[PATCH 4/7] mm/mremap: Replace can_modify_mm with can_modify_vma

[Pedro Falcato <pedro.falcato@gmail.com>]

Delegate all can_modify checks to the proper places. Unmap checks are
done in do_unmap (et al).

This patch allows for mremap partial failure in certain cases (for
instance, when destination VMAs aren't sealed, but the source VMA is).
It shouldn't be too troublesome, as you'd need to go out of your way to
do illegal operations on a VMA.

Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
---
 mm/mremap.c | 33 +++++++--------------------------
 1 file changed, 7 insertions(+), 26 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index e7ae140fc64..8af877d7bb0 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -676,6 +676,9 @@ static unsigned long move_vma(struct vm_area_struct *vma,
 	if (unlikely(flags & MREMAP_DONTUNMAP))
 		to_account = new_len;
 
+	if (!can_modify_vma(vma))
+		return -EPERM;
+
 	if (vma->vm_ops && vma->vm_ops->may_split) {
 		if (vma->vm_start != old_addr)
 			err = vma->vm_ops->may_split(vma, old_addr);
@@ -821,6 +824,10 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
 	if (!vma)
 		return ERR_PTR(-EFAULT);
 
+	/* Don't allow vma expansion when it has already been sealed */
+	if (!can_modify_vma(vma))
+		return ERR_PTR(-EPERM);
+
 	/*
 	 * !old_len is a special case where an attempt is made to 'duplicate'
 	 * a mapping.  This makes no sense for private mappings as it will
@@ -902,19 +909,6 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
 	if ((mm->map_count + 2) >= sysctl_max_map_count - 3)
 		return -ENOMEM;
 
-	/*
-	 * In mremap_to().
-	 * Move a VMA to another location, check if src addr is sealed.
-	 *
-	 * Place can_modify_mm here because mremap_to()
-	 * does its own checking for address range, and we only
-	 * check the sealing after passing those checks.
-	 *
-	 * can_modify_mm assumes we have acquired the lock on MM.
-	 */
-	if (unlikely(!can_modify_mm(mm, addr, addr + old_len)))
-		return -EPERM;
-
 	if (flags & MREMAP_FIXED) {
 		/*
 		 * In mremap_to().
@@ -1079,19 +1073,6 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 		goto out;
 	}
 
-	/*
-	 * Below is shrink/expand case (not mremap_to())
-	 * Check if src address is sealed, if so, reject.
-	 * In other words, prevent shrinking or expanding a sealed VMA.
-	 *
-	 * Place can_modify_mm here so we can keep the logic related to
-	 * shrink/expand together.
-	 */
-	if (unlikely(!can_modify_mm(mm, addr, addr + old_len))) {
-		ret = -EPERM;
-		goto out;
-	}
-
 	/*
 	 * Always allow a shrinking remap: that just unmaps
 	 * the unnecessary pages..
-- 
2.46.0