Commit 4bb5f5d9 authored by David Herrmann's avatar David Herrmann Committed by Linus Torvalds
Browse files

mm: allow drivers to prevent new writable mappings 



This patch (of 6):

The i_mmap_writable field counts existing writable mappings of an
address_space.  To allow drivers to prevent new writable mappings, make
this counter signed and prevent new writable mappings if it is negative.
This is modelled after i_writecount and DENYWRITE.

This will be required by the shmem-sealing infrastructure to prevent any
new writable mappings after the WRITE seal has been set.  In case there
exists a writable mapping, this operation will fail with EBUSY.

Note that we rely on the fact that iff you already own a writable mapping,
you can increase the counter without using the helpers.  This is the same
that we do for i_writecount.

Signed-off-by: default avatarDavid Herrmann <dh.herrmann@gmail.com>
Acked-by: default avatarHugh Dickins <hughd@google.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Ryan Lortie <desrt@desrt.ca>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: Daniel Mack <zonque@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 935e9f02

fs/inode.c

+1 −0
Original line number Diff line number Diff line
@@ -165,6 +165,7 @@ int inode_init_always(struct super_block *sb, struct inode *inode) 
	mapping->a_ops = &empty_aops;

	mapping->host = inode;

	mapping->flags = 0;

	atomic_set(&mapping->i_mmap_writable, 0);

	mapping_set_gfp_mask(mapping, GFP_HIGHUSER_MOVABLE);

	mapping->private_data = NULL;

	mapping->backing_dev_info = &default_backing_dev_info;

include/linux/fs.h

+27 −2
Original line number Diff line number Diff line
@@ -387,7 +387,7 @@ struct address_space { 
	struct inode		*host;		/* owner: inode, block_device */

	struct radix_tree_root	page_tree;	/* radix tree of all pages */

	spinlock_t		tree_lock;	/* and lock protecting it */

	unsigned int		i_mmap_writable;/* count VM_SHARED mappings */

	atomic_t		i_mmap_writable;/* count VM_SHARED mappings */

	struct rb_root		i_mmap;		/* tree of private and shared mappings */

	struct list_head	i_mmap_nonlinear;/*list VM_NONLINEAR mappings */

	struct mutex		i_mmap_mutex;	/* protect tree, count, list */
@@ -470,10 +470,35 @@ static inline int mapping_mapped(struct address_space *mapping) 
 * Note that i_mmap_writable counts all VM_SHARED vmas: do_mmap_pgoff

 * marks vma as VM_SHARED if it is shared, and the file was opened for

 * writing i.e. vma may be mprotected writable even if now readonly.

 *

 * If i_mmap_writable is negative, no new writable mappings are allowed. You

 * can only deny writable mappings, if none exists right now.

 */

static inline int mapping_writably_mapped(struct address_space *mapping)

{

	return mapping->i_mmap_writable != 0;

	return atomic_read(&mapping->i_mmap_writable) > 0;

}



static inline int mapping_map_writable(struct address_space *mapping)

{

	return atomic_inc_unless_negative(&mapping->i_mmap_writable) ?

		0 : -EPERM;

}



static inline void mapping_unmap_writable(struct address_space *mapping)

{

	atomic_dec(&mapping->i_mmap_writable);

}



static inline int mapping_deny_writable(struct address_space *mapping)

{

	return atomic_dec_unless_positive(&mapping->i_mmap_writable) ?

		0 : -EBUSY;

}



static inline void mapping_allow_writable(struct address_space *mapping)

{

	atomic_inc(&mapping->i_mmap_writable);

}



/*

kernel/fork.c

+1 −1
Original line number Diff line number Diff line
@@ -429,7 +429,7 @@ static int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm) 
				atomic_dec(&inode->i_writecount);

			mutex_lock(&mapping->i_mmap_mutex);

			if (tmp->vm_flags & VM_SHARED)

				mapping->i_mmap_writable++;

				atomic_inc(&mapping->i_mmap_writable);

			flush_dcache_mmap_lock(mapping);

			/* insert tmp into the share list, just after mpnt */

			if (unlikely(tmp->vm_flags & VM_NONLINEAR))

mm/mmap.c

+24 −6
Original line number Diff line number Diff line
@@ -221,7 +221,7 @@ static void __remove_shared_vm_struct(struct vm_area_struct *vma, 
	if (vma->vm_flags & VM_DENYWRITE)

		atomic_inc(&file_inode(file)->i_writecount);

	if (vma->vm_flags & VM_SHARED)

		mapping->i_mmap_writable--;

		mapping_unmap_writable(mapping);



	flush_dcache_mmap_lock(mapping);

	if (unlikely(vma->vm_flags & VM_NONLINEAR))
@@ -622,7 +622,7 @@ static void __vma_link_file(struct vm_area_struct *vma) 
		if (vma->vm_flags & VM_DENYWRITE)

			atomic_dec(&file_inode(file)->i_writecount);

		if (vma->vm_flags & VM_SHARED)

			mapping->i_mmap_writable++;

			atomic_inc(&mapping->i_mmap_writable);



		flush_dcache_mmap_lock(mapping);

		if (unlikely(vma->vm_flags & VM_NONLINEAR))
@@ -1577,6 +1577,17 @@ munmap_back: 
			if (error)

				goto free_vma;

		}

		if (vm_flags & VM_SHARED) {

			error = mapping_map_writable(file->f_mapping);

			if (error)

				goto allow_write_and_free_vma;

		}



		/* ->mmap() can change vma->vm_file, but must guarantee that

		 * vma_link() below can deny write-access if VM_DENYWRITE is set

		 * and map writably if VM_SHARED is set. This usually means the

		 * new file must not have been exposed to user-space, yet.

		 */

		vma->vm_file = get_file(file);

		error = file->f_op->mmap(file, vma);

		if (error)
@@ -1616,8 +1627,12 @@ munmap_back: 


	vma_link(mm, vma, prev, rb_link, rb_parent);

	/* Once vma denies write, undo our temporary denial count */

	if (file) {

		if (vm_flags & VM_SHARED)

			mapping_unmap_writable(file->f_mapping);

		if (vm_flags & VM_DENYWRITE)

			allow_write_access(file);

	}

	file = vma->vm_file;

out:

	perf_event_mmap(vma);
@@ -1646,14 +1661,17 @@ out: 
	return addr;



unmap_and_free_vma:

	if (vm_flags & VM_DENYWRITE)

		allow_write_access(file);

	vma->vm_file = NULL;

	fput(file);



	/* Undo any partial mapping done by a device driver. */

	unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end);

	charged = 0;

	if (vm_flags & VM_SHARED)

		mapping_unmap_writable(file->f_mapping);

allow_write_and_free_vma:

	if (vm_flags & VM_DENYWRITE)

		allow_write_access(file);

free_vma:

	kmem_cache_free(vm_area_cachep, vma);

unacct_error:

mm/swap_state.c

+1 −0
Original line number Diff line number Diff line
@@ -39,6 +39,7 @@ static struct backing_dev_info swap_backing_dev_info = { 
struct address_space swapper_spaces[MAX_SWAPFILES] = {

	[0 ... MAX_SWAPFILES - 1] = {

		.page_tree	= RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),

		.i_mmap_writable = ATOMIC_INIT(0),

		.a_ops		= &swap_aops,

		.backing_dev_info = &swap_backing_dev_info,

	}

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