Re: [PATCH v3 09/12] mm: Take placement mappings gap into account

[Christophe Leroy]

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, &current->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, &current->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;
>