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    SubjectRe: [RFC PATCH 3/3] mm: perform the mapping_map_writable() check after call_mmap()
    On Mon 03-04-23 23:28:32, Lorenzo Stoakes wrote:
    > 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.
    >
    > Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
    > ---
    > mm/mmap.c | 12 ++++++------
    > 1 file changed, 6 insertions(+), 6 deletions(-)
    >
    > diff --git a/mm/mmap.c b/mm/mmap.c
    > index c96dcce90772..a166e9f3c474 100644
    > --- a/mm/mmap.c
    > +++ b/mm/mmap.c
    > @@ -2596,17 +2596,17 @@ 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 unmap_and_free_vma;

    Shouldn't we rather jump to close_and_free_vma?

    Honza
    --
    Jan Kara <jack@suse.com>
    SUSE Labs, CR

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