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Date: Thu, 25 Jan 2024 16:44:50 -0800
From: Deepak Gupta <debug@rivosinc.com>
To: Charlie Jenkins <charlie@rivosinc.com>
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Subject: Re: [RFC PATCH v1 15/28] riscv/mm: Implement map_shadow_stack()
 syscall
Message-ID: <ZbMAgl5RqaUEs3Dr@debug.ba.rivosinc.com>
References: <20240125062739.1339782-1-debug@rivosinc.com>
 <20240125062739.1339782-16-debug@rivosinc.com>
 <ZbLRgEVZvh7LE+k/@ghost>
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On Thu, Jan 25, 2024 at 01:24:16PM -0800, Charlie Jenkins wrote:
>On Wed, Jan 24, 2024 at 10:21:40PM -0800, debug@rivosinc.com wrote:
>> From: Deepak Gupta <debug@rivosinc.com>
>>
>> As discussed extensively in the changelog for the addition of this
>> syscall on x86 ("x86/shstk: Introduce map_shadow_stack syscall") the
>> existing mmap() and madvise() syscalls do not map entirely well onto the
>> security requirements for guarded control stacks since they lead to
>> windows where memory is allocated but not yet protected or stacks which
>> are not properly and safely initialised. Instead a new syscall
>> map_shadow_stack() has been defined which allocates and initialises a
>> shadow stack page.
>>
>> This patch implements this syscall for riscv. riscv doesn't require token
>> to be setup by kernel because user mode can do that by itself. However to
>> provide compatiblity and portability with other architectues, user mode can
>> specify token set flag.
>>
>> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
>> ---
>>  arch/riscv/kernel/Makefile      |   2 +
>>  arch/riscv/kernel/usercfi.c     | 150 ++++++++++++++++++++++++++++++++
>>  include/uapi/asm-generic/mman.h |   1 +
>>  3 files changed, 153 insertions(+)
>>  create mode 100644 arch/riscv/kernel/usercfi.c
>>
>> diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
>> index fee22a3d1b53..8c668269e886 100644
>> --- a/arch/riscv/kernel/Makefile
>> +++ b/arch/riscv/kernel/Makefile
>> @@ -102,3 +102,5 @@ obj-$(CONFIG_COMPAT)		+= compat_vdso/
>>
>>  obj-$(CONFIG_64BIT)		+= pi/
>>  obj-$(CONFIG_ACPI)		+= acpi.o
>> +
>> +obj-$(CONFIG_RISCV_USER_CFI) += usercfi.o
>> diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c
>> new file mode 100644
>> index 000000000000..35ede2cbc05b
>> --- /dev/null
>> +++ b/arch/riscv/kernel/usercfi.c
>> @@ -0,0 +1,150 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Copyright (C) 2023 Rivos, Inc.
>Nit: Should be updated to 2024

noted

>> + * Deepak Gupta <debug@rivosinc.com>
>> + */
>> +
>> +#include <linux/sched.h>
>> +#include <linux/bitops.h>
>> +#include <linux/types.h>
>> +#include <linux/mm.h>
>> +#include <linux/mman.h>
>> +#include <linux/uaccess.h>
>> +#include <linux/sizes.h>
>> +#include <linux/user.h>
>> +#include <linux/syscalls.h>
>> +#include <linux/prctl.h>
>> +#include <asm/csr.h>
>> +#include <asm/usercfi.h>
>> +
>> +#define SHSTK_ENTRY_SIZE sizeof(void *)
>> +
>> +/*
>> + * Writes on shadow stack can either be `sspush` or `ssamoswap`. `sspush` can happen
>> + * implicitly on current shadow stack pointed to by CSR_SSP. `ssamoswap` takes pointer to
>> + * shadow stack. To keep it simple, we plan to use `ssamoswap` to perform writes on shadow
>> + * stack.
>> + */
>> +static noinline unsigned long amo_user_shstk(unsigned long *addr, unsigned long val)
>> +{
>> +	/*
>> +	 * In case ssamoswap faults, return -1.
>> +	 * Never expect -1 on shadow stack. Expect return addresses and zero
>> +	 */
>> +	unsigned long swap = -1;
>> +
>> +	__enable_user_access();
>> +	asm_volatile_goto(
>> +				".option push\n"
>> +				".option arch, +zicfiss\n"
>> +#ifdef CONFIG_64BIT
>> +				"1: ssamoswap.d %0, %2, %1\n"
>> +#else
>> +				"1: ssamoswap.w %0, %2, %1\n"
>
>A SSAMOSWAP macro that conditionally defines this would be cleaner

Yes I need to do that. Infact I need to gate CONFIG_RISCV_USER_CFI behind
some riscv-gnu toolchain version as well. Becuase not all toolchain versions will
recognize this.

>
>> +#endif
>> +				_ASM_EXTABLE(1b, %l[fault])
>> +				RISCV_ACQUIRE_BARRIER
>> +				".option pop\n"
>> +				: "=r" (swap), "+A" (*addr)
>
>I just ran into this on one of my patches that not every compiler
>supports output args in asm goto blocks. You need to guard this with the
>kconfig option CC_HAS_ASM_GOTO_TIED_OUTPUT. Unfortunately, that means
>that this code needs two versions, or you can choose to gate CFI behind
>this option, it's supported by recent versions of GCC/CLANG.

Thanks.
I'll gate behind CC_HAS_ASM_GOTO_TIED_OUTPUT. Earlier versions of GCC/CLANG
won't have CFI support in them anyways.

>
>For readability it is also nice to use labels for the asm variables such
>as `"=r" (swap)` can be `[swap] "=r" (swap)` and then replace %0 with
>%[swap].

noted, will do that. I copied it from gcc asm snippet `amoswap` somewhere in
kernel. Goes without saying, I am terrible with gcc asm syntax.

>
>- Charlie
>
>> +				: "r" (val)
>> +				: "memory"
>> +				: fault