From patchwork Sat Oct 3 04:50:53 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jarkko Sakkinen X-Patchwork-Id: 11815041 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 46CDD1668 for ; Sat, 3 Oct 2020 04:52:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 32320235F8 for ; Sat, 3 Oct 2020 04:52:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725919AbgJCEwS (ORCPT ); Sat, 3 Oct 2020 00:52:18 -0400 Received: from mail.kernel.org ([198.145.29.99]:52940 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725827AbgJCEvw (ORCPT ); Sat, 3 Oct 2020 00:51:52 -0400 Received: from localhost (83-245-197-237.elisa-laajakaista.fi [83.245.197.237]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id C1B7A221F0; Sat, 3 Oct 2020 04:51:45 +0000 (UTC) From: Jarkko Sakkinen To: x86@kernel.org, linux-sgx@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Sean Christopherson , Andy Lutomirski , Jethro Beekman , Jarkko Sakkinen , akpm@linux-foundation.org, andriy.shevchenko@linux.intel.com, asapek@google.com, bp@alien8.de, cedric.xing@intel.com, chenalexchen@google.com, conradparker@google.com, cyhanish@google.com, dave.hansen@intel.com, haitao.huang@intel.com, kai.huang@intel.com, kai.svahn@intel.com, kmoy@google.com, ludloff@google.com, luto@kernel.org, nhorman@redhat.com, npmccallum@redhat.com, puiterwijk@redhat.com, rientjes@google.com, tglx@linutronix.de, yaozhangx@google.com, mikko.ylinen@intel.com Subject: [PATCH v39 18/24] x86/vdso: Add support for exception fixup in vDSO functions Date: Sat, 3 Oct 2020 07:50:53 +0300 Message-Id: <20201003045059.665934-19-jarkko.sakkinen@linux.intel.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20201003045059.665934-1-jarkko.sakkinen@linux.intel.com> References: <20201003045059.665934-1-jarkko.sakkinen@linux.intel.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-sgx@vger.kernel.org From: Sean Christopherson The basic concept and implementation is very similar to the kernel's exception fixup mechanism. The key differences are that the kernel handler is hardcoded and the fixup entry addresses are relative to the overall table as opposed to individual entries. Hardcoding the kernel handler avoids the need to figure out how to get userspace code to point at a kernel function. Given that the expected usage is to propagate information to userspace, dumping all fault information into registers is likely the desired behavior for the vast majority of yet-to-be-created functions. Use registers DI, SI and DX to communicate fault information, which follows Linux's ABI for register consumption and hopefully avoids conflict with hardware features that might leverage the fixup capabilities, e.g. register usage for SGX instructions was at least partially designed with calling conventions in mind. Making fixup addresses relative to the overall table allows the table to be stripped from the final vDSO image (it's a kernel construct) without complicating the offset logic, e.g. entry-relative addressing would also need to account for the table's location relative to the image. Regarding stripping the table, modify vdso2c to extract the table from the raw, a.k.a. unstripped, data and dump it as a standalone byte array in the resulting .c file. The original base of the table, its length and a pointer to the byte array are captured in struct vdso_image. Alternatively, the table could be dumped directly into the struct, but because the number of entries can vary per image, that would require either hardcoding a max sized table into the struct definition or defining the table as a flexible length array. The flexible length array approach has zero benefits, e.g. the base/size are still needed, and prevents reusing the extraction code, while hardcoding the max size adds ongoing maintenance just to avoid exporting the explicit size. The immediate use case is for Intel Software Guard Extensions (SGX). SGX introduces a new CPL3-only "enclave" mode that runs as a sort of black box shared object that is hosted by an untrusted "normal" CPl3 process. Entering an enclave can only be done through SGX-specific instructions, EENTER and ERESUME, and is a non-trivial process. Because of the complexity of transitioning to/from an enclave, the vast majority of enclaves are expected to utilize a library to handle the actual transitions. This is roughly analogous to how e.g. libc implementations are used by most applications. Another crucial characteristic of SGX enclaves is that they can generate exceptions as part of their normal (at least as "normal" as SGX can be) operation that need to be handled *in* the enclave and/or are unique to SGX. And because they are essentially fancy shared objects, a process can host any number of enclaves, each of which can execute multiple threads simultaneously. Putting everything together, userspace enclaves will utilize a library that must be prepared to handle any and (almost) all exceptions any time at least one thread may be executing in an enclave. Leveraging signals to handle the enclave exceptions is unpleasant, to put it mildly, e.g. the SGX library must constantly (un)register its signal handler based on whether or not at least one thread is executing in an enclave, and filter and forward exceptions that aren't related to its enclaves. This becomes particularly nasty when using multiple levels of libraries that register signal handlers, e.g. running an enclave via cgo inside of the Go runtime. Enabling exception fixup in vDSO allows the kernel to provide a vDSO function that wraps the low-level transitions to/from the enclave, i.e. the EENTER and ERESUME instructions. The vDSO function can intercept exceptions that would otherwise generate a signal and return the fault information directly to its caller, thus avoiding the need to juggle signal handlers. Note that unlike the kernel's _ASM_EXTABLE_HANDLE implementation, the 'C' version of _ASM_VDSO_EXTABLE_HANDLE doesn't use a pre-compiled assembly macro. Duplicating four lines of code is simpler than adding the necessary infrastructure to generate pre-compiled assembly and the intended benefit of massaging GCC's inlining algorithm is unlikely to realized in the vDSO any time soon, if ever. Suggested-by: Andy Lutomirski Acked-by: Jethro Beekman Signed-off-by: Sean Christopherson Signed-off-by: Jarkko Sakkinen --- arch/x86/entry/vdso/Makefile | 6 ++-- arch/x86/entry/vdso/extable.c | 46 ++++++++++++++++++++++++ arch/x86/entry/vdso/extable.h | 28 +++++++++++++++ arch/x86/entry/vdso/vdso-layout.lds.S | 9 ++++- arch/x86/entry/vdso/vdso2c.h | 50 ++++++++++++++++++++++++++- arch/x86/include/asm/vdso.h | 5 +++ 6 files changed, 139 insertions(+), 5 deletions(-) create mode 100644 arch/x86/entry/vdso/extable.c create mode 100644 arch/x86/entry/vdso/extable.h diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile index 215376d975a2..3f183d0b8826 100644 --- a/arch/x86/entry/vdso/Makefile +++ b/arch/x86/entry/vdso/Makefile @@ -31,7 +31,7 @@ vobjs32-y := vdso32/note.o vdso32/system_call.o vdso32/sigreturn.o vobjs32-y += vdso32/vclock_gettime.o # files to link into kernel -obj-y += vma.o +obj-y += vma.o extable.o KASAN_SANITIZE_vma.o := y UBSAN_SANITIZE_vma.o := y KCSAN_SANITIZE_vma.o := y @@ -130,8 +130,8 @@ $(obj)/%-x32.o: $(obj)/%.o FORCE targets += vdsox32.lds $(vobjx32s-y) -$(obj)/%.so: OBJCOPYFLAGS := -S -$(obj)/%.so: $(obj)/%.so.dbg FORCE +$(obj)/%.so: OBJCOPYFLAGS := -S --remove-section __ex_table +$(obj)/%.so: $(obj)/%.so.dbg $(call if_changed,objcopy) $(obj)/vdsox32.so.dbg: $(obj)/vdsox32.lds $(vobjx32s) FORCE diff --git a/arch/x86/entry/vdso/extable.c b/arch/x86/entry/vdso/extable.c new file mode 100644 index 000000000000..afcf5b65beef --- /dev/null +++ b/arch/x86/entry/vdso/extable.c @@ -0,0 +1,46 @@ +// SPDX-License-Identifier: GPL-2.0 +#include +#include +#include +#include +#include + +struct vdso_exception_table_entry { + int insn, fixup; +}; + +bool fixup_vdso_exception(struct pt_regs *regs, int trapnr, + unsigned long error_code, unsigned long fault_addr) +{ + const struct vdso_image *image = current->mm->context.vdso_image; + const struct vdso_exception_table_entry *extable; + unsigned int nr_entries, i; + unsigned long base; + + /* + * Do not attempt to fixup #DB or #BP. It's impossible to identify + * whether or not a #DB/#BP originated from within an SGX enclave and + * SGX enclaves are currently the only use case for vDSO fixup. + */ + if (trapnr == X86_TRAP_DB || trapnr == X86_TRAP_BP) + return false; + + if (!current->mm->context.vdso) + return false; + + base = (unsigned long)current->mm->context.vdso + image->extable_base; + nr_entries = image->extable_len / (sizeof(*extable)); + extable = image->extable; + + for (i = 0; i < nr_entries; i++) { + if (regs->ip == base + extable[i].insn) { + regs->ip = base + extable[i].fixup; + regs->di = trapnr; + regs->si = error_code; + regs->dx = fault_addr; + return true; + } + } + + return false; +} diff --git a/arch/x86/entry/vdso/extable.h b/arch/x86/entry/vdso/extable.h new file mode 100644 index 000000000000..b56f6b012941 --- /dev/null +++ b/arch/x86/entry/vdso/extable.h @@ -0,0 +1,28 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef __VDSO_EXTABLE_H +#define __VDSO_EXTABLE_H + +/* + * Inject exception fixup for vDSO code. Unlike normal exception fixup, + * vDSO uses a dedicated handler the addresses are relative to the overall + * exception table, not each individual entry. + */ +#ifdef __ASSEMBLY__ +#define _ASM_VDSO_EXTABLE_HANDLE(from, to) \ + ASM_VDSO_EXTABLE_HANDLE from to + +.macro ASM_VDSO_EXTABLE_HANDLE from:req to:req + .pushsection __ex_table, "a" + .long (\from) - __ex_table + .long (\to) - __ex_table + .popsection +.endm +#else +#define _ASM_VDSO_EXTABLE_HANDLE(from, to) \ + ".pushsection __ex_table, \"a\"\n" \ + ".long (" #from ") - __ex_table\n" \ + ".long (" #to ") - __ex_table\n" \ + ".popsection\n" +#endif + +#endif /* __VDSO_EXTABLE_H */ diff --git a/arch/x86/entry/vdso/vdso-layout.lds.S b/arch/x86/entry/vdso/vdso-layout.lds.S index 4d152933547d..dc8da7695859 100644 --- a/arch/x86/entry/vdso/vdso-layout.lds.S +++ b/arch/x86/entry/vdso/vdso-layout.lds.S @@ -75,11 +75,18 @@ SECTIONS * stuff that isn't used at runtime in between. */ - .text : { *(.text*) } :text =0x90909090, + .text : { + *(.text*) + *(.fixup) + } :text =0x90909090, + + .altinstructions : { *(.altinstructions) } :text .altinstr_replacement : { *(.altinstr_replacement) } :text + __ex_table : { *(__ex_table) } :text + /DISCARD/ : { *(.discard) *(.discard.*) diff --git a/arch/x86/entry/vdso/vdso2c.h b/arch/x86/entry/vdso/vdso2c.h index 6f46e11ce539..1c7cfac7e64a 100644 --- a/arch/x86/entry/vdso/vdso2c.h +++ b/arch/x86/entry/vdso/vdso2c.h @@ -5,6 +5,41 @@ * are built for 32-bit userspace. */ +static void BITSFUNC(copy)(FILE *outfile, const unsigned char *data, size_t len) +{ + size_t i; + + for (i = 0; i < len; i++) { + if (i % 10 == 0) + fprintf(outfile, "\n\t"); + fprintf(outfile, "0x%02X, ", (int)(data)[i]); + } +} + + +/* + * Extract a section from the input data into a standalone blob. Used to + * capture kernel-only data that needs to persist indefinitely, e.g. the + * exception fixup tables, but only in the kernel, i.e. the section can + * be stripped from the final vDSO image. + */ +static void BITSFUNC(extract)(const unsigned char *data, size_t data_len, + FILE *outfile, ELF(Shdr) *sec, const char *name) +{ + unsigned long offset; + size_t len; + + offset = (unsigned long)GET_LE(&sec->sh_offset); + len = (size_t)GET_LE(&sec->sh_size); + + if (offset + len > data_len) + fail("section to extract overruns input data"); + + fprintf(outfile, "static const unsigned char %s[%lu] = {", name, len); + BITSFUNC(copy)(outfile, data + offset, len); + fprintf(outfile, "\n};\n\n"); +} + static void BITSFUNC(go)(void *raw_addr, size_t raw_len, void *stripped_addr, size_t stripped_len, FILE *outfile, const char *image_name) @@ -15,7 +50,7 @@ static void BITSFUNC(go)(void *raw_addr, size_t raw_len, ELF(Ehdr) *hdr = (ELF(Ehdr) *)raw_addr; unsigned long i, syms_nr; ELF(Shdr) *symtab_hdr = NULL, *strtab_hdr, *secstrings_hdr, - *alt_sec = NULL; + *alt_sec = NULL, *extable_sec = NULL; ELF(Dyn) *dyn = 0, *dyn_end = 0; const char *secstrings; INT_BITS syms[NSYMS] = {}; @@ -77,6 +112,8 @@ static void BITSFUNC(go)(void *raw_addr, size_t raw_len, if (!strcmp(secstrings + GET_LE(&sh->sh_name), ".altinstructions")) alt_sec = sh; + if (!strcmp(secstrings + GET_LE(&sh->sh_name), "__ex_table")) + extable_sec = sh; } if (!symtab_hdr) @@ -155,6 +192,9 @@ static void BITSFUNC(go)(void *raw_addr, size_t raw_len, (int)((unsigned char *)stripped_addr)[i]); } fprintf(outfile, "\n};\n\n"); + if (extable_sec) + BITSFUNC(extract)(raw_addr, raw_len, outfile, + extable_sec, "extable"); fprintf(outfile, "const struct vdso_image %s = {\n", image_name); fprintf(outfile, "\t.data = raw_data,\n"); @@ -165,6 +205,14 @@ static void BITSFUNC(go)(void *raw_addr, size_t raw_len, fprintf(outfile, "\t.alt_len = %lu,\n", (unsigned long)GET_LE(&alt_sec->sh_size)); } + if (extable_sec) { + fprintf(outfile, "\t.extable_base = %lu,\n", + (unsigned long)GET_LE(&extable_sec->sh_offset)); + fprintf(outfile, "\t.extable_len = %lu,\n", + (unsigned long)GET_LE(&extable_sec->sh_size)); + fprintf(outfile, "\t.extable = extable,\n"); + } + for (i = 0; i < NSYMS; i++) { if (required_syms[i].export && syms[i]) fprintf(outfile, "\t.sym_%s = %" PRIi64 ",\n", diff --git a/arch/x86/include/asm/vdso.h b/arch/x86/include/asm/vdso.h index bbcdc7b8f963..b5d23470f56b 100644 --- a/arch/x86/include/asm/vdso.h +++ b/arch/x86/include/asm/vdso.h @@ -15,6 +15,8 @@ struct vdso_image { unsigned long size; /* Always a multiple of PAGE_SIZE */ unsigned long alt, alt_len; + unsigned long extable_base, extable_len; + const void *extable; long sym_vvar_start; /* Negative offset to the vvar area */ @@ -45,6 +47,9 @@ extern void __init init_vdso_image(const struct vdso_image *image); extern int map_vdso_once(const struct vdso_image *image, unsigned long addr); +extern bool fixup_vdso_exception(struct pt_regs *regs, int trapnr, + unsigned long error_code, + unsigned long fault_addr); #endif /* __ASSEMBLER__ */ #endif /* _ASM_X86_VDSO_H */