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Commit 18693050 authored by Michel Lespinasse's avatar Michel Lespinasse Committed by Linus Torvalds
Browse files

mm: introduce VM_POPULATE flag to better deal with racy userspace programs 



The vm_populate() code populates user mappings without constantly
holding the mmap_sem.  This makes it susceptible to racy userspace
programs: the user mappings may change while vm_populate() is running,
and in this case vm_populate() may end up populating the new mapping
instead of the old one.

In order to reduce the possibility of userspace getting surprised by
this behavior, this change introduces the VM_POPULATE vma flag which
gets set on vmas we want vm_populate() to work on.  This way
vm_populate() may still end up populating the new mapping after such a
race, but only if the new mapping is also one that the user has
requested (using MAP_SHARED, MAP_LOCKED or mlock) to be populated.

Signed-off-by: default avatarMichel Lespinasse <walken@google.com>
Acked-by: default avatarRik van Riel <riel@redhat.com>
Tested-by: default avatarAndy Lutomirski <luto@amacapital.net>
Cc: Greg Ungerer <gregungerer@westnet.com.au>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent cea10a19

include/linux/mm.h

+1 −0
Original line number Diff line number Diff line
@@ -87,6 +87,7 @@ extern unsigned int kobjsize(const void *objp); 
#define VM_PFNMAP	0x00000400	/* Page-ranges managed without "struct page", just pure PFN */

#define VM_DENYWRITE	0x00000800	/* ETXTBSY on write attempts.. */



#define VM_POPULATE     0x00001000

#define VM_LOCKED	0x00002000

#define VM_IO           0x00004000	/* Memory mapped I/O or similar */

include/linux/mman.h

+3 −1
Original line number Diff line number Diff line
@@ -79,6 +79,8 @@ calc_vm_flag_bits(unsigned long flags) 
{

	return _calc_vm_trans(flags, MAP_GROWSDOWN,  VM_GROWSDOWN ) |

	       _calc_vm_trans(flags, MAP_DENYWRITE,  VM_DENYWRITE ) |

	       _calc_vm_trans(flags, MAP_LOCKED,     VM_LOCKED    );

	       ((flags & MAP_LOCKED) ? (VM_LOCKED | VM_POPULATE) : 0) |

	       (((flags & (MAP_POPULATE | MAP_NONBLOCK)) == MAP_POPULATE) ?

							VM_POPULATE : 0);

}

#endif /* _LINUX_MMAN_H */

mm/fremap.c

+10 −2
Original line number Diff line number Diff line
@@ -204,8 +204,10 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size, 
			unsigned long addr;

			struct file *file = get_file(vma->vm_file);



			addr = mmap_region(file, start, size,

					vma->vm_flags, pgoff);

			vm_flags = vma->vm_flags;

			if (!(flags & MAP_NONBLOCK))

				vm_flags |= VM_POPULATE;

			addr = mmap_region(file, start, size, vm_flags, pgoff);

			fput(file);

			if (IS_ERR_VALUE(addr)) {

				err = addr;
@@ -224,6 +226,12 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size, 
		mutex_unlock(&mapping->i_mmap_mutex);

	}



	if (!(flags & MAP_NONBLOCK) && !(vma->vm_flags & VM_POPULATE)) {

		if (!has_write_lock)

			goto get_write_lock;

		vma->vm_flags |= VM_POPULATE;

	}



	if (vma->vm_flags & VM_LOCKED) {

		/*

		 * drop PG_Mlocked flag for over-mapped range

mm/mlock.c

+10 −9
Original line number Diff line number Diff line
@@ -340,9 +340,9 @@ static int do_mlock(unsigned long start, size_t len, int on) 


		/* Here we know that  vma->vm_start <= nstart < vma->vm_end. */



		newflags = vma->vm_flags | VM_LOCKED;

		if (!on)

			newflags &= ~VM_LOCKED;

		newflags = vma->vm_flags & ~VM_LOCKED;

		if (on)

			newflags |= VM_LOCKED | VM_POPULATE;



		tmp = vma->vm_end;

		if (tmp > end)
@@ -402,7 +402,8 @@ int __mm_populate(unsigned long start, unsigned long len, int ignore_errors) 
		 * range with the first VMA. Also, skip undesirable VMA types.

		 */

		nend = min(end, vma->vm_end);

		if (vma->vm_flags & (VM_IO | VM_PFNMAP))

		if ((vma->vm_flags & (VM_IO | VM_PFNMAP | VM_POPULATE)) !=

		    VM_POPULATE)

			continue;

		if (nstart < vma->vm_start)

			nstart = vma->vm_start;
@@ -475,18 +476,18 @@ static int do_mlockall(int flags) 
	struct vm_area_struct * vma, * prev = NULL;



	if (flags & MCL_FUTURE)

		current->mm->def_flags |= VM_LOCKED;

		current->mm->def_flags |= VM_LOCKED | VM_POPULATE;

	else

		current->mm->def_flags &= ~VM_LOCKED;

		current->mm->def_flags &= ~(VM_LOCKED | VM_POPULATE);

	if (flags == MCL_FUTURE)

		goto out;



	for (vma = current->mm->mmap; vma ; vma = prev->vm_next) {

		vm_flags_t newflags;



		newflags = vma->vm_flags | VM_LOCKED;

		if (!(flags & MCL_CURRENT))

			newflags &= ~VM_LOCKED;

		newflags = vma->vm_flags & ~VM_LOCKED;

		if (flags & MCL_CURRENT)

			newflags |= VM_LOCKED | VM_POPULATE;



		/* Ignore errors */

		mlock_fixup(vma, &prev, vma->vm_start, vma->vm_end, newflags);

mm/mmap.c

+1 −3
Original line number Diff line number Diff line
@@ -1306,9 +1306,7 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr, 
	}



	addr = mmap_region(file, addr, len, vm_flags, pgoff);

	if (!IS_ERR_VALUE(addr) &&

	    ((vm_flags & VM_LOCKED) ||

	     (flags & (MAP_POPULATE | MAP_NONBLOCK)) == MAP_POPULATE))

	if (!IS_ERR_VALUE(addr) && (vm_flags & VM_POPULATE))

		*populate = true;

	return addr;

}