Re: [PATCH 2/2] x86/paravirt: add extra clobbers with ZERO_CALL_USED_REGS enabled

[Kees Cook <keescook@chromium.org>]

On Fri, Sep 02, 2022 at 09:37:50PM +0000, Bill Wendling wrote:
> [...]
>         callq   *pv_ops+536(%rip)

Do you know which pv_ops function is this? I can't figure out where
pte_offset_kernel() gets converted into a pv_ops call....

> [...]
> --- a/arch/x86/include/asm/paravirt_types.h
> +++ b/arch/x86/include/asm/paravirt_types.h
> @@ -414,8 +414,17 @@ int paravirt_disable_iospace(void);
>  				"=c" (__ecx)
>  #define PVOP_CALL_CLOBBERS	PVOP_VCALL_CLOBBERS, "=a" (__eax)
>  
> -/* void functions are still allowed [re]ax for scratch */
> +/*
> + * void functions are still allowed [re]ax for scratch.
> + *
> + * The ZERO_CALL_USED REGS feature may end up zeroing out callee-saved
> + * registers. Make sure we model this with the appropriate clobbers.
> + */
> +#ifdef CONFIG_ZERO_CALL_USED_REGS
> +#define PVOP_VCALLEE_CLOBBERS	"=a" (__eax), PVOP_VCALL_CLOBBERS
> +#else
>  #define PVOP_VCALLEE_CLOBBERS	"=a" (__eax)
> +#endif
>  #define PVOP_CALLEE_CLOBBERS	PVOP_VCALLEE_CLOBBERS

I don't think this should depend on CONFIG_ZERO_CALL_USED_REGS; it should
always be present.

I've only been looking at this just now, so many I'm missing
something. The callee clobbers are for functions with return values,
yes?

For example, 32-bit has to manually deal with doing a 64-bit value return,
and even got it wrong originally, fixing it in commit 0eb592dbba40
("x86/paravirt: return full 64-bit result"), with:

-#define PVOP_VCALLEE_CLOBBERS          "=a" (__eax)
+#define PVOP_VCALLEE_CLOBBERS          "=a" (__eax), "=d" (__edx)

But the naming is confusing, since these aren't actually clobbers,
they're input constraints marked as clobbers (the "=" modifier).

Regardless, the note in the comments ...

 ...
 * However, x86_64 also have to clobber all caller saved registers, which
 * unfortunately, are quite a bit (r8 - r11)
 ...

... would indicate that ALL the function argument registers need to be
marked as clobbers (i.e. the compiler can't figure this out on its own).

I was going to say it seems like they're missing from EXTRA_CLOBBERS,
but it's not used with any of the macros using PVOP_VCALLEE_CLOBBERS,
and then I saw the weird alternatives patching that encodes the clobbers
a second time (CLBR_ANY vs CLBR_RET_REG) via:

#define _paravirt_alt(insn_string, type, clobber)       \
        "771:\n\t" insn_string "\n" "772:\n"            \
        ".pushsection .parainstructions,\"a\"\n"        \
        _ASM_ALIGN "\n"                                 \
        _ASM_PTR " 771b\n"                              \
        "  .byte " type "\n"                            \
        "  .byte 772b-771b\n"                           \
        "  .short " clobber "\n"                        \
        ".popsection\n"

And after reading the alternatives patching code which parses this via
the following struct:

/* These all sit in the .parainstructions section to tell us what to patch. */
struct paravirt_patch_site {
        u8 *instr;              /* original instructions */
        u8 type;                /* type of this instruction */
        u8 len;                 /* length of original instruction */
};

... I see it _doesn't use the clobbers_ at all! *head explode* I found
that removal in commit 27876f3882fd ("x86/paravirt: Remove clobbers from
struct paravirt_patch_site")

So, I guess the CLBR_* can all be entirely removed. But back to my other
train of thought...

It seems like all the input registers need to be explicitly listed in
the PVOP_VCALLEE_CLOBBERS list (as you have), but likely should be done
unconditionally and for 32-bit as well.

-Kees

(Also, please CC linux-hardening@vger.kernel.org.)

-- 
Kees Cook