On Mon, Dec 10, 2018 at 3:31 PM Vincenzo Frascino vincenzo.frascino@arm.com wrote:
On arm64 the TCR_EL1.TBI0 bit has been set since Linux 3.x hence the userspace (EL0) is allowed to set a non-zero value in the top byte but the resulting pointers are not allowed at the user-kernel syscall ABI boundary.
This patchset proposes a relaxation of the ABI and a mechanism to advertise it to the userspace via an AT_FLAGS.
The rationale behind the choice of AT_FLAGS is that the Unix System V ABI defines AT_FLAGS as "flags", leaving some degree of freedom in interpretation. There are two previous attempts of using AT_FLAGS in the Linux Kernel for different reasons: the first was more generic and was used to expose the support for the GNU STACK NX feature [1] and the second was done for the MIPS architecture and was used to expose the support of "MIPS ABI Extension for IEEE Std 754 Non-Compliant Interlinking" [2]. Both the changes are currently _not_ merged in mainline. The only architecture that reserves some of the bits in AT_FLAGS is currently MIPS, which introduced the concept of platform specific ABI (psABI) reserving the top-byte [3].
When ARM64_AT_FLAGS_SYSCALL_TBI is set the kernel is advertising to the userspace that a relaxed ABI is supported hence this type of pointers are now allowed to be passed to the syscalls when they are in memory ranges obtained by anonymous mmap() or brk().
The userspace _must_ verify that the flag is set before passing tagged pointers to the syscalls allowed by this relaxation.
More in general, exposing the ARM64_AT_FLAGS_SYSCALL_TBI flag and mandating to the software to check that the feature is present, before using the associated functionality, it provides a degree of control on the decision of disabling such a feature in future without consequently breaking the userspace.
The change required a modification of the elf common code, because in Linux the AT_FLAGS are currently set to zero by default by the kernel.
The newly added flag has been verified on arm64 using the code below. #include <stdio.h> #include <stdbool.h> #include <sys/auxv.h>
#define ARM64_AT_FLAGS_SYSCALL_TBI (1 << 0)
bool arm64_syscall_tbi_is_present(void) { unsigned long at_flags = getauxval(AT_FLAGS); if (at_flags & ARM64_AT_FLAGS_SYSCALL_TBI) return true;
return false;}
void main() { if (arm64_syscall_tbi_is_present()) printf("ARM64_AT_FLAGS_SYSCALL_TBI is present\n"); }
This patchset should be merged together with [4].
[1] https://patchwork.ozlabs.org/patch/579578/ [2] https://lore.kernel.org/patchwork/cover/618280/ [3] ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/psABI_mips3.0.pdf [4] https://patchwork.kernel.org/cover/10674351/
ABI References:
Sco SysV ABI: http://www.sco.com/developers/gabi/2003-12-17/contents.html PowerPC AUXV: http://openpowerfoundation.org/wp-content/uploads/resources/leabi/content/db... AMD64 ABI: https://www.cs.tufts.edu/comp/40-2012f/readings/amd64-abi.pdf x86 ABI: https://www.uclibc.org/docs/psABI-i386.pdf MIPS ABI: ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/psABI_mips3.0.pdf ARM ABI: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0044f/IHI0044F_aaelf.pdf SPARC ABI: http://math-atlas.sourceforge.net/devel/assembly/abi_sysV_sparc.pdf
Cc: Catalin Marinas catalin.marinas@arm.com Cc: Will Deacon will.deacon@arm.com Cc: Mark Rutland mark.rutland@arm.com Cc: Robin Murphy robin.murphy@arm.com Cc: Kees Cook keescook@chromium.org Cc: Kate Stewart kstewart@linuxfoundation.org Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Andrew Morton akpm@linux-foundation.org Cc: Ingo Molnar mingo@kernel.org Cc: "Kirill A . Shutemov" kirill.shutemov@linux.intel.com Cc: Shuah Khan shuah@kernel.org Cc: Chintan Pandya cpandya@codeaurora.org Cc: Jacob Bramley Jacob.Bramley@arm.com Cc: Ruben Ayrapetyan Ruben.Ayrapetyan@arm.com Cc: Andrey Konovalov andreyknvl@google.com Cc: Lee Smith Lee.Smith@arm.com Cc: Kostya Serebryany kcc@google.com Cc: Dmitry Vyukov dvyukov@google.com, Cc: Ramana Radhakrishnan Ramana.Radhakrishnan@arm.com Cc: Luc Van Oostenryck luc.vanoostenryck@gmail.com Cc: Evgeniy Stepanov eugenis@google.com CC: Alexander Viro viro@zeniv.linux.org.uk Signed-off-by: Vincenzo Frascino vincenzo.frascino@arm.com
Vincenzo Frascino (3): elf: Make AT_FLAGS arch configurable arm64: Define Documentation/arm64/elf_at_flags.txt arm64: elf: Advertise relaxed ABI
Documentation/arm64/elf_at_flags.txt | 111 ++++++++++++++++++++++++++ arch/arm64/include/asm/atflags.h | 7 ++ arch/arm64/include/asm/elf.h | 5 ++ arch/arm64/include/uapi/asm/atflags.h | 8 ++ fs/binfmt_elf.c | 6 +- fs/binfmt_elf_fdpic.c | 6 +- fs/compat_binfmt_elf.c | 5 ++ 7 files changed, 146 insertions(+), 2 deletions(-) create mode 100644 Documentation/arm64/elf_at_flags.txt create mode 100644 arch/arm64/include/asm/atflags.h create mode 100644 arch/arm64/include/uapi/asm/atflags.h
-- 2.19.2
Acked-by: Andrey Konovalov andreyknvl@google.com