[PATCH] mm/mmap: Fix MAP_FIXED address return on VMA merge

[Liam Howlett]

mmap should return the start address of newly mapped area when
successful.  On a successful merge of a VMA, the return address was
changed and thus was violating that expectation from userspace.

This is a restoration of functionality provided by 309d08d9b3a3
(mm/mmap.c: fix mmap return value when vma is merged after call_mmap()).
For completeness of fixing MAP_FIXED, implement the comments from the
previous discussion to never update the address and fail if the address
changes.  Leaving the error as a WARN_ON() to avoid crashing the kernel.

Cc: Liu Zixian <liuzixian4@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/all/Y06yk66SKxlrwwfb@lakrids/
Link: https://lore.kernel.org/all/20201203085350.22624-1-liuzixian4@xxxxxxxxxx/
Fixes: 4dd1b84140c1 (mm/mmap: use advanced maple tree API for mmap_region())
Reported-by: Mark Rutland <mark.rutland@xxxxxxx>
Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
---
 mm/mmap.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 42cd2c260898..22010e13f1a1 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2625,14 +2625,14 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		if (error)
 			goto unmap_and_free_vma;
 
-		/* Can addr have changed??
-		 *
-		 * Answer: Yes, several device drivers can do it in their
-		 *         f_op->mmap method. -DaveM
+		/*
+		 * Expansion is handled above, merging is handled below.
+		 * Drivers should not alter the address of the VMA.
 		 */
-		WARN_ON_ONCE(addr != vma->vm_start);
-
-		addr = vma->vm_start;
+		if (WARN_ON((addr != vma->vm_start))) {
+			error = -EINVAL;
+			goto close_and_free_vma;
+		}
 		mas_reset(&mas);
 
 		/*
@@ -2654,7 +2654,6 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 				vm_area_free(vma);
 				vma = merge;
 				/* Update vm_flags to pick up the change. */
-				addr = vma->vm_start;
 				vm_flags = vma->vm_flags;
 				goto unmap_writable;
 			}
-- 
2.35.1