RESEND [PATCH v2 3/3] arm64: compat: Add CONFIG_KUSER_HELPERS

Mark Salyzyn
2018-06-18T15:09:33Z

From: Kevin Brodsky <kevin.brodsky@arm.com>

Make it possible to disable the kuser helpers by adding a KUSER_HELPERS
config option (enabled by default). When disabled, all kuser
helpers-related code is removed from the kernel and no mapping is done
at the fixed high address (0xffff0000); any attempt to use a kuser
helper from a 32-bit process will result in a segfault.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Cc: James Morse <james.morse@arm.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dmitry Safonov <dsafonov@virtuozzo.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Laura Abbott <labbott@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Andy Gross <andy.gross@linaro.org>
Cc: Andrew Pinski <apinski@cavium.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: Jeremy Linton <Jeremy.Linton@arm.com>

v2:
- split off assembler changes to a new previous patch in series to reduce churn
- modify slightly the feature documentation to reduce its reach
- modify slightly the feature documentation to rationalize the yes default.
- There are more ifdefs as a result of the rebase.

v3:
- rebase
---
arch/arm64/Kconfig | 30 ++++++++++++++++++++++++++++++
arch/arm64/kernel/Makefile | 2 +-
arch/arm64/kernel/vdso.c | 10 ++++++++++
3 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 42c090cf0292..11b4c6aef7d7 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1296,6 +1296,36 @@ config COMPAT

If you want to execute 32-bit userspace applications, say Y.

+config KUSER_HELPERS
+ bool "Enable the kuser helpers page in 32-bit processes"
+ depends on COMPAT
+ default y
+ help
+ Warning: disabling this option may break 32-bit applications.
+
+ Provide kuser helpers in a special purpose fixed-address page. The
+ kernel provides helper code to userspace in read-only form at a fixed
+ location to allow userspace to be independent of the CPU type fitted
+ to the system. This permits 32-bit binaries to be run on ARMv6 through
+ to ARMv8 without modification.
+
+ See Documentation/arm/kernel_user_helpers.txt for details.
+
+ However, the fixed-address nature of these helpers can be used by ROP
+ (return-orientated programming) authors when creating exploits.
+
+ If all of the 32-bit binaries and libraries that run on your platform
+ are built specifically for your platform, and make no use of these
+ helpers, then you can turn this option off to hinder such exploits.
+ However, in that case, if a binary or library relying on those helpers
+ is run, it will receive a SIGSEGV signal, which will terminate the
+ program. Typically, binaries compiled for ARMv7 or later do not use
+ the kuser helpers.
+
+ Say N here only if you are absolutely certain that you do not need
+ these helpers; otherwise, the safe option is to say Y (the default
+ for now)
+
config SYSVIPC_COMPAT
def_bool y
depends on COMPAT && SYSVIPC
diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 9851be3ef932..c16be3cf39bf 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -29,7 +29,7 @@ $(obj)/%.stub.o: $(obj)/%.o FORCE
arm64-obj-$(CONFIG_COMPAT) += sys32.o signal32.o \
sys_compat.o entry32.o
arm64-obj-$(CONFIG_COMPAT) += sigreturn32.o
-arm64-obj-$(CONFIG_COMPAT) += kuser32.o
+arm64-obj-$(CONFIG_KUSER_HELPERS) += kuser32.o
arm64-obj-$(CONFIG_FUNCTION_TRACER) += ftrace.o entry-ftrace.o
arm64-obj-$(CONFIG_MODULES) += arm64ksyms.o module.o
arm64-obj-$(CONFIG_ARM64_MODULE_PLTS) += module-plts.o
diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
index 5398f6454ce1..76a94bed4bd5 100644
--- a/arch/arm64/kernel/vdso.c
+++ b/arch/arm64/kernel/vdso.c
@@ -62,18 +62,22 @@ static const struct vm_special_mapping compat_vdso_spec[] = {
.name = "[sigpage]",
.pages = &vectors_page[0],
},
+#ifdef CONFIG_KUSER_HELPERS
{
.name = "[kuserhelpers]",
.pages = &vectors_page[1],
},
+#endif
};
static struct page *vectors_page[ARRAY_SIZE(compat_vdso_spec)] __ro_after_init;

static int __init alloc_vectors_page(void)
{
+#ifdef CONFIG_KUSER_HELPERS
extern char __kuser_helper_start[], __kuser_helper_end[];
size_t kuser_sz = __kuser_helper_end - __kuser_helper_start;
unsigned long kuser_vpage;
+#endif

extern char __aarch32_sigret_code_start[], __aarch32_sigret_code_end[];
size_t sigret_sz =
@@ -84,22 +88,26 @@ static int __init alloc_vectors_page(void)
if (!sigret_vpage)
return -ENOMEM;

+#ifdef CONFIG_KUSER_HELPERS
kuser_vpage = get_zeroed_page(GFP_ATOMIC);
if (!kuser_vpage) {
free_page(sigret_vpage);
return -ENOMEM;
}
+#endif

/* sigreturn code */
memcpy((void *)sigret_vpage, __aarch32_sigret_code_start, sigret_sz);
flush_icache_range(sigret_vpage, sigret_vpage + PAGE_SIZE);
vectors_page[0] = virt_to_page(sigret_vpage);

+#ifdef CONFIG_KUSER_HELPERS
/* kuser helpers */
memcpy((void *)kuser_vpage + 0x1000 - kuser_sz, __kuser_helper_start,
kuser_sz);
flush_icache_range(kuser_vpage, kuser_vpage + PAGE_SIZE);
vectors_page[1] = virt_to_page(kuser_vpage);
+#endif

return 0;
}
@@ -128,11 +136,13 @@ int aarch32_setup_vectors_page(struct linux_binprm *bprm, int uses_interp)

current->mm->context.vdso = (void *)addr;

+#ifdef CONFIG_KUSER_HELPERS
/* Map the kuser helpers at the ABI-defined high address. */
ret = _install_special_mapping(mm, AARCH32_KUSER_HELPERS_BASE,
PAGE_SIZE,
VM_READ|VM_EXEC|VM_MAYREAD|VM_MAYEXEC,
&compat_vdso_spec[1]);
+#endif
out:
up_write(&mm->mmap_sem);

--
2.18.0.rc1.244.gcf134e6275-goog