Re: [PATCH bpf v2 1/4] bpf_trace: check size for overflow in bpf_kprobe_multi_link_attach

Yonghong Song
2022-05-17T07:03:46Z

On 5/16/22 4:04 PM, 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 | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index d8553f4..f1d4e68 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2352,12 +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;
>
> + if (check_mul_overflow(cnt, (u32)sizeof(*syms), &size))
> + return -EOVERFLOW;
> size = cnt * sizeof(*syms);

The statement 'size = cnt * sizeof(*syms)' can be removed right?
Here, the 'size' will be computed properly with check_mul_overflow()
if 0 is returned.

> syms = kvzalloc(size, GFP_KERNEL);
> if (!syms)
> @@ -2382,9 +2385,9 @@ 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;
> @@ -2429,6 +2432,8 @@ int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
> if (!cnt)
> return -EINVAL;
>
> + if (check_mul_overflow(cnt, (u32)sizeof(*addrs), &size))
> + return -EOVERFLOW;
> size = cnt * sizeof(*addrs);

Same here, 'size = ...' is not needed.

> addrs = kvmalloc(size, GFP_KERNEL);
> if (!addrs)