[PATCH v2 1/2] seq_buf: Make seq_buf_puts() null-terminate the buffer

* [PATCH v2 1/2] seq_buf: Make seq_buf_puts() null-terminate the buffer
@ 2018-10-19  4:21 Michael Ellerman
  2018-10-19  4:21 ` [PATCH v2 2/2] seq_buf: Use size_t for len in seq_buf_puts() Michael Ellerman
  2018-12-12 11:21 ` [PATCH v2 1/2] seq_buf: Make seq_buf_puts() null-terminate the buffer Michael Ellerman
  0 siblings, 2 replies; 6+ messages in thread
From: Michael Ellerman @ 2018-10-19  4:21 UTC (permalink / raw)
  To: rostedt; +Cc: linux-kernel, kernel-hardening, jannh, keescook

Currently seq_buf_puts() will happily create a non null-terminated
string for you in the buffer. This is particularly dangerous if the
buffer is on the stack.

For example:

  char buf[8];
  char secret = "secret";
  struct seq_buf s;

  seq_buf_init(&s, buf, sizeof(buf));
  seq_buf_puts(&s, "foo");
  printk("Message is %s\n", buf);

Can result in:

  Message is fooªªªªªsecret

We could require all users to memset() their buffer to zero before
use. But that seems likely to be forgotten and lead to bugs.

Instead we can change seq_buf_puts() to always leave the buffer in a
null-terminated state.

The only downside is that this makes the buffer 1 character smaller
for seq_buf_puts(), but that seems like a good trade off.

Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 lib/seq_buf.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

v2: Fix NULL/null terminology.

diff --git a/lib/seq_buf.c b/lib/seq_buf.c
index 11f2ae0f9099..6aabb609dd87 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -144,9 +144,13 @@ int seq_buf_puts(struct seq_buf *s, const char *str)
 
 	WARN_ON(s->size == 0);
 
+	/* Add 1 to len for the trailing null byte which must be there */
+	len += 1;
+
 	if (seq_buf_can_fit(s, len)) {
 		memcpy(s->buffer + s->len, str, len);
-		s->len += len;
+		/* Don't count the trailing null byte against the capacity */
+		s->len += len - 1;
 		return 0;
 	}
 	seq_buf_set_overflow(s);
-- 
2.17.2


^ permalink raw reply related	[flat|nested] 6+ messages in thread