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

[Dan Carpenter]

On Thu, Feb 10, 2022 at 05:18:52PM +0100, Björn Töpel wrote:
> On Thu, 10 Feb 2022 at 09:35, Daniel Borkmann <daniel@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx>
> > >> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > >> 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@xxxxxxxxxxxxx/
> >
> > There was earlier discussion from Andrew to potentially retire the warning:
> >
> >    https://lore.kernel.org/bpf/20211201202905.b9892171e3f5b9a60f9da251@xxxxxxxxxxxxxxxxxxxx/
> >
> > 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

It would have to be lower than that.  The limit is on "npgs" but we are
allocating npgs * sizeof(struct page *) so it would have to:

	if (npgs > INT_MAX / sizeof(void *))
		return -EINVAL;

Is it normally going to huge?  You could call vmalloc() instead of
kvmalloc().

When Linus added the WARN_ON() for huge kvmalloc sizes, it was as a
reaction to a security bug where the size which was more than UINT_MAX
but not everything was prepared to handle ulong sizes.  He wanted
people who allocated large amounts of RAM to do it in a deliberate way
instead of by accident.

regards,
dan carpenter