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Commit 5fcf7bb7 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
Browse files

[PATCH] read_zero_pagealigned() locking fix 



Ramiro Voicu hits the BUG_ON(!pte_none(*pte)) in zeromap_pte_range: kernel
bugzilla 7645.  Right: read_zero_pagealigned uses down_read of mmap_sem,
but another thread's racing read of /dev/zero, or a normal fault, can
easily set that pte again, in between zap_page_range and zeromap_page_range
getting there.  It's been wrong ever since 2.4.3.

The simple fix is to use down_write instead, but that would serialize reads
of /dev/zero more than at present: perhaps some app would be badly
affected.  So instead let zeromap_page_range return the error instead of
BUG_ON, and read_zero_pagealigned break to the slower clear_user loop in
that case - there's no need to optimize for it.

Use -EEXIST for when a pte is found: BUG_ON in mmap_zero (the other user of
zeromap_page_range), though it really isn't interesting there.  And since
mmap_zero wants -EAGAIN for out-of-memory, the zeromaps better return that
than -ENOMEM.

Signed-off-by: default avatarHugh Dickins <hugh@veritas.com>
Cc: Ramiro Voicu: <Ramiro.Voicu@cern.ch>
Cc: <stable@kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 347a00fb

drivers/char/mem.c

+8 −4
Original line number Diff line number Diff line
@@ -646,7 +646,8 @@ static inline size_t read_zero_pagealigned(char __user * buf, size_t size) 
			count = size;



		zap_page_range(vma, addr, count, NULL);

        	zeromap_page_range(vma, addr, count, PAGE_COPY);

        	if (zeromap_page_range(vma, addr, count, PAGE_COPY))

			break;



		size -= count;

		buf += count;
@@ -713,11 +714,14 @@ static ssize_t read_zero(struct file * file, char __user * buf, 


static int mmap_zero(struct file * file, struct vm_area_struct * vma)

{

	int err;



	if (vma->vm_flags & VM_SHARED)

		return shmem_zero_setup(vma);

	if (zeromap_page_range(vma, vma->vm_start, vma->vm_end - vma->vm_start, vma->vm_page_prot))

		return -EAGAIN;

	return 0;

	err = zeromap_page_range(vma, vma->vm_start,

			vma->vm_end - vma->vm_start, vma->vm_page_prot);

	BUG_ON(err == -EEXIST);

	return err;

}

#else /* CONFIG_MMU */

static ssize_t read_zero(struct file * file, char * buf,

mm/memory.c

+21 −11
Original line number Diff line number Diff line
@@ -1110,23 +1110,29 @@ static int zeromap_pte_range(struct mm_struct *mm, pmd_t *pmd, 
{

	pte_t *pte;

	spinlock_t *ptl;

	int err = 0;



	pte = pte_alloc_map_lock(mm, pmd, addr, &ptl);

	if (!pte)

		return -ENOMEM;

		return -EAGAIN;

	arch_enter_lazy_mmu_mode();

	do {

		struct page *page = ZERO_PAGE(addr);

		pte_t zero_pte = pte_wrprotect(mk_pte(page, prot));



		if (unlikely(!pte_none(*pte))) {

			err = -EEXIST;

			pte++;

			break;

		}

		page_cache_get(page);

		page_add_file_rmap(page);

		inc_mm_counter(mm, file_rss);

		BUG_ON(!pte_none(*pte));

		set_pte_at(mm, addr, pte, zero_pte);

	} while (pte++, addr += PAGE_SIZE, addr != end);

	arch_leave_lazy_mmu_mode();

	pte_unmap_unlock(pte - 1, ptl);

	return 0;

	return err;

}



static inline int zeromap_pmd_range(struct mm_struct *mm, pud_t *pud,
@@ -1134,16 +1140,18 @@ static inline int zeromap_pmd_range(struct mm_struct *mm, pud_t *pud, 
{

	pmd_t *pmd;

	unsigned long next;

	int err;



	pmd = pmd_alloc(mm, pud, addr);

	if (!pmd)

		return -ENOMEM;

		return -EAGAIN;

	do {

		next = pmd_addr_end(addr, end);

		if (zeromap_pte_range(mm, pmd, addr, next, prot))

			return -ENOMEM;

		err = zeromap_pte_range(mm, pmd, addr, next, prot);

		if (err)

			break;

	} while (pmd++, addr = next, addr != end);

	return 0;

	return err;

}



static inline int zeromap_pud_range(struct mm_struct *mm, pgd_t *pgd,
@@ -1151,16 +1159,18 @@ static inline int zeromap_pud_range(struct mm_struct *mm, pgd_t *pgd, 
{

	pud_t *pud;

	unsigned long next;

	int err;



	pud = pud_alloc(mm, pgd, addr);

	if (!pud)

		return -ENOMEM;

		return -EAGAIN;

	do {

		next = pud_addr_end(addr, end);

		if (zeromap_pmd_range(mm, pud, addr, next, prot))

			return -ENOMEM;

		err = zeromap_pmd_range(mm, pud, addr, next, prot);

		if (err)

			break;

	} while (pud++, addr = next, addr != end);

	return 0;

	return err;

}



int zeromap_page_range(struct vm_area_struct *vma,