On Sun, 24 Oct 2021 at 22:12, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Sun, Oct 24, 2021 at 09:54:22AM -1000, Linus Torvalds wrote: > > On Sat, Oct 23, 2021 at 9:35 AM syzbot > > <syzbot+75639e6a0331cd61d3e2@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > syzbot found the following issue on: > > > > > > HEAD commit: 9c0c4d24ac00 Merge tag 'block-5.15-2021-10-22' of git://gi.. > > > git tree: upstream > > > console output: https://syzkaller.appspot.com/x/log.txt?x=115a0328b00000 > > > kernel config: https://syzkaller.appspot.com/x/.config?x=59f3ef2b4077575 > > > dashboard link: https://syzkaller.appspot.com/bug?extid=75639e6a0331cd61d3e2 > > > compiler: Debian clang version 11.0.1-2, GNU ld (GNU Binutils for Debian) 2.35.2 > > > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=13a035c2b00000 > > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=14ae869f300000 > > > > > > The issue was bisected to: > > > > > > commit 110860541f443f950c1274f217a1a3e298670a33 > > > > I think that commit is actually just buggy. > > > > "secretmem_users" is not actually a reference count. There's no "magic > > happens when it goes down to zero". > > > > It's purely a count of the number of existing users, and incrementing > > it from zero is not a probolem at all - it is in fact expected. > > > > Sure, zero means "we can hibernate", so zero and overflow are somewhat > > special, but not special enough to cause these kinds of issues. > > > > I have reverted this commit in my tree, because honestly, the whole > > "try to overflow exactly, and hibernate" threat model just isn't worth > > this all. > > > > If people really care, I can suggest > > > > - use "atomic_long_t" instead. Let's face it, 32-bit isn't > > interesting any more, and 64-bit doesn't overflow. > > > > - make up some new "atomic_inc_nooverflow()" thing or whatever. > > > > but for now this is just reverted. > > There was a separate thread on an earlier version of this report. gcc and clang somehow produce different frames (I guess a tail call). Need to fix parsing. #syz dup: WARNING: refcount bug in sys_memfd_secret > https://lore.kernel.org/linux-mm/YXU7%2FiRjf9v77gon@xxxxxxxxxxxxxxxxxxxx/ > I agree with you and suggested that if anybody really cares (I mean, > you need a multi-TB machine to produce this problem) that we simply do > what we did with the page refcount: > > +++ b/mm/secretmem.c > @@ -203,6 +203,8 @@ SYSCALL_DEFINE1(memfd_secret, unsigned int, flags) > > if (flags & ~(SECRETMEM_FLAGS_MASK | O_CLOEXEC)) > return -EINVAL; > + if (atomic_read(&secretmem_users) < 0) > + return -ENFILE; > > fd = get_unused_fd_flags(flags & O_CLOEXEC); > if (fd < 0) > > Mike didn't particularly like that answer though.
