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userfaultfd: don't pin the user memory in userfaultfd_file_create()
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userfaultfd_file_create() increments mm->mm_users; this means that the
memory won't be unmapped/freed if mm owner exits/execs, and UFFDIO_COPY
after that can populate the orphaned mm more.

Change userfaultfd_file_create() and userfaultfd_ctx_put() to use
mm->mm_count to pin mm_struct.  This means that
atomic_inc_not_zero(mm->mm_users) is needed when we are going to
actually play with this memory.  Except handle_userfault() path doesn't
need this, the caller must already have a reference.

The patch adds the new trivial helper, mmget_not_zero(), it can have
more users.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Oleg Nesterov <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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@oleg-nesterov @torvalds
oleg-nesterov authored and torvalds committed May 21, 2016
1 parent cd33a76 commit d2005e3
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Showing 2 changed files with 34 additions and 14 deletions.
41 changes: 28 additions & 13 deletions fs/userfaultfd.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -137,7 +137,7 @@ static void userfaultfd_ctx_put(struct userfaultfd_ctx *ctx)
VM_BUG_ON(waitqueue_active(&ctx->fault_wqh));
VM_BUG_ON(spin_is_locked(&ctx->fd_wqh.lock));
VM_BUG_ON(waitqueue_active(&ctx->fd_wqh));
mmput(ctx->mm);
mmdrop(ctx->mm);
kmem_cache_free(userfaultfd_ctx_cachep, ctx);
}
}
Expand Down Expand Up @@ -434,6 +434,9 @@ static int userfaultfd_release(struct inode *inode, struct file *file)

ACCESS_ONCE(ctx->released) = true;

if (!mmget_not_zero(mm))
goto wakeup;

/*
* Flush page faults out of all CPUs. NOTE: all page faults
* must be retried without returning VM_FAULT_SIGBUS if
Expand Down Expand Up @@ -466,7 +469,8 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
}
up_write(&mm->mmap_sem);

mmput(mm);
wakeup:
/*
* After no new page faults can wait on this fault_*wqh, flush
* the last page faults that may have been already waiting on
Expand Down Expand Up @@ -760,10 +764,12 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
start = uffdio_register.range.start;
end = start + uffdio_register.range.len;

ret = -ENOMEM;
if (!mmget_not_zero(mm))
goto out;

down_write(&mm->mmap_sem);
vma = find_vma_prev(mm, start, &prev);

ret = -ENOMEM;
if (!vma)
goto out_unlock;

Expand Down Expand Up @@ -864,6 +870,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
} while (vma && vma->vm_start < end);
out_unlock:
up_write(&mm->mmap_sem);
mmput(mm);
if (!ret) {
/*
* Now that we scanned all vmas we can already tell
Expand Down Expand Up @@ -902,10 +909,12 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
start = uffdio_unregister.start;
end = start + uffdio_unregister.len;

ret = -ENOMEM;
if (!mmget_not_zero(mm))
goto out;

down_write(&mm->mmap_sem);
vma = find_vma_prev(mm, start, &prev);

ret = -ENOMEM;
if (!vma)
goto out_unlock;

Expand Down Expand Up @@ -998,6 +1007,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
} while (vma && vma->vm_start < end);
out_unlock:
up_write(&mm->mmap_sem);
mmput(mm);
out:
return ret;
}
Expand Down Expand Up @@ -1067,9 +1077,11 @@ static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
goto out;
if (uffdio_copy.mode & ~UFFDIO_COPY_MODE_DONTWAKE)
goto out;

ret = mcopy_atomic(ctx->mm, uffdio_copy.dst, uffdio_copy.src,
uffdio_copy.len);
if (mmget_not_zero(ctx->mm)) {
ret = mcopy_atomic(ctx->mm, uffdio_copy.dst, uffdio_copy.src,
uffdio_copy.len);
mmput(ctx->mm);
}
if (unlikely(put_user(ret, &user_uffdio_copy->copy)))
return -EFAULT;
if (ret < 0)
Expand Down Expand Up @@ -1110,8 +1122,11 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
if (uffdio_zeropage.mode & ~UFFDIO_ZEROPAGE_MODE_DONTWAKE)
goto out;

ret = mfill_zeropage(ctx->mm, uffdio_zeropage.range.start,
uffdio_zeropage.range.len);
if (mmget_not_zero(ctx->mm)) {
ret = mfill_zeropage(ctx->mm, uffdio_zeropage.range.start,
uffdio_zeropage.range.len);
mmput(ctx->mm);
}
if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage)))
return -EFAULT;
if (ret < 0)
Expand Down Expand Up @@ -1289,12 +1304,12 @@ static struct file *userfaultfd_file_create(int flags)
ctx->released = false;
ctx->mm = current->mm;
/* prevent the mm struct to be freed */
atomic_inc(&ctx->mm->mm_users);
atomic_inc(&ctx->mm->mm_count);

file = anon_inode_getfile("[userfaultfd]", &userfaultfd_fops, ctx,
O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
if (IS_ERR(file)) {
mmput(ctx->mm);
mmdrop(ctx->mm);
kmem_cache_free(userfaultfd_ctx_cachep, ctx);
}
out:
Expand Down
7 changes: 6 additions & 1 deletion include/linux/sched.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2723,12 +2723,17 @@ extern struct mm_struct * mm_alloc(void);

/* mmdrop drops the mm and the page tables */
extern void __mmdrop(struct mm_struct *);
static inline void mmdrop(struct mm_struct * mm)
static inline void mmdrop(struct mm_struct *mm)
{
if (unlikely(atomic_dec_and_test(&mm->mm_count)))
__mmdrop(mm);
}

static inline bool mmget_not_zero(struct mm_struct *mm)
{
return atomic_inc_not_zero(&mm->mm_users);
}

/* mmput gets rid of the mappings and all user-space */
extern void mmput(struct mm_struct *);
/* same as above but performs the slow path from the async kontext. Can
Expand Down

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