On Wed 04-07-18 07:48:27, Zi Yan wrote: > 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. Not really. vm_brk_flags does call mm_populate for mlocked brk which is the case for mlockall. I do not see any len sanitization in that path. Well do_brk_flags does the roundup. I think we should simply remove the bug on and round up there. mm_populate is an internal API and we should trust our callers. Anyway, the minimum fix seems to be the following (untested): diff --git a/mm/mmap.c b/mm/mmap.c index 9859cd4e19b9..56ad19cf2aea 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -186,8 +186,8 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma) return next; } -static int do_brk(unsigned long addr, unsigned long len, struct list_head *uf); - +static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags, + struct list_head *uf); SYSCALL_DEFINE1(brk, unsigned long, brk) { unsigned long retval; @@ -245,7 +245,7 @@ SYSCALL_DEFINE1(brk, unsigned long, brk) goto out; /* Ok, looks good - let it rip. */ - if (do_brk(oldbrk, newbrk-oldbrk, &uf) < 0) + if (do_brk_flags(oldbrk, newbrk-oldbrk, 0, &uf) < 0) goto out; set_brk: @@ -2939,12 +2939,6 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long pgoff_t pgoff = addr >> PAGE_SHIFT; int error; - len = PAGE_ALIGN(request); - if (len < request) - return -ENOMEM; - if (!len) - return 0; - /* Until we need other flags, refuse anything except VM_EXEC. */ if ((flags & (~VM_EXEC)) != 0) return -EINVAL; @@ -3016,18 +3010,20 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long return 0; } -static int do_brk(unsigned long addr, unsigned long len, struct list_head *uf) -{ - return do_brk_flags(addr, len, 0, uf); -} - -int vm_brk_flags(unsigned long addr, unsigned long len, unsigned long flags) +int vm_brk_flags(unsigned long addr, unsigned long request, unsigned long flags) { struct mm_struct *mm = current->mm; + unsigned long len; int ret; bool populate; LIST_HEAD(uf); + len = PAGE_ALIGN(request); + if (len < request) + return -ENOMEM; + if (!len) + return 0; + if (down_write_killable(&mm->mmap_sem)) return -EINTR; -- Michal Hocko SUSE Labs
