mm: perform the mapping_map_writable() check after call_mmap()

In order for a F_SEAL_WRITE sealed memfd mapping to have an opportunity to
clear VM_MAYWRITE, we must be able to invoke the appropriate
vm_ops->mmap() handler to do so.  We would otherwise fail the
mapping_map_writable() check before we had the opportunity to avoid it.

This patch moves this check after the call_mmap() invocation.  Only memfd
actively denies write access causing a potential failure here (in
memfd_add_seals()), so there should be no impact on non-memfd cases.

This patch makes the userland-visible change that MAP_SHARED, PROT_READ
mappings of an F_SEAL_WRITE sealed memfd mapping will now succeed.

There is a delicate situation with cleanup paths assuming that a writable
mapping must have occurred in circumstances where it may now not have.  In
order to ensure we do not accidentally mark a writable file unwritable by
mistake, we explicitly track whether we have a writable mapping and unmap
only if we do.

[lstoakes@gmail.com: do not set writable_file_mapping in inappropriate case]
Link: https://lkml.kernel.org/r/c9eb4cc6-7db4-4c2b-838d-43a0b319a4f0@lucifer.local
Link: https://bugzilla.kernel.org/show_bug.cgi?id=217238
Link: https://lkml.kernel.org/r/55e413d20678a1bb4c7cce889062bbb07b0df892.1697116581.git.lstoakes@gmail.com
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/mmap.c b/mm/mmap.c
index 0041e3631f6cccaf49d7072191b378aa4f4e85c3..8b57e42fd980b8d187a73dbbbf8de9d29533f307 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2752,6 +2752,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
        unsigned long charged = 0;
        unsigned long end = addr + len;
        unsigned long merge_start = addr, merge_end = end;
+       bool writable_file_mapping = false;
        pgoff_t vm_pgoff;
        int error;
        VMA_ITERATOR(vmi, mm, addr);
@@ -2846,17 +2847,19 @@ cannot_expand:
        vma->vm_pgoff = pgoff;
 
        if (file) {
-               if (is_shared_maywrite(vm_flags)) {
-                       error = mapping_map_writable(file->f_mapping);
-                       if (error)
-                               goto free_vma;
-               }
-
                vma->vm_file = get_file(file);
                error = call_mmap(file, vma);
                if (error)
                        goto unmap_and_free_vma;
 
+               if (vma_is_shared_maywrite(vma)) {
+                       error = mapping_map_writable(file->f_mapping);
+                       if (error)
+                               goto close_and_free_vma;
+
+                       writable_file_mapping = true;
+               }
+
                /*
                 * Expansion is handled above, merging is handled below.
                 * Drivers should not alter the address of the VMA.
@@ -2937,7 +2940,7 @@ cannot_expand:
 
        /* Once vma denies write, undo our temporary denial count */
 unmap_writable:
-       if (file && is_shared_maywrite(vm_flags))
+       if (writable_file_mapping)
                mapping_unmap_writable(file->f_mapping);
        file = vma->vm_file;
        ksm_add_vma(vma);
@@ -2985,7 +2988,7 @@ unmap_and_free_vma:
                unmap_region(mm, &vmi.mas, vma, prev, next, vma->vm_start,
                             vma->vm_end, vma->vm_end, true);
        }
-       if (file && is_shared_maywrite(vm_flags))
+       if (writable_file_mapping)
                mapping_unmap_writable(file->f_mapping);
 free_vma:
        vm_area_free(vma);