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The READ_IMPLIES_EXEC work-around was designed for old toolchains that lacked the ELF PT_GNU_STACK marking under the assumption that toolchains that couldn't specify executable permission flags for the stack may not know how to do it correctly for any memory region. This logic is sensible for having ancient binaries coexist in a system with possibly NX memory, but was implemented in a way that equated having a PT_GNU_STACK marked executable as being as "broken" as lacking the PT_GNU_STACK marking entirely. Things like unmarked assembly and stack trampolines may cause PT_GNU_STACK to need an executable bit, but they do not imply all mappings must be executable. This confusion has led to situations where modern programs with explicitly marked executable stack are forced into the READ_IMPLIES_EXEC state when no such thing is needed. (And leads to unexpected failures when mmap()ing regions of device driver memory that wish to disallow VM_EXEC[1].) In looking for other reasons for the READ_IMPLIES_EXEC behavior, Jann Horn noted that glibc thread stacks have always been marked RWX (until 2003 when they started tracking the PT_GNU_STACK flag instead[2]). And musl doesn't support executable stacks at all[3]. As such, no breakage for multithreaded applications is expected from this change. This changes arm32 and arm64 compat together, to keep behavior the same. [1] https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com [2] https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=54ee14b3882 [3] https://lkml.kernel.org/r/20190423192534.GN23599@brightrain.aerifal.cx Suggested-by: Hector Marco-Gisbert <hecmargi@upv.es> Signed-off-by: Kees Cook <keescook@chromium.org> Signed-off-by: Borislav Petkov <bp@suse.de> Reviewed-by: Jason Gunthorpe <jgg@mellanox.com> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> Link: https://lkml.kernel.org/r/20200327064820.12602-6-keescook@chromium.org
133 lines
3.8 KiB
C
133 lines
3.8 KiB
C
// SPDX-License-Identifier: GPL-2.0
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#include <linux/export.h>
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#include <linux/sched.h>
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#include <linux/personality.h>
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#include <linux/binfmts.h>
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#include <linux/elf.h>
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#include <linux/elf-fdpic.h>
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#include <asm/system_info.h>
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int elf_check_arch(const struct elf32_hdr *x)
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{
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unsigned int eflags;
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/* Make sure it's an ARM executable */
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if (x->e_machine != EM_ARM)
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return 0;
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/* Make sure the entry address is reasonable */
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if (x->e_entry & 1) {
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if (!(elf_hwcap & HWCAP_THUMB))
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return 0;
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} else if (x->e_entry & 3)
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return 0;
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eflags = x->e_flags;
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if ((eflags & EF_ARM_EABI_MASK) == EF_ARM_EABI_UNKNOWN) {
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unsigned int flt_fmt;
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/* APCS26 is only allowed if the CPU supports it */
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if ((eflags & EF_ARM_APCS_26) && !(elf_hwcap & HWCAP_26BIT))
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return 0;
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flt_fmt = eflags & (EF_ARM_VFP_FLOAT | EF_ARM_SOFT_FLOAT);
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/* VFP requires the supporting code */
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if (flt_fmt == EF_ARM_VFP_FLOAT && !(elf_hwcap & HWCAP_VFP))
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return 0;
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}
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return 1;
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}
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EXPORT_SYMBOL(elf_check_arch);
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void elf_set_personality(const struct elf32_hdr *x)
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{
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unsigned int eflags = x->e_flags;
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unsigned int personality = current->personality & ~PER_MASK;
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/*
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* We only support Linux ELF executables, so always set the
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* personality to LINUX.
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*/
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personality |= PER_LINUX;
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/*
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* APCS-26 is only valid for OABI executables
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*/
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if ((eflags & EF_ARM_EABI_MASK) == EF_ARM_EABI_UNKNOWN &&
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(eflags & EF_ARM_APCS_26))
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personality &= ~ADDR_LIMIT_32BIT;
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else
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personality |= ADDR_LIMIT_32BIT;
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set_personality(personality);
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/*
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* Since the FPA coprocessor uses CP1 and CP2, and iWMMXt uses CP0
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* and CP1, we only enable access to the iWMMXt coprocessor if the
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* binary is EABI or softfloat (and thus, guaranteed not to use
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* FPA instructions.)
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*/
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if (elf_hwcap & HWCAP_IWMMXT &&
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eflags & (EF_ARM_EABI_MASK | EF_ARM_SOFT_FLOAT)) {
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set_thread_flag(TIF_USING_IWMMXT);
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} else {
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clear_thread_flag(TIF_USING_IWMMXT);
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}
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}
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EXPORT_SYMBOL(elf_set_personality);
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/*
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* An executable for which elf_read_implies_exec() returns TRUE will
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* have the READ_IMPLIES_EXEC personality flag set automatically.
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*
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* The decision process for determining the results are:
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*
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* CPU: | lacks NX* | has NX |
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* ELF: | | |
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* ---------------------|------------|------------|
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* missing PT_GNU_STACK | exec-all | exec-all |
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* PT_GNU_STACK == RWX | exec-all | exec-stack |
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* PT_GNU_STACK == RW | exec-all | exec-none |
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*
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* exec-all : all PROT_READ user mappings are executable, except when
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* backed by files on a noexec-filesystem.
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* exec-none : only PROT_EXEC user mappings are executable.
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* exec-stack: only the stack and PROT_EXEC user mappings are executable.
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*
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* *this column has no architectural effect: NX markings are ignored by
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* hardware, but may have behavioral effects when "wants X" collides with
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* "cannot be X" constraints in memory permission flags, as in
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* https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com
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*
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*/
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int arm_elf_read_implies_exec(int executable_stack)
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{
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if (executable_stack == EXSTACK_DEFAULT)
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return 1;
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if (cpu_architecture() < CPU_ARCH_ARMv6)
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return 1;
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return 0;
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}
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EXPORT_SYMBOL(arm_elf_read_implies_exec);
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#if defined(CONFIG_MMU) && defined(CONFIG_BINFMT_ELF_FDPIC)
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void elf_fdpic_arch_lay_out_mm(struct elf_fdpic_params *exec_params,
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struct elf_fdpic_params *interp_params,
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unsigned long *start_stack,
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unsigned long *start_brk)
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{
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elf_set_personality(&exec_params->hdr);
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exec_params->load_addr = 0x8000;
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interp_params->load_addr = ELF_ET_DYN_BASE;
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*start_stack = TASK_SIZE - SZ_16M;
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if ((exec_params->flags & ELF_FDPIC_FLAG_ARRANGEMENT) == ELF_FDPIC_FLAG_INDEPENDENT) {
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exec_params->flags &= ~ELF_FDPIC_FLAG_ARRANGEMENT;
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exec_params->flags |= ELF_FDPIC_FLAG_CONSTDISP;
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}
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}
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#endif
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