Re: [PATCH 1/6] ksmbd: fix global-out-of-bounds in smb2_find_context_vals

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (23/05/08 21:58), Namjae Jeon wrote:
> 2023-05-08 10:05 GMT+09:00, Sergey Senozhatsky <senozhatsky@chromium.org>:
> > On (23/05/06 00:11), Namjae Jeon wrote:
> >> From: Pumpkin <cc85nod@gmail.com>
> >>
> >> If the length of CreateContext name is larger than the tag, it will
> >> access
> >> the data following the tag and trigger KASAN global-out-of-bounds.
> >>
> >> Currently all CreateContext names are defined as string, so we can use
> >> strcmp instead of memcmp to avoid the out-of-bound access.
> Hi Chih-Yen,
> 
> Please reply to Sergey's review comment. If needed, please send v2
> patch after updating it.

Chih-Yen replied privately, but let me move the discussion back to
public list.

> >> +++ b/fs/ksmbd/oplock.c
> >> @@ -1492,7 +1492,7 @@ struct create_context *smb2_find_context_vals(void
> >> *open_req, const char *tag)
> >>  			return ERR_PTR(-EINVAL);
> >>
> >>  		name = (char *)cc + name_off;
> >> -		if (memcmp(name, tag, name_len) == 0)
> >> +		if (!strcmp(name, tag))
> >>  			return cc;
> >>
> >>  		remain_len -= next;
> >
> > I'm slightly surprised that that huge `if` before memcmp() doesn't catch
> > it
> >
> > 		if ((next & 0x7) != 0 ||
> > 		    next > remain_len ||
> > 		    name_off != offsetof(struct create_context, Buffer) ||
> > 		    name_len < 4 ||
> > 		    name_off + name_len > cc_len ||
> > 		    (value_off & 0x7) != 0 ||
> > 		    (value_off && (value_off < name_off + name_len)) ||
> > 		    ((u64)value_off + value_len > cc_len))
> > 			return ERR_PTR(-EINVAL);

So the question is: why doesn't this `if` catch that problem?
I'd rather add one extra condition here, it doesn't make a lot
of sense to strcmp/memcmp if we know beforehand that two strings
have different sizes. So a simple "name len != context len" should
do the trick. No?