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Date: Sun, 4 Sep 2022 23:02:38 -0700 From: Bill Wendling <morbo@...gle.com> To: Kees Cook <keescook@...omium.org> Cc: Juergen Gross <jgross@...e.com>, "Srivatsa S. Bhat (VMware)" <srivatsa@...il.mit.edu>, Alexey Makhalov <amakhalov@...are.com>, VMware PV-Drivers Reviewers <pv-drivers@...are.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, virtualization@...ts.linux-foundation.org, LKML <linux-kernel@...r.kernel.org>, Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, clang-built-linux <llvm@...ts.linux.dev>, linux-hardening@...r.kernel.org Subject: Re: [PATCH 2/2] x86/paravirt: add extra clobbers with ZERO_CALL_USED_REGS enabled On Sat, Sep 3, 2022 at 12:18 AM Kees Cook <keescook@...omium.org> wrote: > > 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.... > This one is _paravirt_ident_64, I believe. I think that the original issue Nathan was seeing was with another seemingly innocuous function. > > [...] > > --- 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? > Kinda. It seems that the usage here is to let the compiler know that a register may be modified by the callee, not just that it's an "actual" return value. So it's suitable for void functions. > 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). > Right. > 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). > Good point. And there are some forms of these macros that specify those as clobbers. > 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... > [switches stations] > 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. > Possibly, though it may cause significant code degradation when the compiler needs to store a value that's live over the ASM statement, but the register it's in isn't actually modified. I saw that in the example I gave in the description. In the case where a "movq" is used, there's a useless move of "rdi" into "r11". > (Also, please CC linux-hardening@...r.kernel.org.) > Doh! Someday I'll learn email. -bw
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