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Date: Wed, 6 Dec 2017 10:25:35 +0000
From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
To: Will Deacon <will.deacon@....com>
Cc: "linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Catalin Marinas <catalin.marinas@....com>,
Mark Rutland <mark.rutland@....com>,
Stephen Boyd <sboyd@...eaurora.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Kees Cook <keescook@...omium.org>,
Mark Salter <msalter@...hat.com>,
Laura Abbott <labbott@...hat.com>,
"tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH v2 10/18] arm64: entry: Add exception trampoline page for
exceptions from EL0
On 30 November 2017 at 16:39, Will Deacon <will.deacon@....com> wrote:
> To allow unmapping of the kernel whilst running at EL0, we need to
> point the exception vectors at an entry trampoline that can map/unmap
> the kernel on entry/exit respectively.
>
> This patch adds the trampoline page, although it is not yet plugged
> into the vector table and is therefore unused.
>
> Signed-off-by: Will Deacon <will.deacon@....com>
> ---
> arch/arm64/kernel/entry.S | 86 +++++++++++++++++++++++++++++++++++++++++
> arch/arm64/kernel/vmlinux.lds.S | 17 ++++++++
> 2 files changed, 103 insertions(+)
>
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index d454d8ed45e4..dea196f287a0 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -28,6 +28,8 @@
> #include <asm/errno.h>
> #include <asm/esr.h>
> #include <asm/irq.h>
> +#include <asm/memory.h>
> +#include <asm/mmu.h>
> #include <asm/processor.h>
> #include <asm/ptrace.h>
> #include <asm/thread_info.h>
> @@ -943,6 +945,90 @@ __ni_sys_trace:
>
> .popsection // .entry.text
>
> +#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> +/*
> + * Exception vectors trampoline.
> + */
> + .pushsection ".entry.tramp.text", "ax"
> +
> + .macro tramp_map_kernel, tmp
> + mrs \tmp, ttbr1_el1
> + sub \tmp, \tmp, #(SWAPPER_DIR_SIZE + RESERVED_TTBR0_SIZE)
> + bic \tmp, \tmp, #USER_ASID_FLAG
> + msr ttbr1_el1, \tmp
> + .endm
> +
> + .macro tramp_unmap_kernel, tmp
> + mrs \tmp, ttbr1_el1
> + add \tmp, \tmp, #(SWAPPER_DIR_SIZE + RESERVED_TTBR0_SIZE)
> + orr \tmp, \tmp, #USER_ASID_FLAG
> + msr ttbr1_el1, \tmp
> + /*
> + * We avoid running the post_ttbr_update_workaround here because the
> + * user and kernel ASIDs don't have conflicting mappings, so any
> + * "blessing" as described in:
> + *
> + * http://lkml.kernel.org/r/56BB848A.6060603@caviumnetworks.com
> + *
> + * will not hurt correctness. Whilst this may partially defeat the
> + * point of using split ASIDs in the first place, it avoids
> + * the hit of invalidating the entire I-cache on every return to
> + * userspace.
> + */
> + .endm
> +
> + .macro tramp_ventry, regsize = 64
> + .align 7
> +1:
> + .if \regsize == 64
> + msr tpidrro_el0, x30
> + .endif
> + tramp_map_kernel x30
> + ldr x30, =vectors
Could we move this literal into the next page, and only map that in
the kernel page tables? It's the only piece of information in the
trampoline page that can reveal the true location of the kernel, and
moving it out is trivial to implement on top of the changes you are
already making to harden KASLR.
> + prfm plil1strm, [x30, #(1b - tramp_vectors)]
> + msr vbar_el1, x30
> + add x30, x30, #(1b - tramp_vectors)
> + isb
> + br x30
> + .endm
> +
> + .macro tramp_exit, regsize = 64
> + adr x30, tramp_vectors
> + msr vbar_el1, x30
> + tramp_unmap_kernel x30
> + .if \regsize == 64
> + mrs x30, far_el1
> + .endif
> + eret
> + .endm
> +
> + .align 11
> +ENTRY(tramp_vectors)
> + .space 0x400
> +
> + tramp_ventry
> + tramp_ventry
> + tramp_ventry
> + tramp_ventry
> +
> + tramp_ventry 32
> + tramp_ventry 32
> + tramp_ventry 32
> + tramp_ventry 32
> +END(tramp_vectors)
> +
> +ENTRY(tramp_exit_native)
> + tramp_exit
> +END(tramp_exit_native)
> +
> +ENTRY(tramp_exit_compat)
> + tramp_exit 32
> +END(tramp_exit_compat)
> +
> + .ltorg
> + .popsection // .entry.tramp.text
> +#endif /* CONFIG_UNMAP_KERNEL_AT_EL0 */
> +
> /*
> * Special system call wrappers.
> */
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index 7da3e5c366a0..6b4260f22aab 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -57,6 +57,17 @@ jiffies = jiffies_64;
> #define HIBERNATE_TEXT
> #endif
>
> +#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> +#define TRAMP_TEXT \
> + . = ALIGN(PAGE_SIZE); \
> + VMLINUX_SYMBOL(__entry_tramp_text_start) = .; \
> + *(.entry.tramp.text) \
> + . = ALIGN(PAGE_SIZE); \
> + VMLINUX_SYMBOL(__entry_tramp_text_end) = .;
> +#else
> +#define TRAMP_TEXT
> +#endif
> +
> /*
> * The size of the PE/COFF section that covers the kernel image, which
> * runs from stext to _edata, must be a round multiple of the PE/COFF
> @@ -113,6 +124,7 @@ SECTIONS
> HYPERVISOR_TEXT
> IDMAP_TEXT
> HIBERNATE_TEXT
> + TRAMP_TEXT
> *(.fixup)
> *(.gnu.warning)
> . = ALIGN(16);
> @@ -214,6 +226,11 @@ SECTIONS
> . += RESERVED_TTBR0_SIZE;
> #endif
>
> +#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> + tramp_pg_dir = .;
> + . += PAGE_SIZE;
> +#endif
> +
> __pecoff_data_size = ABSOLUTE(. - __initdata_begin);
> _end = .;
>
> --
> 2.1.4
>
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