mm/mmap: fix MAP_FIXED address return on VMA merge

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.

Link: https://lkml.kernel.org/r/20221018191613.4133459-1-Liam.Howlett@oracle.com
Link: https://lore.kernel.org/all/Y06yk66SKxlrwwfb@lakrids/
Link: https://lore.kernel.org/all/20201203085350.22624-1-liuzixian4@huawei.com/
Fixes: 4dd1b84140c1 ("mm/mmap: use advanced maple tree API for mmap_region()")
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reported-by: Mark Rutland <mark.rutland@arm.com>
Cc: Liu Zixian <liuzixian4@huawei.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/mmap.c b/mm/mmap.c
index 721fe5c..e270057 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2626,14 +2626,14 @@ cannot_expand:
                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);
 
                /*
@@ -2655,7 +2655,6 @@ cannot_expand:
                                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;
                        }