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

On Thu, 21 May 2020 17:22:50 +0200 Christoph Hellwig [off-list ref] wrote:

quoted

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 <redacted>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Thomas Gleixner <redacted>
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;
+			}