Re: [PATCH bpf v3 1/2] bpf_trace: check size for overflow in bpf_kprobe_multi_link_attach

[Yonghong Song <yhs@fb.com>]

On 5/18/22 5:22 AM, Eugene Syromiatnikov wrote:
> Check that size would not overflow before calculation (and return
> -EOVERFLOW if it will), to prevent potential out-of-bounds write
> with the following copy_from_user.  Add the same check
> to kprobe_multi_resolve_syms in case it will be called from elsewhere
> in the future.
> 
> Fixes: 0dcac272540613d4 ("bpf: Add multi kprobe link")
> Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
> ---
>   kernel/trace/bpf_trace.c | 13 ++++++++-----
>   1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index d8553f4..212faa4 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2352,13 +2352,15 @@ static int
>   kprobe_multi_resolve_syms(const void __user *usyms, u32 cnt,
>   			  unsigned long *addrs)
>   {
> -	unsigned long addr, size;
> +	unsigned long addr, sym_size;
> +	u32 size;
>   	const char __user **syms;
>   	int err = -ENOMEM;
>   	unsigned int i;
>   	char *func;
>   
> -	size = cnt * sizeof(*syms);
> +	if (check_mul_overflow(cnt, (u32)sizeof(*syms), &size))
> +		return -EOVERFLOW;

In mm/util.c kvmalloc_node(), we have

         /* Don't even allow crazy sizes */
         if (unlikely(size > INT_MAX)) {
                 WARN_ON_ONCE(!(flags & __GFP_NOWARN));
                 return NULL;
         }

Basically the maximum size to be allocated in INT_MAX.

Here, we have 'size' as u32, which means if the size is 0xffff0000,
the check_mul_overflow will return false (no overflow) but
kvzalloc will still have a warning.

I think we should change the type of 'size' to be 'int' which
should catch the above case and be consistent with
what kvmalloc_node() intends to warn.

>   	syms = kvzalloc(size, GFP_KERNEL);
>   	if (!syms)
>   		return -ENOMEM;
> @@ -2382,9 +2384,9 @@ kprobe_multi_resolve_syms(const void __user *usyms, u32 cnt,
>   		addr = kallsyms_lookup_name(func);
>   		if (!addr)
>   			goto error;
> -		if (!kallsyms_lookup_size_offset(addr, &size, NULL))
> +		if (!kallsyms_lookup_size_offset(addr, &sym_size, NULL))
>   			goto error;
> -		addr = ftrace_location_range(addr, addr + size - 1);
> +		addr = ftrace_location_range(addr, addr + sym_size - 1);
>   		if (!addr)
>   			goto error;
>   		addrs[i] = addr;
> @@ -2429,7 +2431,8 @@ int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
>   	if (!cnt)
>   		return -EINVAL;
>   
> -	size = cnt * sizeof(*addrs);
> +	if (check_mul_overflow(cnt, (u32)sizeof(*addrs), &size))
> +		return -EOVERFLOW;
>   	addrs = kvmalloc(size, GFP_KERNEL);
>   	if (!addrs)
>   		return -ENOMEM;