Re: [PATCH bpf v3] net/xdp: fix zero-size allocation warning in xskq_create()

[Martin KaFai Lau <martin.lau@linux.dev>]

On 10/5/23 12:35 PM, Andrew Kanner 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

Please ignore the pw-bot email. A question just came to my mind after applying.

> diff --git a/net/xdp/xsk_queue.c b/net/xdp/xsk_queue.c
> index f8905400ee07..c7e8bbb12752 100644
> --- a/net/xdp/xsk_queue.c
> +++ b/net/xdp/xsk_queue.c
> @@ -34,6 +34,11 @@ struct xsk_queue *xskq_create(u32 nentries, bool umem_queue)
>   	q->ring_mask = nentries - 1;
>   
>   	size = xskq_get_ring_size(q, umem_queue);
> +	if (unlikely(size == SIZE_MAX)) {

What if "size" is SIZE_MAX-1? Would it still overflow the PAGE_ALIGN below?

> +		kfree(q);
> +		return NULL;
> +	}
> +
>   	size = PAGE_ALIGN(size);
>   
>   	q->ring = vmalloc_user(size);