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

Andrew Morton
2020-05-28T02:04:35Z

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