Messages in this thread | | | Date | Tue, 15 Apr 2014 13:49:38 +0300 | Subject | Re: + lib-stringc-strlcpy-might-read-too-far.patch added to -mm tree | From | Alexey Dobriyan <> |
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On Tue, Apr 15, 2014 at 12:47 AM, <akpm@linux-foundation.org> wrote: > Subject: lib/string.c: strlcpy() might read too far > > Imagine you have a user controlled variable at the end of a struct which > is allocated at the end of a page. The strlen() could read beyond the > mapped memory and cause an oops. > > Probably there are two reasons why we have never hit this condition in > real life. First you would have to be really unlucky for all the > variables to line up so the oops can happen. Second we don't do a lot of > fuzzing with invalid strings. > > The strnlen() call is obviously a little bit slower than strlen() but I > have tested it and I think it's probably ok.
> --- a/lib/string.c~lib-stringc-strlcpy-might-read-too-far > +++ a/lib/string.c > @@ -148,10 +148,10 @@ EXPORT_SYMBOL(strncpy); > */ > size_t strlcpy(char *dest, const char *src, size_t size) > { > - size_t ret = strlen(src); > + size_t ret = strnlen(src, size); > > if (size) { > - size_t len = (ret >= size) ? size - 1 : ret; > + size_t len = (ret < size) ? ret : ret - 1; > memcpy(dest, src, len); > dest[len] = '\0'; > }
Return value matters. It may not matter for kernel, because kernel is not heavy string user. But it is better to not diverge from master code: http://www.openbsd.org/cgi-bin/cvsweb/src/lib/libc/string/strlcpy.c?rev=1.11
Counter-rationale: * strlcpy() accepts strings, so if you're giving raw buffer you're doing it wrong. * last byte of last page argument is bogus because kernel copies data from userspace first.
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