Messages in this thread | | | From | Kees Cook <> | Date | Fri, 26 Aug 2016 14:57:58 -0400 | Subject | Re: [kernel-hardening] [PATCH] lib: harden strncpy_from_user |
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On Fri, Aug 26, 2016 at 10:31 AM, Mark Rutland <mark.rutland@arm.com> wrote: > The strncpy_from_user() accessor is effectively a copy_from_user() > specialised to copy strings, terminating early at a NUL byte if > possible. In other respects it is identical, and can be used to copy an > arbitrarily large buffer from userspace into the kernel. Conceptually, > it exposes a similar attack surface. > > As with copy_from_user(), we check the destination range when the kernel > is built with KASAN, but unlike copy_from_user() we do not check the > destination buffer when using HARDENED_USERCOPY. As strncpy_from_user() > calls get_user() in a loop, we must call check_object_size() explicitly. > > This patch adds this instrumentation to strncpy_from_user(), per the > same rationale as with the regular copy_from_user(). In the absence of > hardened usercopy this will have no impact as the instrumentation > expands to an empty static inline function. > > Signed-off-by: Mark Rutland <mark.rutland@arm.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Kees Cook <keescook@chromium.org> > --- > lib/strncpy_from_user.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c > index 9c5fe81..7e35fc4 100644 > --- a/lib/strncpy_from_user.c > +++ b/lib/strncpy_from_user.c > @@ -1,6 +1,7 @@ > #include <linux/compiler.h> > #include <linux/export.h> > #include <linux/kasan-checks.h> > +#include <linux/thread_info.h> > #include <linux/uaccess.h> > #include <linux/kernel.h> > #include <linux/errno.h> > @@ -111,6 +112,7 @@ long strncpy_from_user(char *dst, const char __user *src, long count) > long retval; > > kasan_check_write(dst, count); > + check_object_size(dst, count, false); > user_access_begin(); > retval = do_strncpy_from_user(dst, src, count, max); > user_access_end(); > -- > 2.7.4 >
Ah, yes, good catch! (And to repeat what you mentioned to me in passing in the hall: there appear to be other users of get_user() in a loop in other places in the kernel that will likely need some attention too.)
I'll get this into the hardened usercopy tree when I get back from the Security Summit. This will likely need to grow knowledge about builtin-const "count" arguments like we need to the architectures that are missing them in the copy_*_user calls.
Thanks!
-Kees
-- Kees Cook Nexus Security
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