Date: Tue, 23 Apr 2019 11:12:10 -0700 From: Kees Cook <keescook@...omium.org> To: Andrew Morton <akpm@...ux-foundation.org> Cc: Hector Marco-Gisbert <hecmargi@....es>, Jason Gunthorpe <jgg@...lanox.com>, linux-kernel@...r.kernel.org, x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>, Andy Lutomirski <luto@...capital.net>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Mark Rutland <mark.rutland@....com>, linux-arm-kernel@...ts.infradead.org Subject: [PATCH] binfmt_elf: Update READ_IMPLIES_EXEC logic for modern CPUs The READ_IMPLIES_EXEC work-around was designed for old CPUs lacking NX (to have the visible permission flags on memory regions reflect reality: they are all executable), and for old toolchains that lacked the ELF PT_GNU_STACK marking (under the assumption than toolchains that couldn't even specify memory protection flags may have it wrong for all memory regions). This logic is sensible, 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. This is not a reasonable assumption for CPUs that have had NX support from the start (or very close to the start). This confusion has led to situations where modern 64-bit 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.) To fix this, elf_read_implies_exec() is adjusted on arm64 (where NX has always existed and all toolchains include PT_GNU_STACK), and x86 is adjusted to handle this combination of possible outcomes: CPU: | lacks NX | has NX, ia32 | has NX, x86_64 | ELF: | | | | ------------------------------|------------------|------------------| missing GNU_STACK | needs RIE | needs RIE | no RIE | GNU_STACK == RWX | needs RIE | no RIE: stack X | no RIE: stack X | GNU_STACK == RW | needs RIE | no RIE: stack NX | no RIE: stack NX | This has the effect of making binfmt_elf's EXSTACK_DEFAULT actually take on the correct architecture default of being non-executable on arm64 and x86_64, and being executable on ia32.  https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com Suggested-by: Hector Marco-Gisbert <hecmargi@....es> Signed-off-by: Kees Cook <keescook@...omium.org> --- arch/arm64/include/asm/elf.h | 9 ++++++++- arch/x86/include/asm/elf.h | 24 +++++++++++++++++++++--- 2 files changed, 29 insertions(+), 4 deletions(-) diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h index 6adc1a90e7e6..7fbd295a76d2 100644 --- a/arch/arm64/include/asm/elf.h +++ b/arch/arm64/include/asm/elf.h @@ -107,7 +107,14 @@ */ #define elf_check_arch(x) ((x)->e_machine == EM_AARCH64) -#define elf_read_implies_exec(ex,stk) (stk != EXSTACK_DISABLE_X) +/* + * 64-bit processes should not automatically gain READ_IMPLIES_EXEC. Only + * 32-bit processes without PT_GNU_STACK should trigger READ_IMPLIES_EXEC + * out of an abundance of caution against ancient toolchains not knowing + * how to mark memory protection flags correctly. + */ +#define elf_read_implies_exec(ex, stk) \ + (is_compat_task() && stk == EXSTACK_DEFAULT) #define CORE_DUMP_USE_REGSET #define ELF_EXEC_PAGESIZE PAGE_SIZE diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h index 69c0f892e310..87d9cf7643b4 100644 --- a/arch/x86/include/asm/elf.h +++ b/arch/x86/include/asm/elf.h @@ -280,10 +280,28 @@ extern u32 elf_hwcap2; /* * An executable for which elf_read_implies_exec() returns TRUE will - * have the READ_IMPLIES_EXEC personality flag set automatically. + * have the READ_IMPLIES_EXEC personality flag set automatically. This + * is needed either to show the truth about a memory mapping (i.e. CPUs + * that lack NX have all memory implicitly executable, so this makes + * sure that the visible permissions reflect reality), or to deal with + * old toolchains on new CPUs. Old binaries entirely lacking a GNU_STACK + * indicate they were likely built with a toolchain that has no idea about + * memory permissions, and so we must default to the lowest reasonable + * common denominator for the architecture: on ia32 we assume all memory + * to be executable by default, and on x86_64 we assume all memory to be + * non-executable by default. + * + * CPU: | lacks NX | has NX, ia32 | has NX, x86_64 | + * ELF: | | | | + * ------------------------------|------------------|------------------| + * missing GNU_STACK | needs RIE | needs RIE | no RIE | + * GNU_STACK == RWX | needs RIE | no RIE: stack X | no RIE: stack X | + * GNU_STACK == RW | needs RIE | no RIE: stack NX | no RIE: stack NX | + * */ -#define elf_read_implies_exec(ex, executable_stack) \ - (executable_stack != EXSTACK_DISABLE_X) +#define elf_read_implies_exec(ex, stk) \ + (!(__supported_pte_mask & _PAGE_NX) ? 1 : \ + (mmap_is_ia32() && stk == EXSTACK_DEFAULT)) struct task_struct; -- 2.17.1 -- Kees Cook
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