Re: [PATCH v2] x86/uaccess: Avoid check_object_size() in copy_from_user_nmi()

[Yu Zhao]

On Tue, Sep 20, 2022 at 4:30 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Tue, Sep 20, 2022 at 04:23:00PM -0600, Yu Zhao wrote:
> > On Mon, Sep 19, 2022 at 2:16 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > >
> > > The check_object_size() helper under CONFIG_HARDENED_USERCOPY is
> > > designed to skip any checks where the length is known at compile time as
> > > a reasonable heuristic to avoid "likely known-good" cases. However, it can
> > > only do this when the copy_*_user() helpers are, themselves, inline too.
> > >
> > > Using find_vmap_area() requires taking a spinlock. The check_object_size()
> > > helper can call find_vmap_area() when the destination is in vmap memory.
> > > If show_regs() is called in interrupt context, it will attempt a call to
> > > copy_from_user_nmi(), which may call check_object_size() and then
> > > find_vmap_area(). If something in normal context happens to be in the
> > > middle of calling find_vmap_area() (with the spinlock held), the interrupt
> > > handler will hang forever.
> > >
> > > The copy_from_user_nmi() call is actually being called with a fixed-size
> > > length, so check_object_size() should never have been called in
> > > the first place. Given the narrow constraints, just replace the
> > > __copy_from_user_inatomic() call with an open-coded version that calls
> > > only into the sanitizers and not check_object_size(), followed by a call
> > > to raw_copy_from_user().
> > >
> > > Reported-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> > > Link: https://lore.kernel.org/all/CAOUHufaPshtKrTWOz7T7QFYUNVGFm0JBjvM700Nhf9qEL9b3EQ@xxxxxxxxxxxxxx
> > > Reported-by: dev@xxxxxxxxxxx
> > > Suggested-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > > Cc: x86@xxxxxxxxxx
> > > Fixes: 0aef499f3172 ("mm/usercopy: Detect vmalloc overruns")
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > ---
> > > v2: drop the call explicitly instead of using inline to do it
> > > v1: https://lore.kernel.org/lkml/20220916135953.1320601-1-keescook@xxxxxxxxxxxx
> > > ---
> > >  arch/x86/lib/usercopy.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/lib/usercopy.c b/arch/x86/lib/usercopy.c
> > > index ad0139d25401..d2aff9b176cf 100644
> > > --- a/arch/x86/lib/usercopy.c
> > > +++ b/arch/x86/lib/usercopy.c
> > > @@ -44,7 +44,8 @@ copy_from_user_nmi(void *to, const void __user *from, unsigned long n)
> > >          * called from other contexts.
> > >          */
> > >         pagefault_disable();
> > > -       ret = __copy_from_user_inatomic(to, from, n);
> > > +       instrument_copy_from_user(to, from, n);
> >
> > Got a build error on top of mm-unstable:
> >
> > arch/x86/lib/usercopy.c:47:2: error: call to undeclared function
> > 'instrument_copy_from_user'; ISO C99 and later do not support implicit
> > function declarations [-Wimplicit-function-declaration]
> >         instrument_copy_from_user(to, from, n);
> >         ^
> > arch/x86/lib/usercopy.c:47:2: note: did you mean 'instrument_copy_to_user'?
> > ./include/linux/instrumented.h:117:1: note: 'instrument_copy_to_user'
> > declared here
> > instrument_copy_to_user(void __user *to, const void *from, unsigned long n)
> > ^
> > 1 error generated.
>
> Hm, I did test builds of this before sending. I wonder why this passed
> for me. I suppose this is needed explicitly in arch/x86/lib/usercopy.c:
>instrument_copy_to_user()
> #include <linux/instrumented.h>

It seems mm-unstable has switched to instrument_copy_from_user_{before,after}().

Adding Alex.