linux-kernel - Re: [PATCH 8/9] RISC-V: User-facing API

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Date:   Wed, 5 Jul 2017 11:24:33 +0100
From:   James Hogan <james.hogan@...tec.com>
To:     Palmer Dabbelt <palmer@...belt.com>
CC:     <peterz@...radead.org>, <mingo@...hat.com>, <mcgrof@...nel.org>,
        <viro@...iv.linux.org.uk>, <sfr@...b.auug.org.au>,
        <nicolas.dichtel@...nd.com>, <rmk+kernel@...linux.org.uk>,
        <msalter@...hat.com>, <tklauser@...tanz.ch>, <will.deacon@....com>,
        <paul.gortmaker@...driver.com>, <linux@...ck-us.net>,
        <linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>,
        <albert@...ive.com>, <patches@...ups.riscv.org>
Subject: Re: [PATCH 8/9] RISC-V: User-facing API

On Tue, Jul 04, 2017 at 12:51:01PM -0700, Palmer Dabbelt wrote:
> diff --git a/arch/riscv/kernel/ptrace.c b/arch/riscv/kernel/ptrace.c
> new file mode 100644
> index 000000000000..2720d5e97354
> --- /dev/null
> +++ b/arch/riscv/kernel/ptrace.c
> @@ -0,0 +1,138 @@

> +/* Put registers back to task. */
> +static void putregs(struct task_struct *child, struct pt_regs *uregs)
> +{
> +	struct pt_regs *regs = task_pt_regs(child);
> +	*regs = *uregs;
> +}
> +
> +static int riscv_gpr_get(struct task_struct *target,
> +			 const struct user_regset *regset,
> +			 unsigned int pos, unsigned int count,
> +			 void *kbuf, void __user *ubuf)
> +{
> +	struct pt_regs *regs;
> +
> +	regs = task_pt_regs(target);
> +	return user_regset_copyout(&pos, &count, &kbuf, &ubuf, regs, 0,
> +				   sizeof(*regs));

sizeof(struct pt_regs) > sizeof(struct user_regs_struct), which allows
supervisor registers to be copied too. I think you should be using
sizeof(struct user_regs_struct) instead.

> +}
> +
> +static int riscv_gpr_set(struct task_struct *target,
> +			 const struct user_regset *regset,
> +			 unsigned int pos, unsigned int count,
> +			 const void *kbuf, const void __user *ubuf)
> +{
> +	int ret;
> +	struct pt_regs regs;
> +
> +	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &regs, 0,
> +				 sizeof(regs));

same

> +	if (ret)
> +		return ret;
> +
> +	putregs(target, &regs);

you're still copying via the stack without initialising the non-written
fields. If userland does a short PTRACE_SETREGSET the remaining fields
will be copied from the uninitialised kernel stack and accessible to
userland via PTRACE_GETREGSET.

Even if the user does a full sizeof(struct user_regs_struct) (not
pt_regs) PTRACE_SETREGSET the supervisor registers will be overwritten
with uninitialised stack content.

> diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
> new file mode 100644
> index 000000000000..4419604ff46c
> --- /dev/null
> +++ b/arch/riscv/kernel/sys_riscv.c

> +SYSCALL_DEFINE3(sysriscv_cmpxchg32, u32 __user *, ptr, u32, new, u32, old)
> +{
> +	u32 prev;
> +	unsigned int err;
> +
> +	if (!access_ok(VERIFY_WRITE, ptr, sizeof(*ptr)))
> +		return -EFAULT;
> +
> +#ifdef CONFIG_ISA_A
> +	err = 0;
> +	prev = cmpxchg32(ptr, old, new);

I think this needs a special version of cmpxchg (or for cmpxchg to be
modified) with fixup protection to return -EFAULT in case the page isn't
mapped or is paged out/read only.

> +#else
> +	preempt_disable();
> +	err = __get_user(prev, ptr);
> +	if (likely(!err && prev == old))
> +		err = __put_user(new, ptr);
> +	preempt_enable();
> +#endif
> +
> +	return unlikely(err) ? err : prev;
> +}
> +
> +SYSCALL_DEFINE3(sysriscv_cmpxchg64, u64 __user *, ptr, u64, new, u64, old)
> +{
> +#ifdef CONFIG_64BIT
> +	u64 prev;
> +	unsigned int err;
> +
> +	if (!access_ok(VERIFY_WRITE, ptr, sizeof(*ptr)))
> +		return -EFAULT;
> +
> +#ifdef CONFIG_ISA_A
> +	err = 0;
> +	prev = cmpxchg64(ptr, old, new);

Likewise

> +#else
> +	preempt_disable();
> +	err = __get_user(prev, ptr);
> +	if (likely(!err && prev == old))
> +		err = __put_user(new, ptr);
> +	preempt_enable();
> +#endif
> +	return unlikely(err) ? err : prev;
> +#else
> +	return -ENOTSUPP;

I think -ENOSYS is more standard for missing/unimplemented system calls.

A better way IMO would be to #ifdef the definitions in unistd.h, then
the __NR_* definitions could also be more accurately extracted from the
kernel headers, and you could just ifdef CONFIG_64BIT the whole syscall
implementation, i.e.:

> diff --git a/arch/riscv/include/uapi/asm/unistd.h b/arch/riscv/include/uapi/asm/unistd.h
> new file mode 100644
> index 000000000000..37a5429cc896
> --- /dev/null
> +++ b/arch/riscv/include/uapi/asm/unistd.h
> @@ -0,0 +1,23 @@

> +/*
> + * These system calls add support for AMOs on RISC-V systems without support
> + * for the A extension.
> + */
> +#define __NR_sysriscv_cmpxchg32		(__NR_arch_specific_syscall + 0)
> +__SYSCALL(__NR_sysriscv_cmpxchg32, sys_sysriscv_cmpxchg32)

+ifdef WHATEVER_BUILTIN_GCC_DEFINES_FOR_64BIT_RISCV_ABI

In case its helpful the following should list builtin preprocessor
defines for given CFLAGS:
${CROSS_COMPILE}gcc ${CFLAGS} -dM -E -</dev/null

> +#define __NR_sysriscv_cmpxchg64		(__NR_arch_specific_syscall + 1)
> +__SYSCALL(__NR_sysriscv_cmpxchg64, sys_sysriscv_cmpxchg64)

+endif

Cheers
James

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