From: Szabolcs Nagy <Szabolcs.Nagy@arm.com>
To: Vincenzo Frascino <Vincenzo.Frascino@arm.com>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
"linux-kselftest@vger.kernel.org"
<linux-kselftest@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Cc: Catalin Marinas <Catalin.Marinas@arm.com>, nd <nd@arm.com>,
Will Deacon <Will.Deacon@arm.com>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Andrey Konovalov <andreyknvl@google.com>
Subject: Re: [PATCH v4 1/2] arm64: Define Documentation/arm64/tagged-address-abi.txt
Date: Wed, 12 Jun 2019 16:30:34 +0000 [thread overview]
Message-ID: <a90da586-8ff6-4bed-d940-9306d517a18c@arm.com> (raw)
In-Reply-To: <20190612142111.28161-2-vincenzo.frascino@arm.com>
On 12/06/2019 15:21, Vincenzo Frascino wrote:
> On arm64 the TCR_EL1.TBI0 bit has been always enabled hence
> the userspace (EL0) is allowed to set a non-zero value in the
> top byte but the resulting pointers are not allowed at the
> user-kernel syscall ABI boundary.
>
> With the relaxed ABI proposed through this document, it is now possible
> to pass tagged pointers to the syscalls, when these pointers are in
> memory ranges obtained by an anonymous (MAP_ANONYMOUS) mmap().
>
> This change in the ABI requires a mechanism to requires the userspace
> to opt-in to such an option.
>
> Specify and document the way in which sysctl and prctl() can be used
> in combination to allow the userspace to opt-in this feature.
>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will.deacon@arm.com>
> CC: Andrey Konovalov <andreyknvl@google.com>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
> ---
> Documentation/arm64/tagged-address-abi.txt | 111 +++++++++++++++++++++
> 1 file changed, 111 insertions(+)
> create mode 100644 Documentation/arm64/tagged-address-abi.txt
>
> diff --git a/Documentation/arm64/tagged-address-abi.txt b/Documentation/arm64/tagged-address-abi.txt
> new file mode 100644
> index 000000000000..96e149e2c55c
> --- /dev/null
> +++ b/Documentation/arm64/tagged-address-abi.txt
> @@ -0,0 +1,111 @@
> +ARM64 TAGGED ADDRESS ABI
> +========================
> +
> +This document describes the usage and semantics of the Tagged Address
> +ABI on arm64.
> +
> +1. Introduction
> +---------------
> +
> +On arm64 the TCR_EL1.TBI0 bit has been always enabled on the arm64 kernel,
> +hence the userspace (EL0) is allowed to set a non-zero value in the top
> +byte but the resulting pointers are not allowed at the user-kernel syscall
> +ABI boundary.
> +
> +This document describes a relaxation of the ABI with which it is possible
> +to pass tagged tagged pointers to the syscalls, when these pointers are in
^^^^^^^^^^^^^
typo.
> +memory ranges obtained as described in paragraph 2.
> +
> +Since it is not desirable to relax the ABI to allow tagged user addresses
> +into the kernel indiscriminately, arm64 provides a new sysctl interface
> +(/proc/sys/abi/tagged_addr) that is used to prevent the applications from
> +enabling the relaxed ABI and a new prctl() interface that can be used to
> +enable or disable the relaxed ABI.
> +
> +The sysctl is meant also for testing purposes in order to provide a simple
> +way for the userspace to verify the return error checking of the prctl()
> +command without having to reconfigure the kernel.
> +
> +The ABI properties are inherited by threads of the same application and
> +fork()'ed children but cleared when a new process is spawn (execve()).
OK.
> +
> +2. ARM64 Tagged Address ABI
> +---------------------------
> +
> +From the kernel syscall interface prospective, we define, for the purposes
^^^^^^^^^^^
perspective
> +of this document, a "valid tagged pointer" as a pointer that either it has
> +a zero value set in the top byte or it has a non-zero value, it is in memory
> +ranges privately owned by a userspace process and it is obtained in one of
> +the following ways:
> + - mmap() done by the process itself, where either:
> + * flags = MAP_PRIVATE | MAP_ANONYMOUS
> + * flags = MAP_PRIVATE and the file descriptor refers to a regular
> + file or "/dev/zero"
this does not make it clear if MAP_FIXED or other
flags are valid (there are many map flags i don't
know, but at least fixed should work and stack/growsdown.
i'd expect anything that's not incompatible with
private|anon to work).
> + - a mapping below sbrk(0) done by the process itself
doesn't the mmap rule cover this?
> + - any memory mapped by the kernel in the process's address space during
> + creation and following the restrictions presented above (i.e. data, bss,
> + stack).
OK.
Can a null pointer have a tag?
(in case NULL is valid to pass to a syscall)
> +
> +The ARM64 Tagged Address ABI is an opt-in feature, and an application can
> +control it using the following prctl()s:
> + - PR_SET_TAGGED_ADDR_CTRL: can be used to enable the Tagged Address ABI.
> + - PR_GET_TAGGED_ADDR_CTRL: can be used to check the status of the Tagged
> + Address ABI.
> +
> +As a consequence of invoking PR_SET_TAGGED_ADDR_CTRL prctl() by an applications,
> +the ABI guarantees the following behaviours:
> +
> + - Every current or newly introduced syscall can accept any valid tagged
> + pointers.
> +
> + - If a non valid tagged pointer is passed to a syscall then the behaviour
> + is undefined.
> +
> + - Every valid tagged pointer is expected to work as an untagged one.
> +
> + - The kernel preserves any valid tagged pointers and returns them to the
> + userspace unchanged in all the cases except the ones documented in the
> + "Preserving tags" paragraph of tagged-pointers.txt.
OK.
i guess pointers of another process are not "valid tagged
pointers" for the current one, so e.g. in ptrace the
ptracer has to clear the tags before PEEK etc.
> +
> +A definition of the meaning of tagged pointers on arm64 can be found in:
> +Documentation/arm64/tagged-pointers.txt.
> +
> +3. ARM64 Tagged Address ABI Exceptions
> +--------------------------------------
> +
> +The behaviours described in paragraph 2, with particular reference to the
> +acceptance by the syscalls of any valid tagged pointer are not applicable
> +to the following cases:
> + - mmap() addr parameter.
> + - mremap() new_address parameter.
> + - prctl_set_mm() struct prctl_map fields.
> + - prctl_set_mm_map() struct prctl_map fields.
i don't understand the exception: does it mean
that passing a tagged address to these syscalls
is undefined?
> +
> +4. Example of correct usage
> +---------------------------
> +
> +void main(void)
> +{
> + static int tbi_enabled = 0;
> + unsigned long tag = 0;
> +
> + char *ptr = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE,
> + MAP_ANONYMOUS, -1, 0);
> +
> + if (prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE,
> + 0, 0, 0) == 0)
> + tbi_enabled = 1;
> +
> + if (!ptr)
> + return -1;
mmap returns MAP_FAILED on failure.
> +
> + if (tbi_enabled)
> + tag = rand() & 0xff;
> +
> + ptr = (char *)((unsigned long)ptr | (tag << TAG_SHIFT));
> +
> + *ptr = 'a';
> +
> + ...
> +}
> +
>
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next prev parent reply other threads:[~2019-06-12 16:30 UTC|newest]
Thread overview: 78+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-12 11:43 [PATCH v17 00/15] arm64: untag user pointers passed to the kernel Andrey Konovalov
2019-06-12 11:43 ` [PATCH v17 01/15] arm64: untag user pointers in access_ok and __uaccess_mask_ptr Andrey Konovalov
2019-06-12 14:26 ` Vincenzo Frascino
2019-06-12 11:43 ` [PATCH v17 02/15] lib, arm64: untag user pointers in strn*_user Andrey Konovalov
2019-06-12 14:28 ` Vincenzo Frascino
2019-06-12 11:43 ` [PATCH v17 03/15] arm64: Introduce prctl() options to control the tagged user addresses ABI Andrey Konovalov
2019-06-12 14:30 ` Vincenzo Frascino
2019-06-12 14:55 ` Catalin Marinas
2019-06-13 11:02 ` Dave Martin
2019-06-13 15:26 ` Catalin Marinas
2019-06-14 5:13 ` Kees Cook
2019-06-18 9:18 ` Dave Martin
2019-06-13 11:16 ` Dave Martin
2019-06-13 15:35 ` Catalin Marinas
2019-06-13 15:45 ` Vincenzo Frascino
2019-06-13 15:57 ` Catalin Marinas
2019-06-13 16:15 ` Vincenzo Frascino
2019-06-17 13:56 ` Catalin Marinas
2019-06-17 16:56 ` Szabolcs Nagy
2019-06-17 16:57 ` Evgenii Stepanov
2019-06-17 17:18 ` Catalin Marinas
2019-06-17 21:59 ` Evgenii Stepanov
2019-06-19 14:45 ` Andrey Konovalov
2019-06-19 15:29 ` Catalin Marinas
2019-06-12 11:43 ` [PATCH v17 04/15] mm, arm64: untag user pointers passed to memory syscalls Andrey Konovalov
2019-06-12 14:31 ` Vincenzo Frascino
2019-06-19 15:55 ` Khalid Aziz
2019-06-19 16:46 ` Khalid Aziz
2019-06-24 14:22 ` Andrey Konovalov
2019-06-12 11:43 ` [PATCH v17 05/15] mm, arm64: untag user pointers in mm/gup.c Andrey Konovalov
2019-06-12 14:33 ` Vincenzo Frascino
2019-06-19 16:41 ` Khalid Aziz
2019-06-12 11:43 ` [PATCH v17 06/15] mm, arm64: untag user pointers in get_vaddr_frames Andrey Konovalov
2019-06-12 14:34 ` Vincenzo Frascino
2019-06-19 16:48 ` Khalid Aziz
2019-06-12 11:43 ` [PATCH v17 07/15] fs, arm64: untag user pointers in copy_mount_options Andrey Konovalov
2019-06-12 14:35 ` Vincenzo Frascino
2019-06-19 20:01 ` Khalid Aziz
2019-06-12 11:43 ` [PATCH v17 08/15] userfaultfd, arm64: untag user pointers Andrey Konovalov
2019-06-12 14:40 ` Vincenzo Frascino
2019-06-12 11:43 ` [PATCH v17 09/15] drm/amdgpu, " Andrey Konovalov
2019-06-12 11:43 ` [PATCH v17 10/15] drm/radeon, arm64: untag user pointers in radeon_gem_userptr_ioctl Andrey Konovalov
2019-06-12 11:43 ` [PATCH v17 11/15] IB/mlx4, arm64: untag user pointers in mlx4_get_umem_mr Andrey Konovalov
2019-06-12 11:43 ` [PATCH v17 12/15] media/v4l2-core, arm64: untag user pointers in videobuf_dma_contig_user_get Andrey Konovalov
2019-06-19 20:05 ` Khalid Aziz
2019-06-12 11:43 ` [PATCH v17 13/15] tee/shm, arm64: untag user pointers in tee_shm_register Andrey Konovalov
2019-06-12 11:43 ` [PATCH v17 14/15] vfio/type1, arm64: untag user pointers in vaddr_get_pfn Andrey Konovalov
2019-06-12 14:41 ` Vincenzo Frascino
2019-06-12 15:58 ` Auger Eric
2019-06-12 11:43 ` [PATCH v17 15/15] selftests, arm64: add a selftest for passing tagged pointers to kernel Andrey Konovalov
2019-06-12 12:30 ` Szabolcs Nagy
2019-06-12 15:00 ` Catalin Marinas
2019-06-19 14:42 ` Andrey Konovalov
2019-06-12 14:21 ` [PATCH v4 0/2] arm64 relaxed ABI Vincenzo Frascino
2019-06-12 14:21 ` [PATCH v4 1/2] arm64: Define Documentation/arm64/tagged-address-abi.txt Vincenzo Frascino
2019-06-12 15:35 ` Catalin Marinas
2019-06-13 10:15 ` Vincenzo Frascino
2019-06-13 11:37 ` Dave Martin
2019-06-13 12:28 ` Catalin Marinas
2019-06-13 13:23 ` Dave Martin
2019-06-13 15:39 ` Catalin Marinas
2019-06-12 16:30 ` Szabolcs Nagy [this message]
2019-06-13 9:20 ` Catalin Marinas
2019-06-13 10:14 ` Szabolcs Nagy
2019-06-13 11:16 ` Vincenzo Frascino
2019-06-13 12:28 ` Szabolcs Nagy
2019-06-13 14:03 ` Vincenzo Frascino
2019-06-13 15:32 ` Szabolcs Nagy
2019-06-13 15:35 ` Vincenzo Frascino
2019-06-12 14:21 ` [PATCH v4 2/2] arm64: Relax Documentation/arm64/tagged-pointers.txt Vincenzo Frascino
2019-06-12 15:56 ` Catalin Marinas
2019-06-12 16:37 ` Szabolcs Nagy
2019-06-13 15:51 ` [PATCH v5 0/2] arm64 relaxed ABI Vincenzo Frascino
2019-06-13 15:51 ` [PATCH v5 1/2] arm64: Define Documentation/arm64/tagged-address-abi.txt Vincenzo Frascino
2019-06-18 11:02 ` Szabolcs Nagy
2019-06-18 13:13 ` Kevin Brodsky
2019-06-21 15:16 ` Catalin Marinas
2019-06-13 15:51 ` [PATCH v5 2/2] arm64: Relax Documentation/arm64/tagged-pointers.txt Vincenzo Frascino
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