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

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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>
This commit is contained in:
Lorenzo Stoakes 2023-10-12 18:04:30 +01:00 committed by Andrew Morton
parent 28464bbb2d
commit 158978945f
1 changed files with 11 additions and 8 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

19
mm/mmap.c
View file

@ -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 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
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 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
/* 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 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
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);