* Vlastimil Babka <vbabka@xxxxxxx> [241023 08:59]:
> On 10/22/24 22:40, Lorenzo Stoakes wrote:
> > The mmap_region() function is somewhat terrifying, with spaghetti-like
> > control flow and numerous means by which issues can arise and incomplete
> > state, memory leaks and other unpleasantness can occur.
> >
> > A large amount of the complexity arises from trying to handle errors late
> > in the process of mapping a VMA, which forms the basis of recently observed
> > issues with resource leaks and observable inconsistent state.
> >
> > Taking advantage of previous patches in this series we move a number of
> > checks earlier in the code, simplifying things by moving the core of the
> > logic into a static internal function __mmap_region().
> >
> > Doing this allows us to perform a number of checks up front before we do
> > any real work, and allows us to unwind the writable unmap check
> > unconditionally as required and to perform a CONFIG_DEBUG_VM_MAPLE_TREE
> > validation unconditionally also.
> >
> > We move a number of things here:
> >
> > 1. We preallocate memory for the iterator before we call the file-backed
> > memory hook, allowing us to exit early and avoid having to perform
> > complicated and error-prone close/free logic. We carefully free
> > iterator state on both success and error paths.
> >
> > 2. The enclosing mmap_region() function handles the mapping_map_writable()
> > logic early. Previously the logic had the mapping_map_writable() at the
> > point of mapping a newly allocated file-backed VMA, and a matching
> > mapping_unmap_writable() on success and error paths.
> >
> > We now do this unconditionally if this is a file-backed, shared writable
> > mapping. If a driver changes the flags to eliminate VM_MAYWRITE, however
> > doing so does not invalidate the seal check we just performed, and we in
> > any case always decrement the counter in the wrapper.
> >
> > We perform a debug assert to ensure a driver does not attempt to do the
> > opposite.
> >
> > 3. We also move arch_validate_flags() up into the mmap_region()
> > function. This is only relevant on arm64 and sparc64, and the check is
> > only meaningful for SPARC with ADI enabled. We explicitly add a warning
> > for this arch if a driver invalidates this check, though the code ought
> > eventually to be fixed to eliminate the need for this.
> >
> > With all of these measures in place, we no longer need to explicitly close
> > the VMA on error paths, as we place all checks which might fail prior to a
> > call to any driver mmap hook.
> >
> > This eliminates an entire class of errors, makes the code easier to reason
> > about and more robust.
> >
> > Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> > Fixes: deb0f6562884 ("mm/mmap: undo ->mmap() when arch_validate_flags() fails")
> > Cc: stable <stable@xxxxxxxxxx>
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
>
> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
>
> some nits below
>
> > ---
> > mm/mmap.c | 120 ++++++++++++++++++++++++++++++------------------------
> > 1 file changed, 66 insertions(+), 54 deletions(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 66edf0ebba94..7d02b47a1895 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1361,20 +1361,18 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
> > return do_vmi_munmap(&vmi, mm, start, len, uf, false);
> > }
> >
> > -unsigned long mmap_region(struct file *file, unsigned long addr,
> > +static unsigned long __mmap_region(struct file *file, unsigned long addr,
> > unsigned long len, vm_flags_t vm_flags, unsigned long pgoff,
> > struct list_head *uf)
> > {
> > struct mm_struct *mm = current->mm;
> > struct vm_area_struct *vma = NULL;
> > pgoff_t pglen = PHYS_PFN(len);
> > - struct vm_area_struct *merge;
> > unsigned long charged = 0;
> > struct vma_munmap_struct vms;
> > struct ma_state mas_detach;
> > struct maple_tree mt_detach;
> > unsigned long end = addr + len;
> > - bool writable_file_mapping = false;
> > int error;
> > VMA_ITERATOR(vmi, mm, addr);
> > VMG_STATE(vmg, mm, &vmi, addr, end, vm_flags, pgoff);
> > @@ -1448,28 +1446,26 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> > vm_flags_init(vma, vm_flags);
> > vma->vm_page_prot = vm_get_page_prot(vm_flags);
> >
> > + if (vma_iter_prealloc(&vmi, vma)) {
> > + error = -ENOMEM;
> > + goto free_vma;
> > + }
> > +
> > if (file) {
> > vma->vm_file = get_file(file);
> > error = mmap_file(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;
> > - }
> > + goto unmap_and_free_file_vma;
> >
> > + /* Drivers cannot alter the address of the VMA. */
> > + WARN_ON_ONCE(addr != vma->vm_start);
> > /*
> > - * Expansion is handled above, merging is handled below.
> > - * Drivers should not alter the address of the VMA.
> > + * Drivers should not permit writability when previously it was
> > + * disallowed.
> > */
> > - if (WARN_ON((addr != vma->vm_start))) {
> > - error = -EINVAL;
> > - goto close_and_free_vma;
> > - }
> > + VM_WARN_ON_ONCE(vm_flags != vma->vm_flags &&
> > + !(vm_flags & VM_MAYWRITE) &&
> > + (vma->vm_flags & VM_MAYWRITE));
> >
> > vma_iter_config(&vmi, addr, end);
>
> I wonder if this one could be removed, earlier above we did the same config
> and neither parameters changed? But it was true before this patch as well,
> and maybe it's further refactored away later in the series, just noting.
Yes, this was here in case the vma changed address, so it's probably not
necessary.
>
> > /*
> > @@ -1477,6 +1473,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> > * vma again as we may succeed this time.
> > */
> > if (unlikely(vm_flags != vma->vm_flags && vmg.prev)) {
> > + struct vm_area_struct *merge;
> > +
> > vmg.flags = vma->vm_flags;
> > /* If this fails, state is reset ready for a reattempt. */
> > merge = vma_merge_new_range(&vmg);
> > @@ -1491,10 +1489,11 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> > */
> > fput(vma->vm_file);
> > vm_area_free(vma);
> > + vma_iter_free(&vmi);
>
> If we merged successfully, I think this is not necessary? But doesn't hurt?
Yes, it will use the allocations (and re-allocate more if necessary)
then free the unused allocations once this call path reaches
commit_merge() with the same vmi, which is nice.
And yes, it is safe to do regardless.
To be honest, this whole block is so rare that I want to delete it
anyways.
>
> > vma = merge;
> > /* Update vm_flags to pick up the change. */
> > vm_flags = vma->vm_flags;
> > - goto unmap_writable;
> > + goto file_expanded;
> > }
> > vma_iter_config(&vmi, addr, end);
> > }
> > @@ -1503,26 +1502,15 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> > } else if (vm_flags & VM_SHARED) {
> > error = shmem_zero_setup(vma);
> > if (error)
> > - goto free_vma;
> > + goto free_iter_vma;
> > } else {
> > vma_set_anonymous(vma);
> > }
> >
> > - if (map_deny_write_exec(vma->vm_flags, vma->vm_flags)) {
> > - error = -EACCES;
> > - goto close_and_free_vma;
> > - }
> > -
> > - /* Allow architectures to sanity-check the vm_flags */
> > - if (!arch_validate_flags(vma->vm_flags)) {
> > - error = -EINVAL;
> > - goto close_and_free_vma;
> > - }
> > -
> > - if (vma_iter_prealloc(&vmi, vma)) {
> > - error = -ENOMEM;
> > - goto close_and_free_vma;
> > - }
> > +#ifdef CONFIG_SPARC64
> > + /* TODO: Fix SPARC ADI! */
> > + WARN_ON_ONCE(!arch_validate_flags(vm_flags));
> > +#endif
> >
> > /* Lock the VMA since it is modified after insertion into VMA tree */
> > vma_start_write(vma);
> > @@ -1536,10 +1524,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> > */
> > khugepaged_enter_vma(vma, vma->vm_flags);
> >
> > - /* Once vma denies write, undo our temporary denial count */
> > -unmap_writable:
> > - if (writable_file_mapping)
> > - mapping_unmap_writable(file->f_mapping);
> > +file_expanded:
> > file = vma->vm_file;
> > ksm_add_vma(vma);
> > expanded:
> > @@ -1572,23 +1557,17 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >
> > vma_set_page_prot(vma);
> >
> > - validate_mm(mm);
> > return addr;
> >
> > -close_and_free_vma:
> > - vma_close(vma);
> > -
> > - if (file || vma->vm_file) {
> > -unmap_and_free_vma:
> > - fput(vma->vm_file);
> > - vma->vm_file = NULL;
> > +unmap_and_free_file_vma:
> > + fput(vma->vm_file);
> > + vma->vm_file = NULL;
> >
> > - vma_iter_set(&vmi, vma->vm_end);
> > - /* Undo any partial mapping done by a device driver. */
> > - unmap_region(&vmi.mas, vma, vmg.prev, vmg.next);
> > - }
> > - if (writable_file_mapping)
> > - mapping_unmap_writable(file->f_mapping);
> > + vma_iter_set(&vmi, vma->vm_end);
> > + /* Undo any partial mapping done by a device driver. */
> > + unmap_region(&vmi.mas, vma, vmg.prev, vmg.next);
> > +free_iter_vma:
> > + vma_iter_free(&vmi);
> > free_vma:
> > vm_area_free(vma);
> > unacct_error:
> > @@ -1598,10 +1577,43 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> > abort_munmap:
> > vms_abort_munmap_vmas(&vms, &mas_detach);
> > gather_failed:
> > - validate_mm(mm);
> > return error;
> > }
> >
> > +unsigned long mmap_region(struct file *file, unsigned long addr,
> > + unsigned long len, vm_flags_t vm_flags, unsigned long pgoff,
> > + struct list_head *uf)
> > +{
> > + unsigned long ret;
> > + bool writable_file_mapping = false;
> > +
> > + /* Allow architectures to sanity-check the vm_flags. */
> > + if (!arch_validate_flags(vm_flags))
> > + return -EINVAL;
> > +
> > + /* Check to see if MDWE is applicable. */
> > + if (map_deny_write_exec(vm_flags, vm_flags))
> > + return -EACCES;
>
> The two checks above used to be in the opposite order. Can we keep that just
> to be sure we don't change user observable behavior unnecessarily?
>
> > + /* Map writable and ensure this isn't a sealed memfd. */
> > + if (file && is_shared_maywrite(vm_flags)) {
> > + int error = mapping_map_writable(file->f_mapping);
> > +
> > + if (error)
> > + return error;
> > + writable_file_mapping = true;
> > + }
> > +
> > + ret = __mmap_region(file, addr, len, vm_flags, pgoff, uf);
> > +
> > + /* Clear our write mapping regardless of error. */
> > + if (writable_file_mapping)
> > + mapping_unmap_writable(file->f_mapping);
> > +
> > + validate_mm(current->mm);
> > + return ret;
> > +}
> > +
> > static int __vm_munmap(unsigned long start, size_t len, bool unlock)
> > {
> > int ret;
> > --
> > 2.47.0
>
