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

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[ Upstream commit 158978945f ]

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>
Cc: stable@vger.kernel.org
[isaacmanjarres: added error handling to cleanup the work done by the
mmap() callback and removed unused label.]
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Lorenzo Stoakes 2025-07-29 17:58:08 -07:00 committed by Greg Kroah-Hartman
parent 55933e9538
commit f11d31371b
1 changed files with 14 additions and 8 deletions
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22
mm/mmap.c
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@ -1718,6 +1718,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
{
struct mm_struct *mm = current->mm;
struct vm_area_struct *vma, *prev;
bool writable_file_mapping = false;
int error;
struct rb_node **rb_link, *rb_parent;
unsigned long charged = 0;
@ -1785,11 +1786,6 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
if (error)
goto free_vma;
}
if (is_shared_maywrite(vm_flags)) {
error = mapping_map_writable(file->f_mapping);
if (error)
goto allow_write_and_free_vma;
}
/* ->mmap() can change vma->vm_file, but must guarantee that
* vma_link() below can deny write-access if VM_DENYWRITE is set
@ -1801,6 +1797,14 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
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;
}
/* Can addr have changed??
*
* Answer: Yes, several device drivers can do it in their
@ -1823,7 +1827,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
vma_link(mm, vma, prev, rb_link, rb_parent);
/* Once vma denies write, undo our temporary denial count */
if (file) {
if (is_shared_maywrite(vm_flags))
if (writable_file_mapping)
mapping_unmap_writable(file->f_mapping);
if (vm_flags & VM_DENYWRITE)
allow_write_access(file);
@ -1858,15 +1862,17 @@ out:
return addr;
close_and_free_vma:
if (vma->vm_ops && vma->vm_ops->close)
vma->vm_ops->close(vma);
unmap_and_free_vma:
vma->vm_file = NULL;
fput(file);
/* Undo any partial mapping done by a device driver. */
unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end);
if (is_shared_maywrite(vm_flags))
if (writable_file_mapping)
mapping_unmap_writable(file->f_mapping);
allow_write_and_free_vma:
if (vm_flags & VM_DENYWRITE)
allow_write_access(file);
free_vma: