linux-kernel - [PATCH 0/1] uprobes: Kill __replace_page(), change uprobe_write

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Date:	Mon, 9 Dec 2013 22:18:24 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	Borislav Petkov <bp@...en8.de>,
	Hugh Dickins <hughd@...gle.com>,
	Ingo Molnar <mingo@...nel.org>, Jiri Kosina <jkosina@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 0/1] uprobes: Kill __replace_page(), change
	uprobe_write_opcode() to rely on gup(WRITE)

Hello.

It is not clear to me if Linus still dislikes this change or not.
Let me send the patch "officially" so that it can be nacked if I
misunderstood the result of discussion.

Changes:

	- add a huge comment above gup(WRITE | FORCE)

	- add WARN_ON(!(PageAnon() && page_mapcount() == 1))
	  to ensure it works as expected

If (say, on x86) we can avoid the pte games, we can simply add

	if (IS_ENABLED(CONFIG_WHATEVER)) {
		copy_to_page(...);
		set_page_dirty_locked(page);
		goto put;
	}

right after the 2nd get_user_pages().

In any case I believe it would be very nice to kill __replace_page(),
and even the fact this patch removes include(mm/internal.h) makes me
think this patch makes sense. Assuming it is correct.

Oleg.
---

int uprobe_write_opcode(struct mm_struct *mm, unsigned long vaddr,
			uprobe_opcode_t opcode)
{
	struct page *page;
	struct vm_area_struct *vma;
	pte_t *ptep, entry;
	spinlock_t *ptlp;
	int ret;

	/* Read the page with vaddr into memory */
	ret = get_user_pages(NULL, mm, vaddr, 1, 0, 1, &page, NULL);
	if (ret < 0)
		return ret;

	ret = verify_opcode(page, vaddr, &opcode);
	if (ret <= 0)
		goto put;

 retry:
	put_page(page);
	/*
	 * Break the mapping unless the page is already anonymous and
	 * unshare the page, see the WARN_ON() below.
	 *
	 * We never write to the VM_SHARED vma, every caller must check
	 * valid_vma(). FOLL_WRITE | FOLL_FORCE should anonymize this
	 * page unless uprobe_write_opcode() was already called in the
	 * past or the application itself did mprotect(PROT_WRITE) and
	 * wrote into this page.
	 *
	 * If it was already anonymous it can be shared due to dup_mm(),
	 * in this case do_wp_page() or do_swap_page() will do another
	 * cow to unshare, so we can safely modify it.
	 */
	ret = get_user_pages(NULL, mm, vaddr, 1, 1, 1, &page, &vma);
	if (ret < 0)
		return ret;

	ptep = page_check_address(page, mm, vaddr, &ptlp, 0);
	if (!ptep)
		goto retry;

	ret = 0;
	if (WARN_ON(!PageAnon(page) || page_mapcount(page) != 1)) {
		dump_page(page);
		ret = -EFAULT;
		goto unlock;
	}

	/* Unmap this page to ensure that nobody can execute it */
	flush_cache_page(vma, vaddr, pte_pfn(*ptep));
	entry = ptep_clear_flush(vma, vaddr, ptep);

	/* Nobody can fault in this page, modify it */
	copy_to_page(page, vaddr, &opcode, UPROBE_SWBP_INSN_SIZE);

	/* Restore the old mapping */
	entry = pte_mkdirty(entry);
	flush_icache_page(vma, page);
	set_pte_at(mm, vaddr, ptep, entry);
	update_mmu_cache(vma, vaddr, ptep);
 unlock:
	pte_unmap_unlock(ptep, ptlp);
 put:
	put_page(page);
	return ret;
}

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