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From: Dave Hansen <dave.hansen@intel.com>
To: Yu-cheng Yu <yu-cheng.yu@intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-mm@kvack.org, linux-arch@vger.kernel.org,
	linux-api@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Andy Lutomirski <luto@kernel.org>,
	Balbir Singh <bsingharora@gmail.com>,
	Borislav Petkov <bp@alien8.de>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Eugene Syromiatnikov <esyr@redhat.com>,
	Florian Weimer <fweimer@redhat.com>,
	"H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
	Dave Martin <Dave.Martin@arm.com>,
	Weijiang Yang <weijiang.yang@intel.com>,
	Pengfei Xu <pengfei.xu@intel.com>
Subject: Re: [PATCH v18 24/25] x86/cet/shstk: Add arch_prctl functions for shadow stack
Date: Fri, 29 Jan 2021 09:07:01 -0800	[thread overview]
Message-ID: <ba39586d-25b6-6ea5-19c3-adf17b59f910@intel.com> (raw)
In-Reply-To: <20210127212524.10188-25-yu-cheng.yu@intel.com>

On 1/27/21 1:25 PM, Yu-cheng Yu wrote:
> arch_prctl(ARCH_X86_CET_STATUS, u64 *args)
>     Get CET feature status.
> 
>     The parameter 'args' is a pointer to a user buffer.  The kernel returns
>     the following information:
> 
>     *args = shadow stack/IBT status
>     *(args + 1) = shadow stack base address
>     *(args + 2) = shadow stack size

What's the deal for 32-bit binaries?  The in-kernel code looks 64-bit
only, but I don't see anything restricting the interface to 64-bit.

> +static int copy_status_to_user(struct cet_status *cet, u64 arg2)

This has static scope, but it's still awfully generically named.  A cet_
prefix would be nice.

> +{
> +	u64 buf[3] = {0, 0, 0};
> +
> +	if (cet->shstk_size) {
> +		buf[0] |= GNU_PROPERTY_X86_FEATURE_1_SHSTK;
> +		buf[1] = (u64)cet->shstk_base;
> +		buf[2] = (u64)cet->shstk_size;

What's the casting for?

> +	}
> +
> +	return copy_to_user((u64 __user *)arg2, buf, sizeof(buf));
> +}
> +
> +int prctl_cet(int option, u64 arg2)
> +{
> +	struct cet_status *cet;
> +	unsigned int features;
> +
> +	/*
> +	 * GLIBC's ENOTSUPP == EOPNOTSUPP == 95, and it does not recognize
> +	 * the kernel's ENOTSUPP (524).  So return EOPNOTSUPP here.
> +	 */
> +	if (!IS_ENABLED(CONFIG_X86_CET))
> +		return -EOPNOTSUPP;

Let's ignore glibc for a moment.  What error code *should* the kernel be
returning here?  errno(3) says:

       EOPNOTSUPP      Operation not supported on socket (POSIX.1)
...
       ENOTSUP         Operation not supported (POSIX.1)


> +	cet = &current->thread.cet;
> +
> +	if (option == ARCH_X86_CET_STATUS)
> +		return copy_status_to_user(cet, arg2);

What's the point of doing copy_status_to_user() if the processor doesn't
support CET?  In other words, shouldn't this be below the CPU feature check?

Also, please cast arg2 *here*.  It becomes a user pointer here, not at
the copy_to_user().

> +	if (!static_cpu_has(X86_FEATURE_CET))
> +		return -EOPNOTSUPP;

So, you went to the trouble of adding a disabled-features.h entry for
this.  Why not just do:

	if (cpu_feature_enabled(X86_FEATURE_CET))
		...

instead of the IS_ENABLED() check above?  That should get rid of one of
these if's.

> +	switch (option) {
> +	case ARCH_X86_CET_DISABLE:
> +		if (cet->locked)
> +			return -EPERM;
> +
> +		features = (unsigned int)arg2;

What's the purpose of this cast?

> +		if (features & ~GNU_PROPERTY_X86_FEATURE_1_VALID)
> +			return -EINVAL;
> +		if (features & GNU_PROPERTY_X86_FEATURE_1_SHSTK)
> +			cet_disable_shstk();
> +		return 0;

This doesn't enforce that the high bits of arg2 be 0.  Shouldn't we call
them reserved and enforce that they be 0?

> +	case ARCH_X86_CET_LOCK:
> +		cet->locked = 1;
> +		return 0;

This needs to check for and enforce that arg2==0.

> +	default:
> +		return -ENOSYS;
> +	}
> +}


  reply	other threads:[~2021-01-29 17:07 UTC|newest]

Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-27 21:24 [PATCH v18 00/25] Control-flow Enforcement: Shadow Stack Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 01/25] Documentation/x86: Add CET description Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 02/25] x86/cet/shstk: Add Kconfig option for user-mode control-flow protection Yu-cheng Yu
2021-01-29 19:42   ` Dave Hansen
2021-01-29 19:58     ` Andy Lutomirski
2021-01-29 20:33       ` Dave Hansen
2021-01-29 20:46         ` Borislav Petkov
2021-01-29 21:13           ` Yu, Yu-cheng
2021-01-29 20:00     ` Yu, Yu-cheng
2021-01-27 21:25 ` [PATCH v18 03/25] x86/cpufeatures: Add CET CPU feature flags for Control-flow Enforcement Technology (CET) Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 04/25] x86/cpufeatures: Introduce X86_FEATURE_CET and setup functions Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 05/25] x86/fpu/xstate: Introduce CET MSR and XSAVES supervisor states Yu-cheng Yu
2021-01-29 21:00   ` [NEEDS-REVIEW] " Dave Hansen
2021-01-29 22:35     ` Yu, Yu-cheng
2021-01-29 22:53       ` Dave Hansen
2021-02-01 22:43         ` Yu, Yu-cheng
2021-02-01 22:59           ` Dave Hansen
2021-02-01 23:05             ` Yu, Yu-cheng
2021-02-01 23:12               ` Dave Hansen
2021-02-01 23:14                 ` Yu, Yu-cheng
2021-01-27 21:25 ` [PATCH v18 06/25] x86/cet: Add control-protection fault handler Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 07/25] x86/mm: Remove _PAGE_DIRTY from kernel RO pages Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 08/25] x86/mm: Introduce _PAGE_COW Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 09/25] drm/i915/gvt: Change _PAGE_DIRTY to _PAGE_DIRTY_BITS Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 10/25] x86/mm: Update pte_modify for _PAGE_COW Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 11/25] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 12/25] mm: Introduce VM_SHSTK for shadow stack memory Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 13/25] x86/mm: Shadow Stack page fault error checking Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 14/25] x86/mm: Update maybe_mkwrite() for shadow stack Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 15/25] mm: Fixup places that call pte_mkwrite() directly Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 16/25] mm: Add guard pages around a shadow stack Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 17/25] mm/mmap: Add shadow stack pages to memory accounting Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 18/25] mm: Update can_follow_write_pte() for shadow stack Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 19/25] mm: Re-introduce vm_flags to do_mmap() Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 20/25] x86/cet/shstk: User-mode shadow stack support Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 21/25] x86/cet/shstk: Handle signals for shadow stack Yu-cheng Yu
2021-02-01 22:53   ` Dave Hansen
2021-02-01 22:58     ` Yu, Yu-cheng
2021-01-27 21:25 ` [PATCH v18 22/25] ELF: Introduce arch_setup_elf_property() Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 23/25] x86/cet/shstk: Handle thread shadow stack Yu-cheng Yu
2021-01-27 21:25 ` [PATCH v18 24/25] x86/cet/shstk: Add arch_prctl functions for " Yu-cheng Yu
2021-01-29 17:07   ` Dave Hansen [this message]
2021-01-29 18:56     ` Yu, Yu-cheng
2021-01-29 19:15       ` Dave Hansen
2021-01-29 19:53         ` Yu, Yu-cheng
2021-02-03 21:54       ` Yu, Yu-cheng
2021-02-03 22:11         ` Dave Hansen
2021-02-03 22:28           ` Yu, Yu-cheng
2021-01-27 21:25 ` [PATCH v18 25/25] mm: Introduce PROT_SHSTK " Yu-cheng Yu

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