From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: xen/link: Move .data.rel.ro sections into .rodata for final link Date: Mon, 31 Jul 2017 16:56:38 +0100 Message-ID: <1501516597.4771.328.camel@infradead.org> References: <1500564043.4400.15.camel@infradead.org> <597D79BD0200007800102F92@prv-mh.provo.novell.com> <1501498940.4771.251.camel@infradead.org> <597F2D860200007800103049@prv-mh.provo.novell.com> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============3529629189456546055==" Return-path: Received: from mail6.bemta3.messagelabs.com ([195.245.230.39]) by lists.xenproject.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dcD3l-0004b0-4r for xen-devel@lists.xenproject.org; Mon, 31 Jul 2017 15:56:45 +0000 In-Reply-To: <597F2D860200007800103049@prv-mh.provo.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xen.org Sender: "Xen-devel" To: Jan Beulich Cc: xen-devel@lists.xenproject.org, jiewen.yao@intel.com, jeff.fan@intel.com List-Id: xen-devel@lists.xenproject.org --===============3529629189456546055== Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-JJdq1glJHIAkgxpxu5LQ" --=-JJdq1glJHIAkgxpxu5LQ Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2017-07-31 at 07:15 -0600, Jan Beulich wrote: > > > > David Woodhouse 07/31/17 1:02 PM >>> > > On Sun, 2017-07-30 at 00:16 -0600, Jan Beulich wrote: > > > > > > David Woodhouse 07/20/17 5:22 PM >>> > > > > This includes stuff lke the hypercall tables which we really want > > > > to be read-only. And they were going into .data.read-mostly. > > > > > > Yes, we'd like them to be read-only, but what if EFI properly assigne= d r/o > > > permissions to the .rodata section when loading xen.efi? We'd then be > > > unable to apply relocations when switching from 1:1 to virtual mappin= gs > > > (see efi_arch_relocate_image()). > >=20 > > FWIW it does look like TianoCore has gained the ability to mark > > sections as read-only, in January of this year: > > https://github.com/tianocore/edk2/commit/d0e92aad46 > >=20 > > It doesn't actually seem to be complete =E2=80=94 even with subsequent = fixes > > since that commit, it doesn't look like it catches the case of data > > sections without EFI_IMAGE_SCN_MEM_WRITE, such as .rodata.=C2=A0 > >=20 > > And even if/when that gets fixed you'll note that the protection is > > deliberately torn down in ExitBootServices(), specifically for the case > > you're concerned about below =E2=80=94 because you'll need to do the > > relocations. > > As said in an earlier reply, a first pass over relocations is being done > long before the call to ExitBootServices(). A minimal adjustment to > efi_arch_relocate_image() will be needed anyway, afaict. Ah, right. I think more "implied" than "said" but I understand now. :) At least, I understand what you're saying... I have no bloody clue what's actually going on though. There is a first call to efi_arch_relocate_image(0) before the ExitBootServices() call. However I'm missing something because I can't see how that call achieves anything *other* than to trigger the fault we're concerned about. There are three types of relocations =E2=80=94 PE_BASE_RELOC_ABS, PE_BASE_RELOC_HIGHLOW, and PE_BASE_RELOC_DIR64. The first (ABS) doesn't seem to do anything, ever. And is never emitted by mkrelocs.c.=C2=A0 The second (HIGHLOW) does nothing if (!delta). The third (DIR64) simply adds 'delta' to the target address. We could potentially stop it faulting on that pointless '*addr +=3D 0' by doing this... --- a/xen/arch/x86/efi/efi-boot.h +++ b/xen/arch/x86/efi/efi-boot.h @@ -87,7 +87,8 @@ static void __init efi_arch_relocate_image(unsigned long = delta) =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0case PE_BASE_RELOC_DIR64: =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0if ( in_page_tables(addr) ) =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0blexit(L"Unexpected relo= cation type"); -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0*(u64 *)addr +=3D delta; +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0if ( delta ) +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0*(u64 *)addr +=3D delta; =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0break; =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0default: =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0blexit(L"Unsupported relocation type"); ... but then again, if the whole function is really doing nothing at all when invoked with delta=3D=3D0 then perhaps it would just be easier to remove the first pass altogether. I feel sure I'm missing something, but what? Is it still supposed to be adding xen_phys_start in the PE_BASE_RELOC_HIGHLOW case even when delta=3D=3D0? Because it isn't... Either way, this is still broken before my patch though, right? Especially if UEFI learns to do it for non-executable sections, but AFAICT even before that. These are the sections I see the PE section headers of a local build: =C2=A0Name =C2=A0 =C2=A0 Characteristics =C2=A0 Relocations .text =C2=A0 =C2=A00xe0d00020 (WRX) =C2=A0 =C2=A0=E2=9C=93 .rodata =C2=A00x40600040 ( R ) =C2=A0 =C2=A0=E2=9C=93 .buildid 0x40300040 ( R ) .init.te 0x60500020 ( RX) =C2=A0 =C2=A0=E2=9C=93 .init.da 0xc0d00040 (WR ) =C2=A0 =C2=A0=E2=9C=93 .data.re 0xc0800040 (WR ) =C2=A0 =C2=A0=E2=9C=93 .data =C2=A0 =C2=A00xc0d00040 (WR ) =C2=A0 =C2=A0=E2=9C=93 .bss =C2=A0 =C2=A0 0xc1000080 (WR ) .reloc =C2=A0 0x42300040 ( R ) .pad =C2=A0 =C2=A0 0xc0300080 (WR ) So there are (again, before my patch) relocations in .init.da(ta) and .rodata sections which UEFI *might* start marking read-only, and also in .init.te(xt) which is R+X and could be marked read-only today. And the .init.te(xt) relocations include PE_BASE_RELOC_DIR64 relocations, which *would* cause a fault in the !delta case. 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