On Wed, Oct 23, 2024 at 01:19:35PM -0400, Liam R. Howlett wrote:
> * Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> [241022 16:41]:
> > We have seen bugs and resource leaks arise from the complexity of the
> > __mmap_region() function. This, and the generally deeply fragile error
> > handling logic and complexity which makes understanding the function
> > difficult make it highly desirable to refactor it into something readable.
> >
> > Achieve this by separating the function into smaller logical parts which
> > are easier to understand and follow, and which importantly very
> > significantly simplify the error handling.
> >
> > Note that we now call vms_abort_munmap_vmas() in more error paths than we
> > used to, however in cases where no abort need occur, vms->nr_pages will be
> > equal to zero and we simply exit this function without doing more than we
> > would have done previously.
> >
> > Importantly, the invocation of the driver mmap hook via mmap_file() now has
> > very simple and obvious handling (this was previously the most problematic
> > part of the mmap() operation).
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > ---
> > mm/vma.c | 380 +++++++++++++++++++++++++++++++++++--------------------
> > 1 file changed, 240 insertions(+), 140 deletions(-)
> >
> > diff --git a/mm/vma.c b/mm/vma.c
> > index 7617f9d50d62..a271e2b406ab 100644
> > --- a/mm/vma.c
> > +++ b/mm/vma.c
> > @@ -7,6 +7,31 @@
> > #include "vma_internal.h"
> > #include "vma.h"
> >
> > +struct mmap_state {
> > + struct mm_struct *mm;
> > + struct vma_iterator *vmi;
> > + struct vma_merge_struct *vmg;
> > + struct list_head *uf;
> > +
> > + struct vma_munmap_struct vms;
> > + struct ma_state mas_detach;
> > + struct maple_tree mt_detach;
> > +
> > + unsigned long flags;
> > + unsigned long pglen;
> > + unsigned long charged;
> > +};
> > +
> > +#define MMAP_STATE(name, mm_, vmi_, vmg_, uf_, flags_, len_) \
> > + struct mmap_state name = { \
> > + .mm = mm_, \
> > + .vmi = vmi_, \
> > + .vmg = vmg_, \
> > + .uf = uf_, \
> > + .flags = flags_, \
> > + .pglen = PHYS_PFN(len_), \
> > + }
> > +
> > static inline bool is_mergeable_vma(struct vma_merge_struct *vmg, bool merge_next)
> > {
> > struct vm_area_struct *vma = merge_next ? vmg->next : vmg->prev;
> > @@ -2169,189 +2194,247 @@ static void vms_abort_munmap_vmas(struct vma_munmap_struct *vms,
> > vms_complete_munmap_vmas(vms, mas_detach);
> > }
> >
> > -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)
> > +/*
> > + * __mmap_prepare() - Prepare to gather any overlapping VMAs that need to be
> > + * unmapped once the map operation is completed, check limits,
> > + * account mapping and clean up any pre-existing VMAs.
> > + *
>
> nit: formatting seems wrong here?
But I like it this way :( will change though.
>
> > + * @map: Mapping state.
> > + *
> > + * Returns: 0 on success, error code otherwise.
> > + */
> > +static int __mmap_prepare(struct mmap_state *map)
> > {
> > - struct mm_struct *mm = current->mm;
> > - struct vm_area_struct *vma = NULL;
> > - pgoff_t pglen = PHYS_PFN(len);
> > - unsigned long charged = 0;
> > - struct vma_munmap_struct vms;
> > - struct ma_state mas_detach;
> > - struct maple_tree mt_detach;
> > - unsigned long end = addr + len;
> > int error;
> > - VMA_ITERATOR(vmi, mm, addr);
> > - VMG_STATE(vmg, mm, &vmi, addr, end, vm_flags, pgoff);
> > -
> > - vmg.file = file;
> > - /* Find the first overlapping VMA */
> > - vma = vma_find(&vmi, end);
> > - init_vma_munmap(&vms, &vmi, vma, addr, end, uf, /* unlock = */ false);
> > - if (vma) {
> > - mt_init_flags(&mt_detach, vmi.mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
> > - mt_on_stack(mt_detach);
> > - mas_init(&mas_detach, &mt_detach, /* addr = */ 0);
> > + struct vma_iterator *vmi = map->vmi;
> > + struct vma_merge_struct *vmg = map->vmg;
> > + struct vma_munmap_struct *vms = &map->vms;
> > +
> > + /* Find the first overlapping VMA and initialise unmap state. */
> > + vms->vma = vma_find(vmi, vmg->end);
> > + init_vma_munmap(vms, vmi, vms->vma, vmg->start, vmg->end, map->uf,
> > + /* unlock = */ false);
> > +
> > + /* OK, we have overlapping VMAs - prepare to unmap them. */
> > + if (vms->vma) {
> > + mt_init_flags(&map->mt_detach, vmi->mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
>
> Nit: line is too long.
Yeah think this is possibly pre-existing but will fix either way.
>
> > + mt_on_stack(map->mt_detach);
> > + mas_init(&map->mas_detach, &map->mt_detach, /* addr = */ 0);
> > /* Prepare to unmap any existing mapping in the area */
> > - error = vms_gather_munmap_vmas(&vms, &mas_detach);
> > + error = vms_gather_munmap_vmas(vms, &map->mas_detach);
> > if (error)
> > - goto gather_failed;
> > + return error;
>
> As Vlastimil pointed out, there is an issue just returning the error.
Yeah have addressed it, thanks!
>
> >
> > - vmg.next = vms.next;
> > - vmg.prev = vms.prev;
> > - vma = NULL;
> > + vmg->next = vms->next;
> > + vmg->prev = vms->prev;
> > } else {
> > - vmg.next = vma_iter_next_rewind(&vmi, &vmg.prev);
> > + vmg->next = vma_iter_next_rewind(vmi, &vmg->prev);
> > }
> >
> > /* Check against address space limit. */
> > - if (!may_expand_vm(mm, vm_flags, pglen - vms.nr_pages)) {
> > - error = -ENOMEM;
> > - goto abort_munmap;
> > - }
> > + if (!may_expand_vm(map->mm, map->flags, map->pglen - vms->nr_pages))
> > + return -ENOMEM;
> >
> > - /*
> > - * Private writable mapping: check memory availability
> > - */
> > - if (accountable_mapping(file, vm_flags)) {
> > - charged = pglen;
> > - charged -= vms.nr_accounted;
> > - if (charged) {
> > - error = security_vm_enough_memory_mm(mm, charged);
> > + /* Private writable mapping: check memory availability. */
> > + if (accountable_mapping(vmg->file, map->flags)) {
> > + map->charged = map->pglen;
> > + map->charged -= vms->nr_accounted;
> > + if (map->charged) {
> > + error = security_vm_enough_memory_mm(map->mm, map->charged);
> > if (error)
> > - goto abort_munmap;
> > + return error;
> > }
> >
> > - vms.nr_accounted = 0;
> > - vm_flags |= VM_ACCOUNT;
> > - vmg.flags = vm_flags;
> > + vms->nr_accounted = 0;
> > + map->flags |= VM_ACCOUNT;
> > }
> >
> > /*
> > - * clear PTEs while the vma is still in the tree so that rmap
> > + * Clear PTEs while the vma is still in the tree so that rmap
> > * cannot race with the freeing later in the truncate scenario.
> > * This is also needed for mmap_file(), which is why vm_ops
> > * close function is called.
> > */
> > - vms_clean_up_area(&vms, &mas_detach);
> > - vma = vma_merge_new_range(&vmg);
> > - if (vma)
> > - goto expanded;
> > + vms_clean_up_area(vms, &map->mas_detach);
> > +
> > + return 0;
> > +}
> > +
> > +static int __mmap_new_file_vma(struct mmap_state *map, struct vm_area_struct *vma,
> > + struct vm_area_struct **mergep)
> > +{
> > + struct vma_iterator *vmi = map->vmi;
> > + struct vma_merge_struct *vmg = map->vmg;
> > + int error;
> > +
> > + vma->vm_file = get_file(vmg->file);
> > + error = mmap_file(vma->vm_file, vma);
> > + if (error) {
> > + 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);
> > +
> > + return error;
> > + }
> > +
> > + /* Drivers cannot alter the address of the VMA. */
> > + WARN_ON_ONCE(vmg->start != vma->vm_start);
> > + /*
> > + * Drivers should not permit writability when previously it was
> > + * disallowed.
> > + */
> > + VM_WARN_ON_ONCE(map->flags != vma->vm_flags &&
> > + !(map->flags & VM_MAYWRITE) &&
> > + (vma->vm_flags & VM_MAYWRITE));
> > +
> > + vma_iter_config(vmi, vmg->start, vmg->end);
> > + /*
> > + * If flags changed after mmap_file(), we should try merge
> > + * vma again as we may succeed this time.
> > + */
> > + if (unlikely(map->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);
> > +
> > + if (merge) {
> > + /*
> > + * ->mmap() can change vma->vm_file and fput
> > + * the original file. So fput the vma->vm_file
> > + * here or we would add an extra fput for file
> > + * and cause general protection fault
> > + * ultimately.
> > + */
> > + fput(vma->vm_file);
> > + vm_area_free(vma);
> > + vma_iter_free(vmi);
> > + *mergep = merge;
> > + } else {
> > + vma_iter_config(vmi, vmg->start, vmg->end);
> > + }
> > + }
> > +
> > + map->flags = vma->vm_flags;
> > + return 0;
> > +}
> > +
> > +/*
> > + * __mmap_new_vma() - Allocate a new VMA for the region, as merging was not
> > + * possible.
> > + *
> > + * An exception to this is if the mapping is file-backed, and
> > + * the underlying driver changes the VMA flags, permitting a
> > + * subsequent merge of the VMA, in which case the returned
> > + * VMA is one that was merged on a second attempt.
>
> It seems all the descriptions have indented lines.
I like it that way :( will change :'(
>
> > + *
> > + * @map: Mapping state.
> > + * @vmap: Output pointer for the new VMA.
> > + *
> > + * Returns: Zero on success, or an error.
> > + */
> > +static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
> > +{
> > + struct vma_iterator *vmi = map->vmi;
> > + struct vma_merge_struct *vmg = map->vmg;
> > + struct vm_area_struct *merge = NULL;
> > + int error = 0;
> > + struct vm_area_struct *vma;
> > +
> > /*
> > * Determine the object being mapped and call the appropriate
> > * specific mapper. the address has already been validated, but
> > * not unmapped, but the maps are removed from the list.
> > */
> > - vma = vm_area_alloc(mm);
> > - if (!vma) {
> > - error = -ENOMEM;
> > - goto unacct_error;
> > - }
> > + vma = vm_area_alloc(map->mm);
> > + if (!vma)
> > + return -ENOMEM;
> >
> > - vma_iter_config(&vmi, addr, end);
> > - vma_set_range(vma, addr, end, pgoff);
> > - vm_flags_init(vma, vm_flags);
> > - vma->vm_page_prot = vm_get_page_prot(vm_flags);
> > + vma_iter_config(vmi, vmg->start, vmg->end);
>
> This function is only called from __mmap_region() after an
> __mmap_prepare() and vma_merge_new_range(). The state should be fine,
> so maybe this could be WARN_ONs? Although, it's probably safer to just
> leave it.
Yeah perhaps one for the laterbase?
>
> > + vma_set_range(vma, vmg->start, vmg->end, vmg->pgoff);
> > + vm_flags_init(vma, map->flags);
> > + vma->vm_page_prot = vm_get_page_prot(map->flags);
> >
> > - if (vma_iter_prealloc(&vmi, vma)) {
> > + 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_file_vma;
> > -
> > - /* Drivers cannot alter the address of the VMA. */
> > - WARN_ON_ONCE(addr != vma->vm_start);
> > - /*
> > - * Drivers should not permit writability when previously it was
> > - * disallowed.
> > - */
> > - VM_WARN_ON_ONCE(vm_flags != vma->vm_flags &&
> > - !(vm_flags & VM_MAYWRITE) &&
> > - (vma->vm_flags & VM_MAYWRITE));
> > -
> > - vma_iter_config(&vmi, addr, end);
> > - /*
> > - * If vm_flags changed after mmap_file(), we should try merge
> > - * 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);
> > -
> > - if (merge) {
> > - /*
> > - * ->mmap() can change vma->vm_file and fput
> > - * the original file. So fput the vma->vm_file
> > - * here or we would add an extra fput for file
> > - * and cause general protection fault
> > - * ultimately.
> > - */
> > - fput(vma->vm_file);
> > - vm_area_free(vma);
> > - vma_iter_free(&vmi);
> > - vma = merge;
> > - /* Update vm_flags to pick up the change. */
> > - vm_flags = vma->vm_flags;
> > - goto file_expanded;
> > - }
> > - vma_iter_config(&vmi, addr, end);
> > - }
> > -
> > - vm_flags = vma->vm_flags;
> > - } else if (vm_flags & VM_SHARED) {
> > + if (vmg->file)
> > + error = __mmap_new_file_vma(map, vma, &merge);
> > + else if (map->flags & VM_SHARED)
> > error = shmem_zero_setup(vma);
> > - if (error)
> > - goto free_iter_vma;
> > - } else {
> > + else
> > vma_set_anonymous(vma);
> > - }
> > +
> > + if (error)
> > + goto free_iter_vma;
> > +
> > + if (merge)
> > + goto file_expanded;
> >
> > #ifdef CONFIG_SPARC64
> > /* TODO: Fix SPARC ADI! */
> > - WARN_ON_ONCE(!arch_validate_flags(vm_flags));
> > + WARN_ON_ONCE(!arch_validate_flags(map->flags));
> > #endif
> >
> > /* Lock the VMA since it is modified after insertion into VMA tree */
> > vma_start_write(vma);
> > - vma_iter_store(&vmi, vma);
> > - mm->map_count++;
> > + vma_iter_store(vmi, vma);
> > + map->mm->map_count++;
> > vma_link_file(vma);
> >
> > /*
> > * vma_merge_new_range() calls khugepaged_enter_vma() too, the below
> > * call covers the non-merge case.
> > */
> > - khugepaged_enter_vma(vma, vma->vm_flags);
> > + khugepaged_enter_vma(vma, map->flags);
> >
> > file_expanded:
> > - file = vma->vm_file;
> > ksm_add_vma(vma);
>
> __mmap_new_file_vma() may free the vma. I assume this is what you
> mentioned elsewhere about UAF.
Yeah no this is a new one, let me totally rework this thing and make it
operate on a single VMA and return a merged boolean and reduce the window
between 'pointer freed and pointer reassgined to literally ZERO LINES.
Sorry, I was probably feverish when I did this bit...
>
> > -expanded:
> > +
>
> Extra whitespace.
You hate that whitespace :(
>
> > + *vmap = vma;
> > + return 0;
> > +
> > +free_iter_vma:
> > + vma_iter_free(vmi);
> > +free_vma:
> > + vm_area_free(vma);
> > + return error;
> > +}
> > +
> > +/*
> > + * __mmap_complete() - Unmap any VMAs we overlap, account memory mapping
> > + * statistics, handle locking and finalise the VMA.
> > + *
> > + * @map: Mapping state.
> > + * @vma: Merged or newly allocated VMA for the mmap()'d region.
> > + */
> > +static void __mmap_complete(struct mmap_state *map, struct vm_area_struct *vma)
> > +{
> > + struct mm_struct *mm = map->mm;
> > + unsigned long vm_flags = vma->vm_flags;
> > +
> > perf_event_mmap(vma);
> >
> > - /* Unmap any existing mapping in the area */
> > - vms_complete_munmap_vmas(&vms, &mas_detach);
> > + /* Unmap any existing mapping in the area. */
> > + vms_complete_munmap_vmas(&map->vms, &map->mas_detach);
> >
> > - vm_stat_account(mm, vm_flags, pglen);
> > + vm_stat_account(mm, vma->vm_flags, map->pglen);
> > if (vm_flags & VM_LOCKED) {
> > if ((vm_flags & VM_SPECIAL) || vma_is_dax(vma) ||
> > is_vm_hugetlb_page(vma) ||
> > - vma == get_gate_vma(current->mm))
> > + vma == get_gate_vma(mm))
> > vm_flags_clear(vma, VM_LOCKED_MASK);
> > else
> > - mm->locked_vm += pglen;
> > + mm->locked_vm += map->pglen;
> > }
> >
> > - if (file)
> > + if (vma->vm_file)
> > uprobe_mmap(vma);
> >
> > /*
> > @@ -2364,26 +2447,43 @@ unsigned long __mmap_region(struct file *file, unsigned long addr,
> > vm_flags_set(vma, VM_SOFTDIRTY);
> >
> > vma_set_page_prot(vma);
> > +}
> >
> > - return addr;
> > +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;
> > + int error;
> > + VMA_ITERATOR(vmi, mm, addr);
> > + VMG_STATE(vmg, mm, &vmi, addr, addr + len, vm_flags, pgoff);
> > + MMAP_STATE(map, mm, &vmi, &vmg, uf, vm_flags, len);
> >
> > -unmap_and_free_file_vma:
> > - fput(vma->vm_file);
> > - vma->vm_file = NULL;
> > + vmg.file = file;
> >
> > - 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:
> > - if (charged)
> > - vm_unacct_memory(charged);
> > + error = __mmap_prepare(&map);
> > + if (error)
> > + goto abort_munmap;
> > +
> > + /* Attempt to merge with adjacent VMAs... */
> > + vmg.flags = map.flags;
> > + vma = vma_merge_new_range(&vmg);
> > + if (!vma) {
> > + /* ...but if we can't, allocate a new VMA. */
> > + error = __mmap_new_vma(&map, &vma);
> > + if (error)
> > + goto unacct_error;
> > + }
> > +
> > + __mmap_complete(&map, vma);
> >
> > + return addr;
> > +
> > +unacct_error:
> > + if (map.charged)
> > + vm_unacct_memory(map.charged);
>
> So this is effectively undoing __mmap_prepare()'s accounting. I don't
> have a better label for it, but it's not obvious by the label that the
> accounting was done in __mmap_prepare().
There's a comment in the description of the function. I'll add a comment
here too to be clear about it.
>
> > abort_munmap:
> > - vms_abort_munmap_vmas(&vms, &mas_detach);
> > -gather_failed:
> > + vms_abort_munmap_vmas(&map.vms, &map.mas_detach);
> > return error;
> > }
> > --
> > 2.47.0
