Subject: Re: [PATCH v3 19/23] arm64: mte: Add PTRACE_{PEEK,POKE}MTETAGS support

From: Dave Martin <[email-masked]>
Date: Wed, 29 Apr 2020 17:46:07 +0100

On Tue, Apr 21, 2020 at 03:25:59PM +0100, Catalin Marinas wrote:
> Add support for bulk setting/getting of the MTE tags in a tracee's
> address space at 'addr' in the ptrace() syscall prototype. 'data' points
> to a struct iovec in the tracer's address space with iov_base
> representing the address of a tracer's buffer of length iov_len. The
> tags to be copied to/from the tracer's buffer are stored as one tag per
> byte.
> 
> On successfully copying at least one tag, ptrace() returns 0 and updates
> the tracer's iov_len with the number of tags copied. In case of error,
> either -EIO or -EFAULT is returned, trying to follow the ptrace() man
> page.
> 
> Note that the tag copying functions are not performance critical,
> therefore they lack optimisations found in typical memory copy routines.

Doesn't quite belong here, but:

Can we dump the tags and possible the faulting mode etc. when dumping
core?

That information seems potentially valuable for debugging.
Tweaking the fault mode from a debugger may also be useful (which is
quite easy to achieve if coredump support is done by wrapping the MTE
control word in a regset).

These could probably be added later, though.

> Signed-off-by: Catalin Marinas <[email-masked]>
> Cc: Will Deacon <[email-masked]>
> Cc: Alan Hayward <[email-masked]>
> Cc: Luis Machado <[email-masked]>
> Cc: Omair Javaid <[email-masked]>
> ---
> 
> Notes:
>     New in v3.
> 
>  arch/arm64/include/asm/mte.h         |  17 ++++
>  arch/arm64/include/uapi/asm/ptrace.h |   3 +
>  arch/arm64/kernel/mte.c              | 127 +++++++++++++++++++++++++++
>  arch/arm64/kernel/ptrace.c           |  15 +++-
>  arch/arm64/lib/mte.S                 |  50 +++++++++++
>  5 files changed, 211 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
> index 22eb3e06f311..0ca2aaff07a1 100644
> --- a/arch/arm64/include/asm/mte.h
> +++ b/arch/arm64/include/asm/mte.h
> @@ -2,12 +2,21 @@
>  #ifndef __ASM_MTE_H
>  #define __ASM_MTE_H
>  
> +#define MTE_ALLOC_SIZE	UL(16)
> +#define MTE_ALLOC_MASK	(~(MTE_ALLOC_SIZE - 1))
> +#define MTE_TAG_SHIFT	(56)
> +#define MTE_TAG_SIZE	(4)
> +

Nit: pointless () on the last two #defines.

[...]

> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index fa4a4196b248..0cb496ed9bf9 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -3,12 +3,17 @@
>   * Copyright (C) 2020 ARM Ltd.
>   */
>  
> +#include <linux/kernel.h>
> +#include <linux/mm.h>
>  #include <linux/prctl.h>
>  #include <linux/sched.h>
> +#include <linux/sched/mm.h>
>  #include <linux/thread_info.h>
> +#include <linux/uio.h>
>  
>  #include <asm/cpufeature.h>
>  #include <asm/mte.h>
> +#include <asm/ptrace.h>
>  #include <asm/sysreg.h>
>  
>  static void update_sctlr_el1_tcf0(u64 tcf0)
> @@ -133,3 +138,125 @@ long get_mte_ctrl(void)
>  
>  	return ret;
>  }
> +
> +/*
> + * Access MTE tags in another process' address space as given in mm. Update
> + * the number of tags copied. Return 0 if any tags copied, error otherwise.
> + * Inspired by __access_remote_vm().
> + */
> +static int __access_remote_tags(struct task_struct *tsk, struct mm_struct *mm,
> +				unsigned long addr, struct iovec *kiov,
> +				unsigned int gup_flags)
> +{
> +	struct vm_area_struct *vma;
> +	void __user *buf = kiov->iov_base;
> +	size_t len = kiov->iov_len;
> +	int ret;
> +	int write = gup_flags & FOLL_WRITE;
> +
> +	if (down_read_killable(&mm->mmap_sem))
> +		return -EIO;
> +
> +	if (!access_ok(buf, len))
> +		return -EFAULT;

Leaked down_read()?

> +
> +	while (len) {
> +		unsigned long tags, offset;
> +		void *maddr;
> +		struct page *page = NULL;
> +
> +		ret = get_user_pages_remote(tsk, mm, addr, 1, gup_flags,
> +					    &page, &vma, NULL);
> +		if (ret <= 0)
> +			break;
> +
> +		/* limit access to the end of the page */
> +		offset = offset_in_page(addr);
> +		tags = min(len, (PAGE_SIZE - offset) / MTE_ALLOC_SIZE);
> +
> +		maddr = page_address(page);
> +		if (write) {
> +			tags = mte_copy_tags_from_user(maddr + offset, buf, tags);
> +			set_page_dirty_lock(page);
> +		} else {
> +			tags = mte_copy_tags_to_user(buf, maddr + offset, tags);
> +		}
> +		put_page(page);
> +
> +		/* error accessing the tracer's buffer */
> +		if (!tags)
> +			break;
> +
> +		len -= tags;
> +		buf += tags;
> +		addr += tags * MTE_ALLOC_SIZE;
> +	}
> +	up_read(&mm->mmap_sem);
> +
> +	/* return an error if no tags copied */
> +	kiov->iov_len = buf - kiov->iov_base;
> +	if (!kiov->iov_len) {
> +		/* check for error accessing the tracee's address space */
> +		if (ret <= 0)
> +			return -EIO;
> +		else
> +			return -EFAULT;
> +	}
> +
> +	return 0;
> +}
> +
> +/*
> + * Copy MTE tags in another process' address space at 'addr' to/from tracer's
> + * iovec buffer. Return 0 on success. Inspired by ptrace_access_vm().
> + */
> +static int access_remote_tags(struct task_struct *tsk, unsigned long addr,
> +			      struct iovec *kiov, unsigned int gup_flags)
> +{
> +	struct mm_struct *mm;
> +	int ret;
> +
> +	mm = get_task_mm(tsk);
> +	if (!mm)
> +		return -EPERM;
> +
> +	if (!tsk->ptrace || (current != tsk->parent) ||
> +	    ((get_dumpable(mm) != SUID_DUMP_USER) &&
> +	     !ptracer_capable(tsk, mm->user_ns))) {
> +		mmput(mm);
> +		return -EPERM;
> +	}
> +
> +	ret = __access_remote_tags(tsk, mm, addr, kiov, gup_flags);
> +	mmput(mm);
> +
> +	return ret;
> +}
> +
> +int mte_ptrace_copy_tags(struct task_struct *child, long request,
> +			 unsigned long addr, unsigned long data)
> +{
> +	int ret;
> +	struct iovec kiov;
> +	struct iovec __user *uiov = (void __user *)data;
> +	unsigned int gup_flags = FOLL_FORCE;
> +
> +	if (!system_supports_mte())
> +		return -EIO;
> +
> +	if (get_user(kiov.iov_base, &uiov->iov_base) ||
> +	    get_user(kiov.iov_len, &uiov->iov_len))
> +		return -EFAULT;
> +
> +	if (request == PTRACE_POKEMTETAGS)
> +		gup_flags |= FOLL_WRITE;
> +
> +	/* align addr to the MTE tag granule */
> +	addr &= MTE_ALLOC_MASK;
> +
> +	ret = access_remote_tags(child, addr, &kiov, gup_flags);
> +	if (!ret)
> +		ret = __put_user(kiov.iov_len, &uiov->iov_len);

Should this be put_user()?  We didn't use __get_user() above, and I
don't see what guards the access.

> +
> +	return ret;
> +}
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index 077e352495eb..1fdb841ad536 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -34,6 +34,7 @@
>  #include <asm/cpufeature.h>
>  #include <asm/debug-monitors.h>
>  #include <asm/fpsimd.h>
> +#include <asm/mte.h>
>  #include <asm/pgtable.h>
>  #include <asm/pointer_auth.h>
>  #include <asm/stacktrace.h>
> @@ -1797,7 +1798,19 @@ const struct user_regset_view *task_user_regset_view(struct task_struct *task)
>  long arch_ptrace(struct task_struct *child, long request,
>  		 unsigned long addr, unsigned long data)
>  {
> -	return ptrace_request(child, request, addr, data);
> +	int ret;
> +
> +	switch (request) {
> +	case PTRACE_PEEKMTETAGS:
> +	case PTRACE_POKEMTETAGS:
> +		ret = mte_ptrace_copy_tags(child, request, addr, data);
> +		break;

Nit: return mte_trace_copy_tags()?

This is a new function, so we don't need to follow the verbose style of
the core code.  Not everyone likes returning out of switches though.

> +	default:
> +		ret = ptrace_request(child, request, addr, data);
> +		break;
> +	}
> +
> +	return ret;
>  }
>  
>  enum ptrace_syscall_dir {
> diff --git a/arch/arm64/lib/mte.S b/arch/arm64/lib/mte.S
> index bd51ea7e2fcb..45be04a8c73c 100644
> --- a/arch/arm64/lib/mte.S
> +++ b/arch/arm64/lib/mte.S
> @@ -5,6 +5,7 @@
>  #include <linux/linkage.h>
>  
>  #include <asm/assembler.h>
> +#include <asm/mte.h>
>  
>  /*
>   * Compare tags of two pages
> @@ -44,3 +45,52 @@ SYM_FUNC_START(mte_memcmp_pages)
>  
>  	ret
>  SYM_FUNC_END(mte_memcmp_pages)
> +
> +/*
> + * Read tags from a user buffer (one tag per byte) and set the corresponding
> + * tags at the given kernel address. Used by PTRACE_POKEMTETAGS.
> + *   x0 - kernel address (to)
> + *   x1 - user buffer (from)
> + *   x2 - number of tags/bytes (n)

Is it worth checking for x2 == 0?  Currently, x2 will underflow and
we'll try to loop 2^64 times (until a fault stops us).

I don't think callers currently pass 0 here, but it feels like an
accident waiting to happen.  Things like memcpy() usually try to close
this loophole.

Similarly for _to_user().

Cheers
---Dave

> + * Returns:
> + *   x0 - number of tags read/set
> + */
> +SYM_FUNC_START(mte_copy_tags_from_user)
> +	mov	x3, x1
> +1:
> +USER(2f, ldtrb	w4, [x1])
> +	lsl	x4, x4, #MTE_TAG_SHIFT
> +	stg	x4, [x0], #MTE_ALLOC_SIZE
> +	add	x1, x1, #1
> +	subs	x2, x2, #1
> +	b.ne	1b
> +
> +	// exception handling and function return
> +2:	sub	x0, x1, x3		// update the number of tags set
> +	ret
> +SYM_FUNC_END(mte_copy_tags_from_user)
> +
> +/*
> + * Get the tags from a kernel address range and write the tag values to the
> + * given user buffer (one tag per byte). Used by PTRACE_PEEKMTETAGS.
> + *   x0 - user buffer (to)
> + *   x1 - kernel address (from)
> + *   x2 - number of tags/bytes (n)
> + * Returns:
> + *   x0 - number of tags read/set
> + */
> +SYM_FUNC_START(mte_copy_tags_to_user)
> +	mov	x3, x0
> +1:
> +	ldg	x4, [x1]
> +	ubfx	x4, x4, #MTE_TAG_SHIFT, #MTE_TAG_SIZE
> +USER(2f, sttrb	w4, [x0])
> +	add	x0, x0, #1
> +	add	x1, x1, #MTE_ALLOC_SIZE
> +	subs	x2, x2, #1
> +	b.ne	1b
> +
> +	// exception handling and function return
> +2:	sub	x0, x0, x3		// update the number of tags copied
> +	ret
> +SYM_FUNC_END(mte_copy_tags_from_user)
> 
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