Re: [PATCH bpf v4] net/xdp: fix zero-size allocation warning in xskq_create()

[Magnus Karlsson <magnus.karlsson@gmail.com>]

On Sat, 7 Oct 2023 at 09:52, Andrew Kanner <andrew.kanner@gmail.com> wrote:
>
> Syzkaller reported the following issue:
>  ------------[ cut here ]------------
>  WARNING: CPU: 0 PID: 2807 at mm/vmalloc.c:3247 __vmalloc_node_range (mm/vmalloc.c:3361)
>  Modules linked in:
>  CPU: 0 PID: 2807 Comm: repro Not tainted 6.6.0-rc2+ #12
>  Hardware name: Generic DT based system
>  unwind_backtrace from show_stack (arch/arm/kernel/traps.c:258)
>  show_stack from dump_stack_lvl (lib/dump_stack.c:107 (discriminator 1))
>  dump_stack_lvl from __warn (kernel/panic.c:633 kernel/panic.c:680)
>  __warn from warn_slowpath_fmt (./include/linux/context_tracking.h:153 kernel/panic.c:700)
>  warn_slowpath_fmt from __vmalloc_node_range (mm/vmalloc.c:3361 (discriminator 3))
>  __vmalloc_node_range from vmalloc_user (mm/vmalloc.c:3478)
>  vmalloc_user from xskq_create (net/xdp/xsk_queue.c:40)
>  xskq_create from xsk_setsockopt (net/xdp/xsk.c:953 net/xdp/xsk.c:1286)
>  xsk_setsockopt from __sys_setsockopt (net/socket.c:2308)
>  __sys_setsockopt from ret_fast_syscall (arch/arm/kernel/entry-common.S:68)
>
> xskq_get_ring_size() uses struct_size() macro to safely calculate the
> size of struct xsk_queue and q->nentries of desc members. But the
> syzkaller repro was able to set q->nentries with the value initially
> taken from copy_from_sockptr() high enough to return SIZE_MAX by
> struct_size(). The next PAGE_ALIGN(size) is such case will overflow
> the size_t value and set it to 0. This will trigger WARN_ON_ONCE in
> vmalloc_user() -> __vmalloc_node_range().
>
> The issue is reproducible on 32-bit arm kernel.
>
> Reported-and-tested-by: syzbot+fae676d3cf469331fc89@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/000000000000c84b4705fb31741e@google.com/T/
> Link: https://syzkaller.appspot.com/bug?extid=fae676d3cf469331fc89
> Reported-by: syzbot+b132693e925cbbd89e26@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/000000000000e20df20606ebab4f@google.com/T/
> Fixes: 9f78bf330a66 ("xsk: support use vaddr as ring")
> Signed-off-by: Andrew Kanner <andrew.kanner@gmail.com>

Thanks Andrew for fixing this.

Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>

> ---
>
> Notes (akanner):
>     v4:
>       - add explanation about SIZE_MAX, suggested by Martin KaFai Lau
>         <martin.lau@linux.dev>
>     v3: https://lore.kernel.org/all/20231005193548.515-1-andrew.kanner@gmail.com/T/
>       - free kzalloc-ed memory before return, the leak was noticed by
>         Daniel Borkmann <daniel@iogearbox.net>
>     v2: https://lore.kernel.org/all/20231002222939.1519-1-andrew.kanner@gmail.com/raw
>       - use unlikely() optimization for the case with SIZE_MAX return from
>         struct_size(), suggested by Alexander Lobakin
>         <aleksander.lobakin@intel.com>
>       - cc-ed 4 more maintainers, mentioned by cc_maintainers patchwork
>         test
>
>     v1: https://lore.kernel.org/all/20230928204440.543-1-andrew.kanner@gmail.com/T/
>       - RFC notes:
>         It was found that net/xdp/xsk.c:xsk_setsockopt() uses
>         copy_from_sockptr() to get the number of entries (int) for cases
>         with XDP_RX_RING / XDP_TX_RING and XDP_UMEM_FILL_RING /
>         XDP_UMEM_COMPLETION_RING.
>
>         Next in xsk_init_queue() there're 2 sanity checks (entries == 0)
>         and (!is_power_of_2(entries)) for which -EINVAL will be returned.
>
>         After that net/xdp/xsk_queue.c:xskq_create() will calculate the
>         size multipling the number of entries (int) with the size of u64,
>         at least.
>
>         I wonder if there should be the upper bound (e.g. the 3rd sanity
>         check inside xsk_init_queue()). It seems that without the upper
>         limit it's quiet easy to overflow the allocated size (SIZE_MAX),
>         especially for 32-bit architectures, for example arm nodes which
>         were used by the syzkaller.
>
>         In this patch I added a naive check for SIZE_MAX which helped to
>         skip zero-size allocation after overflow, but maybe it's not quite
>         right. Please, suggest if you have any thoughts about the
>         appropriate limit for the size of these xdp rings.
>
>         PS: the initial number of entries is 0x20000000 in syzkaller
>         repro: syscall(__NR_setsockopt, (intptr_t)r[0], 0x11b, 3,
>         0x20000040, 0x20);
>
>         Link:
>         https://syzkaller.appspot.com/text?tag=ReproC&x=10910f18280000
>
>  net/xdp/xsk_queue.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/net/xdp/xsk_queue.c b/net/xdp/xsk_queue.c
> index f8905400ee07..d2c264030017 100644
> --- a/net/xdp/xsk_queue.c
> +++ b/net/xdp/xsk_queue.c
> @@ -34,6 +34,16 @@ struct xsk_queue *xskq_create(u32 nentries, bool umem_queue)
>         q->ring_mask = nentries - 1;
>
>         size = xskq_get_ring_size(q, umem_queue);
> +
> +       /* size which is overflowing or close to SIZE_MAX will become 0 in
> +        * PAGE_ALIGN(), checking SIZE_MAX is enough due to the previous
> +        * is_power_of_2(), the rest will be handled by vmalloc_user()
> +        */
> +       if (unlikely(size == SIZE_MAX)) {
> +               kfree(q);
> +               return NULL;
> +       }
> +
>         size = PAGE_ALIGN(size);
>
>         q->ring = vmalloc_user(size);
> --
> 2.39.3
>
>