Re: [PATCH 12/23] bpf: handle the compat string in bpf_trace_copy_string better

Yonghong Song
2020-05-28T02:27:21Z

On 5/27/20 7:04 PM, Andrew Morton wrote:
> On Thu, 21 May 2020 17:22:50 +0200 Christoph Hellwig <hch@lst.de> wrote:
>
>> User the proper helper for kernel or userspace addresses based on
>> TASK_SIZE instead of the dangerous strncpy_from_unsafe function.
>>
>> ...
>>
>> --- a/kernel/trace/bpf_trace.c
>> +++ b/kernel/trace/bpf_trace.c
>> @@ -331,8 +331,11 @@ static void bpf_trace_copy_string(char *buf, void *unsafe_ptr, char fmt_ptype,
>> switch (fmt_ptype) {
>> case 's':
>> #ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
>> - strncpy_from_unsafe(buf, unsafe_ptr, bufsz);
>> - break;
>> + if ((unsigned long)unsafe_ptr < TASK_SIZE) {
>> + strncpy_from_user_nofault(buf, user_ptr, bufsz);
>> + break;
>> + }
>> + fallthrough;
>> #endif
>> case 'k':
>> strncpy_from_kernel_nofault(buf, unsafe_ptr, bufsz);
>
> Another user of strncpy_from_unsafe() has popped up in linux-next's
> bpf. I did the below, but didn't try very hard - it's probably wrong
> if CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE=n?
>
> Anyway, please take a look at all the bpf_trace.c changes in
> linux-next.
>
>
> From: Andrew Morton <akpm@linux-foundation.org>
> Subject: bpf:bpf_seq_printf(): handle potentially unsafe format string better
>
> User the proper helper for kernel or userspace addresses based on
> TASK_SIZE instead of the dangerous strncpy_from_unsafe function.
>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>
> kernel/trace/bpf_trace.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> --- a/kernel/trace/bpf_trace.c~xxx
> +++ a/kernel/trace/bpf_trace.c
> @@ -588,15 +588,22 @@ BPF_CALL_5(bpf_seq_printf, struct seq_fi
> }
>
> if (fmt[i] == 's') {
> + void *unsafe_ptr;
> +
> /* try our best to copy */
> if (memcpy_cnt >= MAX_SEQ_PRINTF_MAX_MEMCPY) {
> err = -E2BIG;
> goto out;
> }
>
> - err = strncpy_from_unsafe(bufs->buf[memcpy_cnt],
> - (void *) (long) args[fmt_cnt],
> - MAX_SEQ_PRINTF_STR_LEN);
> + unsafe_ptr = (void *)(long)args[fmt_cnt];
> + if ((unsigned long)unsafe_ptr < TASK_SIZE) {
> + err = strncpy_from_user_nofault(
> + bufs->buf[memcpy_cnt], unsafe_ptr,
> + MAX_SEQ_PRINTF_STR_LEN);
> + } else {
> + err = -EFAULT;
> + }

This probably not right.
The pointer stored at args[fmt_cnt] is a kernel pointer,
but it could be an invalid address and we do not want to fault.
Not sure whether it exists or not, we should use
strncpy_from_kernel_nofault()?

> if (err < 0)
> bufs->buf[memcpy_cnt][0] = '\0';
> params[fmt_cnt] = (u64)(long)bufs->buf[memcpy_cnt];
> _
>