From mboxrd@z Thu Jan 1 00:00:00 1970 Reply-To: kernel-hardening@lists.openwall.com Date: Fri, 22 Jul 2016 12:45:51 -0500 From: Josh Poimboeuf Message-ID: <20160722174551.jddle6mf7zlq6xmb@treble> References: <1468619065-3222-1-git-send-email-keescook@chromium.org> <1468619065-3222-3-git-send-email-keescook@chromium.org> <5790711f.2350420a.b4287.2cc0SMTPIN_ADDED_BROKEN@mx.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Subject: [kernel-hardening] Re: [PATCH v3 02/11] mm: Hardened usercopy To: Kees Cook Cc: Michael Ellerman , LKML , Balbir Singh , Daniel Micay , Rik van Riel , Casey Schaufler , PaX Team , Brad Spengler , Russell King , Catalin Marinas , Will Deacon , Ard Biesheuvel , Benjamin Herrenschmidt , Tony Luck , Fenghua Yu , "David S. Miller" , "x86@kernel.org" , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , Andy Lutomirski , Borislav Petkov , Mathias Krause , Jan Kara , Vitaly Wool , Andrea Arcangeli , Dmitry Vyukov , Laura Abbott , "linux-arm-kernel@lists.infradead.org" , linux-ia64@vger.kernel.org, "linuxppc-dev@lists.ozlabs.org" , sparclinux , linux-arch , Linux-MM , "kernel-hardening@lists.openwall.com" List-ID: On Thu, Jul 21, 2016 at 11:34:25AM -0700, Kees Cook wrote: > On Wed, Jul 20, 2016 at 11:52 PM, Michael Ellerman wrote: > > Kees Cook writes: > > > >> diff --git a/mm/usercopy.c b/mm/usercopy.c > >> new file mode 100644 > >> index 000000000000..e4bf4e7ccdf6 > >> --- /dev/null > >> +++ b/mm/usercopy.c > >> @@ -0,0 +1,234 @@ > > ... > >> + > >> +/* > >> + * Checks if a given pointer and length is contained by the current > >> + * stack frame (if possible). > >> + * > >> + * 0: not at all on the stack > >> + * 1: fully within a valid stack frame > >> + * 2: fully on the stack (when can't do frame-checking) > >> + * -1: error condition (invalid stack position or bad stack frame) > >> + */ > >> +static noinline int check_stack_object(const void *obj, unsigned long len) > >> +{ > >> + const void * const stack = task_stack_page(current); > >> + const void * const stackend = stack + THREAD_SIZE; > > > > That allows access to the entire stack, including the struct thread_info, > > is that what we want - it seems dangerous? Or did I miss a check > > somewhere else? > > That seems like a nice improvement to make, yeah. > > > We have end_of_stack() which computes the end of the stack taking > > thread_info into account (end being the opposite of your end above). > > Amusingly, the object_is_on_stack() check in sched.h doesn't take > thread_info into account either. :P Regardless, I think using > end_of_stack() may not be best. To tighten the check, I think we could > add this after checking that the object is on the stack: > > #ifdef CONFIG_STACK_GROWSUP > stackend -= sizeof(struct thread_info); > #else > stack += sizeof(struct thread_info); > #endif > > e.g. then if the pointer was in the thread_info, the second test would > fail, triggering the protection. FWIW, this won't work right on x86 after Andy's CONFIG_THREAD_INFO_IN_TASK patches get merged. -- Josh