Re: [PATCH v3 10/12] x86/mm: Implement HAVE_ARCH_UNMAPPED_AREA_VMFLAGS

[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.
> 
> Add x86 arch implementations of arch_get_unmapped_area_vmflags/_topdown()
> so future changes can allow the guard gap of type of vma being placed to
> be taken into account. This will be used for shadow stack memory.
> 
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
> v3:
>   - Commit log grammar
> 
> v2:
>   - Remove unnecessary added extern
> ---
>   arch/x86/include/asm/pgtable_64.h |  1 +
>   arch/x86/kernel/sys_x86_64.c      | 25 ++++++++++++++++++++-----
>   2 files changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pgtable_64.h b/arch/x86/include/asm/pgtable_64.h
> index 24af25b1551a..13dcaf436efd 100644
> --- a/arch/x86/include/asm/pgtable_64.h
> +++ b/arch/x86/include/asm/pgtable_64.h
> @@ -244,6 +244,7 @@ extern void cleanup_highmap(void);
>   
>   #define HAVE_ARCH_UNMAPPED_AREA
>   #define HAVE_ARCH_UNMAPPED_AREA_TOPDOWN
> +#define HAVE_ARCH_UNMAPPED_AREA_VMFLAGS
>   
>   #define PAGE_AGP    PAGE_KERNEL_NOCACHE
>   #define HAVE_PAGE_AGP 1
> diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c
> index b3278e4f7e59..d6fbc4dd08ef 100644
> --- a/arch/x86/kernel/sys_x86_64.c
> +++ b/arch/x86/kernel/sys_x86_64.c
> @@ -120,8 +120,8 @@ static void find_start_end(unsigned long addr, unsigned long flags,
>   }
>   
>   unsigned long
> -arch_get_unmapped_area(struct file *filp, unsigned long addr,
> -		unsigned long len, unsigned long pgoff, unsigned long flags)
> +arch_get_unmapped_area_vmflags(struct file *filp, unsigned long addr, unsigned long len,
> +		       unsigned long pgoff, unsigned long flags, vm_flags_t vm_flags)
>   {
>   	struct mm_struct *mm = current->mm;
>   	struct vm_area_struct *vma;
> @@ -156,9 +156,9 @@ arch_get_unmapped_area(struct file *filp, unsigned long addr,
>   }
>   
>   unsigned long
> -arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
> -			  const unsigned long len, const unsigned long pgoff,
> -			  const unsigned long flags)
> +arch_get_unmapped_area_topdown_vmflags(struct file *filp, unsigned long addr0,
> +			  unsigned long len, unsigned long pgoff,
> +			  unsigned long flags, vm_flags_t vm_flags)
>   {
>   	struct vm_area_struct *vma;
>   	struct mm_struct *mm = current->mm;
> @@ -227,3 +227,18 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
>   	 */
>   	return arch_get_unmapped_area(filp, addr0, len, pgoff, flags);
>   }
> +
> +unsigned long
> +arch_get_unmapped_area(struct file *filp, unsigned long addr,
> +		unsigned long len, unsigned long pgoff, unsigned long flags)
> +{
> +	return arch_get_unmapped_area_vmflags(filp, addr, len, pgoff, flags, 0);
> +}
> +
> +unsigned long
> +arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr,
> +			  const unsigned long len, const unsigned long pgoff,
> +			  const unsigned long flags)
> +{
> +	return arch_get_unmapped_area_topdown_vmflags(filp, addr, len, pgoff, flags, 0);
> +}

Wouldn't it be better to define those two as static inlines ?