Date: Tue, 23 Apr 2019 11:51:28 -0700 From: akpm@...ux-foundation.org To: a.p.zijlstra@...llo.nl, hecmargi@....es, hpa@...or.com, jgg@...lanox.com, keescook@...omium.org, kernel-hardening@...ts.openwall.com, luto@...capital.net, mark.rutland@....com, mingo@...e.hu, mm-commits@...r.kernel.org, tglx@...utronix.de Subject: + binfmt_elf-update-read_implies_exec-logic-for-modern-cpus.patch added to -mm tree The patch titled Subject: binfmt_elf: Update READ_IMPLIES_EXEC logic for modern CPUs has been added to the -mm tree. Its filename is binfmt_elf-update-read_implies_exec-logic-for-modern-cpus.patch This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/binfmt_elf-update-read_implies_exec-logic-for-modern-cpus.patch and later at http://ozlabs.org/~akpm/mmotm/broken-out/binfmt_elf-update-read_implies_exec-logic-for-modern-cpus.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Kees Cook <keescook@...omium.org> Subject: 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 Link: http://lkml.kernel.org/r/20190423181210.GA2443@beast Signed-off-by: Kees Cook <keescook@...omium.org> Suggested-by: Hector Marco-Gisbert <hecmargi@....es> Cc: Jason Gunthorpe <jgg@...lanox.com> Cc: Thomas Gleixner <tglx@...utronix.de> Cc: Andy Lutomirski <luto@...capital.net> Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com> Cc: Mark Rutland <mark.rutland@....com> Cc: Ingo Molnar <mingo@...e.hu> Cc: "H. Peter Anvin" <hpa@...or.com> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org> --- arch/arm64/include/asm/elf.h | 9 ++++++++- arch/x86/include/asm/elf.h | 24 +++++++++++++++++++++--- 2 files changed, 29 insertions(+), 4 deletions(-) --- a/arch/arm64/include/asm/elf.h~binfmt_elf-update-read_implies_exec-logic-for-modern-cpus +++ a/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 --- a/arch/x86/include/asm/elf.h~binfmt_elf-update-read_implies_exec-logic-for-modern-cpus +++ a/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; _ Patches currently in -mm which might be from keescook@...omium.org are binfmt_elf-move-brk-out-of-mmap-when-doing-direct-loader-exec.patch binfmt_elf-update-read_implies_exec-logic-for-modern-cpus.patch
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