello, > >>> > >>> syzbot found the following crash on: > >>> > >>> HEAD commit: b2d44d145d2a Merge tag '4.18-rc3-smb3fixes' of git://git.s.. > >>> git tree: upstream > >>> console output: https://syzkaller.appspot.com/x/log.txt?x=11d07748400000 > >>> kernel config: https://syzkaller.appspot.com/x/.config?x=2ca6c7a31d407f86 > >>> dashboard link: https://syzkaller.appspot.com/bug?extid=3f84280d52be9b7083cc > >>> compiler: gcc (GCC) 8.0.1 20180413 (experimental) > >>> > >>> Unfortunately, I don't have any reproducer for this crash yet. > >>> > >>> IMPORTANT: if you fix the bug, please add the following tag to the commit: > >>> Reported-by: syzbot+3f84280d52be9b7083cc@xxxxxxxxxxxxxxxxxxxxxxxxx > >>> > >>> next ffff8801ce5e7040 prev ffff8801d20eca50 mm ffff88019c1e13c0 > >>> prot 27 anon_vma ffff88019680cdd8 vm_ops 0000000000000000 > >>> pgoff 0 file ffff8801b2ec2d00 private_data 0000000000000000 > >>> flags: 0xff(read|write|exec|shared|mayread|maywrite|mayexec|mayshare) > >>> ------------[ cut here ]------------ > >>> kernel BUG at mm/memory.c:1422! > >> > >> Looks like vma_is_anonymous() false-positive. > >> > >> Any clues what file is it? I would guess some kind of socket, but it's not > >> clear from log which exactly. > > > > > > From the log it looks like it was this program (number 3 matches Comm: > > syz-executor3): > > > > 08:39:32 executing program 3: > > r0 = socket$nl_route(0x10, 0x3, 0x0) > > bind$netlink(r0, &(0x7f00000002c0)={0x10, 0x0, 0x0, 0x100000}, 0xc) > > getsockname(r0, &(0x7f0000000000)=@pppol2tpv3in6={0x0, 0x0, {0x0, > > <r1=>0xffffffffffffffff, 0x0, 0x0, 0x0, 0x0, {0x0, 0x0, 0x0, > > @loopback}}}, &(0x7f00000000c0)=0x3a) > > mmap(&(0x7f0000e00000/0x200000)=nil, 0x200000, 0x7fdff, 0x11, r1, 0x0) > > ioctl$FIBMAP(r0, 0x1, &(0x7f0000000100)=0x9) > > r2 = socket$inet6(0xa, 0x1000000000002, 0x0) > > ioctl(r2, 0x8912, &(0x7f00000001c0)="796d05ad441e829115ac7fd77200") > > r3 = syz_open_dev$vcsa(&(0x7f0000000140)='/dev/vcsa#\x00', 0x3, 0x2) > > ioctl$VHOST_SET_VRING_ENDIAN(r3, 0x4008af13, &(0x7f0000000180)={0x0, 0x8}) > > sendto$inet(0xffffffffffffffff, &(0x7f0000a88f88), 0xffffffffffffff31, > > 0x0, &(0x7f0000e68000)={0x2, 0x0, @multicast2=0xe0000002}, > > 0xfffffffffffffeb3) > > ftruncate(r1, 0x6) > > mmap(&(0x7f0000e00000/0x200000)=nil, 0x200000, 0x0, 0x11, r0, 0x0) > > setsockopt$SO_TIMESTAMPING(r1, 0x1, 0x25, &(0x7f0000000080)=0x804, 0x4) > > > > But take what happens here with a grain of salt, it can pretend that > > it's doing one thing, but actually do something different. > > So that r1 passed to ftruncate is something that getsockname returned > > somewhere in the middle of address. And since the socket is not > > actually ppp, it can be just some bytes in the middle of netlink > > address, that than happened to be small and match some existing fd... > > > This also happened only once so far: > https://syzkaller.appspot.com/bug?extid=3f84280d52be9b7083cc > and I can't reproduce it rerunning this program. So it's either a very > subtle race, or fd in the middle of netlink address magically matched > some fd once, or something else... Okay, I've got it reproduced. See below. The problem is that kcov doesn't set vm_ops for the VMA and it makes kernel think that the VMA is anonymous. It's not necessary the way it was triggered by syzkaller. I just found that kcov's ->mmap doesn't set vm_ops. There can more such cases. vma_is_anonymous() is what we need to fix. ( Although, I found logic around mmaping the file second time questinable at best. It seems broken to me. ) It is known that vma_is_anonymous() can produce false-positives. It tried to fix it once[1], but it back-fired[2]. I'll look at this again. [1] https://lkml.kernel.org/r/1437133993-91885-1-git-send-email-kirill.shutemov@xxxxxxxxxxxxxxx [2] https://lkml.kernel.org/r/20150914105346.GB23878@xxxxxxx #include <stdio.h> #include <stddef.h> #include <stdint.h> #include <stdlib.h> #include <string.h> #include <sys/types.h> #include <sys/stat.h> #include <sys/ioctl.h> #include <sys/mman.h> #include <unistd.h> #include <fcntl.h> #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long) #define KCOV_ENABLE _IO('c', 100) #define KCOV_DISABLE _IO('c', 101) #define COVER_SIZE (1024<<10) #define KCOV_TRACE_PC 0 #define KCOV_TRACE_CMP 1 int main(int argc, char **argv) { int fd; unsigned long *cover, n, i; system("mount -t debugfs none /sys/kernel/debug"); fd = open("/sys/kernel/debug/kcov", O_RDWR); ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE); cover = mmap(NULL, COVER_SIZE * sizeof(unsigned long), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); munmap(cover, COVER_SIZE * sizeof(unsigned long)); cover = mmap(NULL, COVER_SIZE * sizeof(unsigned long), PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0); memset(cover, 0, COVER_SIZE * sizeof(unsigned long)); ftruncate(fd, 3UL << 20); return 0; } -- Kirill A. Shutemov
