Re: [syzbot] WARNING: kmalloc bug in xdp_umem_create (2)

["Björn Töpel" <bjorn.topel@gmail.com>]

On Thu, 10 Feb 2022 at 09:35, Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On 2/10/22 9:11 AM, Willy Tarreau wrote:
> > On Wed, Feb 09, 2022 at 10:08:07PM -0800, syzbot wrote:
> >> syzbot has bisected this issue to:
> >>
> >> commit 7661809d493b426e979f39ab512e3adf41fbcc69
> >> Author: Linus Torvalds <torvalds@linux-foundation.org>
> >> Date:   Wed Jul 14 16:45:49 2021 +0000
> >>
> >>      mm: don't allow oversized kvmalloc() calls
> >>
> >> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=13bc74c2700000
> >> start commit:   f4bc5bbb5fef Merge tag 'nfsd-5.17-2' of git://git.kernel.o..
> >> git tree:       upstream
> >> final oops:     https://syzkaller.appspot.com/x/report.txt?x=107c74c2700000
> >> console output: https://syzkaller.appspot.com/x/log.txt?x=17bc74c2700000
> >> kernel config:  https://syzkaller.appspot.com/x/.config?x=5707221760c00a20
> >> dashboard link: https://syzkaller.appspot.com/bug?extid=11421fbbff99b989670e
> >> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12e514a4700000
> >> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15fcdf8a700000
> >>
> >> Reported-by: syzbot+11421fbbff99b989670e@syzkaller.appspotmail.com
> >> Fixes: 7661809d493b ("mm: don't allow oversized kvmalloc() calls")
> >>
> >> For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> >
> > Interesting, so in fact syzkaller has shown that the aforementioned
> > patch does its job well and has spotted a call path by which a single
> > userland setsockopt() can request more than 2 GB allocation in the
> > kernel. Most likely that's in fact what needs to be addressed.
> >
> > FWIW the call trace at the URL above is:
> >
> > Call Trace:
> >   kvmalloc include/linux/mm.h:806 [inline]
> >   kvmalloc_array include/linux/mm.h:824 [inline]
> >   kvcalloc include/linux/mm.h:829 [inline]
> >   xdp_umem_pin_pages net/xdp/xdp_umem.c:102 [inline]
> >   xdp_umem_reg net/xdp/xdp_umem.c:219 [inline]
> >   xdp_umem_create+0x6a5/0xf00 net/xdp/xdp_umem.c:252
> >   xsk_setsockopt+0x604/0x790 net/xdp/xsk.c:1068
> >   __sys_setsockopt+0x1fd/0x4e0 net/socket.c:2176
> >   __do_sys_setsockopt net/socket.c:2187 [inline]
> >   __se_sys_setsockopt net/socket.c:2184 [inline]
> >   __x64_sys_setsockopt+0xb5/0x150 net/socket.c:2184
> >   do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> >   do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> >   entry_SYSCALL_64_after_hwframe+0x44/0xae
> >
> > and the meaningful part of the repro is:
> >
> >    syscall(__NR_mmap, 0x1ffff000ul, 0x1000ul, 0ul, 0x32ul, -1, 0ul);
> >    syscall(__NR_mmap, 0x20000000ul, 0x1000000ul, 7ul, 0x32ul, -1, 0ul);
> >    syscall(__NR_mmap, 0x21000000ul, 0x1000ul, 0ul, 0x32ul, -1, 0ul);
> >    intptr_t res = 0;
> >    res = syscall(__NR_socket, 0x2cul, 3ul, 0);
> >    if (res != -1)
> >      r[0] = res;
> >    *(uint64_t*)0x20000080 = 0;
> >    *(uint64_t*)0x20000088 = 0xfff02000000;
> >    *(uint32_t*)0x20000090 = 0x800;
> >    *(uint32_t*)0x20000094 = 0;
> >    *(uint32_t*)0x20000098 = 0;
> >    syscall(__NR_setsockopt, r[0], 0x11b, 4, 0x20000080ul, 0x20ul);
>
> Bjorn had a comment back then when the issue was first raised here:
>
>    https://lore.kernel.org/bpf/3f854ca9-f5d6-4065-c7b1-5e5b25ea742f@iogearbox.net/
>
> There was earlier discussion from Andrew to potentially retire the warning:
>
>    https://lore.kernel.org/bpf/20211201202905.b9892171e3f5b9a60f9da251@linux-foundation.org/
>
> Bjorn / Magnus / Andrew, anyone planning to follow-up on this issue?
>

Honestly, I would need some guidance on how to progress. I could just
change from U32_MAX to INT_MAX, but as I stated earlier (lore-link
above), that has a hacky feeling to it. Andrew's mail didn't really
land in a consensus. From my perspective, the code isn't broken, with
the memcg limits in consideration. Introducing a LARGE flag or a new
"_yes_this_can_be_huge_but_it_is_ok()" version would make sense if
this problem is applicable to more users in the kernel.

So, thoughts? ;-)


Björn