Re: [PATCH v4 04/45] x86: asm: instrument usercopy in get_user() and __put_user_size()

[Alexander Potapenko]

On Thu, Jul 7, 2022 at 12:13 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> On Fri, 1 Jul 2022 at 16:23, Alexander Potapenko <glider@xxxxxxxxxx> wrote:
> >
> > Use hooks from instrumented.h to notify bug detection tools about
> > usercopy events in get_user() and put_user_size().
> >
> > It's still unclear how to instrument put_user(), which assumes that
> > instrumentation code doesn't clobber RAX.
>
> do_put_user_call() has a comment about KASAN clobbering %ax, doesn't
> this also apply to KMSAN? If not, could we have a <asm/instrumented.h>
> that provides helpers to push registers on the stack and pop them back
> on return?

In fact, yes, it is rather simple to not clobber %ax.
A more important aspect of instrumenting get_user()/put_user() is to
always evaluate `x` and `ptr` only once, because sometimes these
macros get called like `put_user(v, sp++)`.
I might have confused the effects of evaluating sp++ twice with some
register clobbering.

> Also it seems the test robot complained about this patch.
Will fix in v5.
>
> > Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> > ---
> > Link: https://linux-review.googlesource.com/id/Ia9f12bfe5832623250e20f1859fdf5cc485a2fce
> > ---
> >  arch/x86/include/asm/uaccess.h | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> > index 913e593a3b45f..1a8b5a234474f 100644
> > --- a/arch/x86/include/asm/uaccess.h
> > +++ b/arch/x86/include/asm/uaccess.h
> > @@ -5,6 +5,7 @@
> >   * User space memory access functions
> >   */
> >  #include <linux/compiler.h>
> > +#include <linux/instrumented.h>
> >  #include <linux/kasan-checks.h>
> >  #include <linux/string.h>
> >  #include <asm/asm.h>
> > @@ -99,11 +100,13 @@ extern int __get_user_bad(void);
> >         int __ret_gu;                                                   \
> >         register __inttype(*(ptr)) __val_gu asm("%"_ASM_DX);            \
> >         __chk_user_ptr(ptr);                                            \
> > +       instrument_copy_from_user_before((void *)&(x), ptr, sizeof(*(ptr))); \
> >         asm volatile("call __" #fn "_%P4"                               \
> >                      : "=a" (__ret_gu), "=r" (__val_gu),                \
> >                         ASM_CALL_CONSTRAINT                             \
> >                      : "0" (ptr), "i" (sizeof(*(ptr))));                \
> >         (x) = (__force __typeof__(*(ptr))) __val_gu;                    \
> > +       instrument_copy_from_user_after((void *)&(x), ptr, sizeof(*(ptr)), 0); \
> >         __builtin_expect(__ret_gu, 0);                                  \
> >  })
> >
> > @@ -248,7 +251,9 @@ extern void __put_user_nocheck_8(void);
> >
> >  #define __put_user_size(x, ptr, size, label)                           \
> >  do {                                                                   \
> > +       __typeof__(*(ptr)) __pus_val = x;                               \
> >         __chk_user_ptr(ptr);                                            \
> > +       instrument_copy_to_user(ptr, &(__pus_val), size);               \
> >         switch (size) {                                                 \
> >         case 1:                                                         \
> >                 __put_user_goto(x, ptr, "b", "iq", label);              \
> > @@ -286,6 +291,7 @@ do {                                                                        \
> >  #define __get_user_size(x, ptr, size, label)                           \
> >  do {                                                                   \
> >         __chk_user_ptr(ptr);                                            \
> > +       instrument_copy_from_user_before((void *)&(x), ptr, size);      \
> >         switch (size) {                                                 \
> >         case 1: {                                                       \
> >                 unsigned char x_u8__;                                   \
> > @@ -305,6 +311,7 @@ do {                                                                        \
> >         default:                                                        \
> >                 (x) = __get_user_bad();                                 \
> >         }                                                               \
> > +       instrument_copy_from_user_after((void *)&(x), ptr, size, 0);    \
> >  } while (0)
> >
> >  #define __get_user_asm(x, addr, itype, ltype, label)                   \
> > --
> > 2.37.0.rc0.161.g10f37bed90-goog
> >



-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Paul Manicle, Liana Sebastian
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg