Le 12/03/2024 à 23:28, Rick Edgecombe a écrit :
> When memory is being placed, mmap() will take care to respect the guard
> gaps of certain types of memory (VM_SHADOWSTACK, VM_GROWSUP and
> VM_GROWSDOWN). In order to ensure guard gaps between mappings, mmap()
> needs to consider two things:
> 1. That the new mapping isn’t placed in an any existing mappings guard
> gaps.
> 2. That the new mapping isn’t placed such that any existing mappings
> are not in *its* guard gaps.
>
> The long standing behavior of mmap() is to ensure 1, but not take any care
> around 2. So for example, if there is a PAGE_SIZE free area, and a
> mmap() with a PAGE_SIZE size, and a type that has a guard gap is being
> placed, mmap() may place the shadow stack in the PAGE_SIZE free area. Then
> the mapping that is supposed to have a guard gap will not have a gap to
> the adjacent VMA.
>
> For MAP_GROWSDOWN/VM_GROWSDOWN and MAP_GROWSUP/VM_GROWSUP this has not
> been a problem in practice because applications place these kinds of
> mappings very early, when there is not many mappings to find a space
> between. But for shadow stacks, they may be placed throughout the lifetime
> of the application.
>
> Use the start_gap field to find a space that includes the guard gap for
> the new mapping. Take care to not interfere with the alignment.
>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
> v3:
> - Spelling fix in comment
>
> v2:
> - Remove VM_UNMAPPED_START_GAP_SET and have struct vm_unmapped_area_info
> initialized with zeros (in another patch). (Kirill)
> - Drop unrelated space change (Kirill)
> - Add comment around interactions of alignment and start gap step
> (Kirill)
> ---
> include/linux/mm.h | 1 +
> mm/mmap.c | 12 +++++++++---
> 2 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index d91cde79aaee..deade7be00d0 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3418,6 +3418,7 @@ struct vm_unmapped_area_info {
> unsigned long high_limit;
> unsigned long align_mask;
> unsigned long align_offset;
> + unsigned long start_gap;
Only a start_gap is needed ? No need of an end_gap ?
> };
>
> extern unsigned long vm_unmapped_area(struct vm_unmapped_area_info *info);
> diff --git a/mm/mmap.c b/mm/mmap.c
> index b889c79d11bd..634e706fd97e 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1582,7 +1582,7 @@ static unsigned long unmapped_area(struct vm_unmapped_area_info *info)
> MA_STATE(mas, ¤t->mm->mm_mt, 0, 0);
>
> /* Adjust search length to account for worst case alignment overhead */
> - length = info->length + info->align_mask;
> + length = info->length + info->align_mask + info->start_gap;
> if (length < info->length)
> return -ENOMEM;
>
> @@ -1594,7 +1594,13 @@ static unsigned long unmapped_area(struct vm_unmapped_area_info *info)
> if (mas_empty_area(&mas, low_limit, high_limit - 1, length))
> return -ENOMEM;
>
> - gap = mas.index;
> + /*
> + * Adjust for the gap first so it doesn't interfere with the
> + * later alignment. The first step is the minimum needed to
> + * fulill the start gap, the next steps is the minimum to align
> + * that. It is the minimum needed to fulill both.
> + */
> + gap = mas.index + info->start_gap;
> gap += (info->align_offset - gap) & info->align_mask;
> tmp = mas_next(&mas, ULONG_MAX);
> if (tmp && (tmp->vm_flags & VM_STARTGAP_FLAGS)) { /* Avoid prev check if possible */
> @@ -1633,7 +1639,7 @@ static unsigned long unmapped_area_topdown(struct vm_unmapped_area_info *info)
>
> MA_STATE(mas, ¤t->mm->mm_mt, 0, 0);
> /* Adjust search length to account for worst case alignment overhead */
> - length = info->length + info->align_mask;
> + length = info->length + info->align_mask + info->start_gap;
> if (length < info->length)
> return -ENOMEM;
>
