Date: Wed, 24 Apr 2019 21:51:17 +0100 From: Will Deacon <will.deacon@....com> To: Kees Cook <keescook@...omium.org> Cc: Andrew Morton <akpm@...ux-foundation.org>, Hector Marco-Gisbert <hecmargi@....es>, Marc Gonzalez <marc.w.gonzalez@...e.fr>, Jason Gunthorpe <jgg@...lanox.com>, x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>, Andy Lutomirski <luto@...capital.net>, Stephen Rothwell <sfr@...b.auug.org.au>, Catalin Marinas <catalin.marinas@....com>, Mark Rutland <mark.rutland@....com>, Arnd Bergmann <arnd@...db.de>, Linux ARM <linux-arm-kernel@...ts.infradead.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH v2] binfmt_elf: Update READ_IMPLIES_EXEC logic for modern CPUs On Wed, Apr 24, 2019 at 01:34:08PM -0700, Kees Cook wrote: > 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 that 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 toolchains have implemented PT_GNU_STACK for a while), > 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> > --- > v2: adjust arm64 to avoid is_compat_task() (marc.w.gonzalez) > --- > arch/arm64/include/asm/elf.h | 8 +++++++- > arch/x86/include/asm/elf.h | 24 +++++++++++++++++++++--- > 2 files changed, 28 insertions(+), 4 deletions(-) > > diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h > index 6adc1a90e7e6..f1bb4b388b8f 100644 > --- a/arch/arm64/include/asm/elf.h > +++ b/arch/arm64/include/asm/elf.h > @@ -107,7 +107,13 @@ > */ > #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 compat_elf_read_implies_exec(ex, stk) (stk == EXSTACK_DEFAULT) Don't you need to hack fs/compat_binfmt_elf.c to pick this up, or am I missing some trick? Should just be something like below. Will --->8 diff --git a/fs/compat_binfmt_elf.c b/fs/compat_binfmt_elf.c index 15f6e96b3bd9..694bc3ee77eb 100644 --- a/fs/compat_binfmt_elf.c +++ b/fs/compat_binfmt_elf.c @@ -116,6 +116,11 @@ #define arch_setup_additional_pages compat_arch_setup_additional_pages #endif +#ifdef compat_elf_read_implies_exec +#undef elf_read_implies_exec +#define elf_read_implies_exec compat_elf_read_implies_exec +#endif + /* * Rename a few of the symbols that binfmt_elf.c will define. * These are all local so the names don't really matter, but it
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