Re: [PATCH v3 03/12] mm: Use get_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.
> 
> Use mm_get_unmapped_area_vmflags() in the do_mmap() so future changes
> can cause shadow stack mappings to be placed with a guard gap. Also use
> the THP variant that takes vm_flags, such that THP shadow stack can get the
> same treatment. Adjust the vm_flags calculation to happen earlier so that
> the vm_flags can be passed into __get_unmapped_area().
> 
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
> v2:
>   - Make get_unmapped_area() a static inline (Kirill)
> ---
>   include/linux/mm.h | 11 ++++++++++-
>   mm/mmap.c          | 34 ++++++++++++++++------------------
>   2 files changed, 26 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f5a97dec5169..d91cde79aaee 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3363,7 +3363,16 @@ extern int install_special_mapping(struct mm_struct *mm,
>   unsigned long randomize_stack_top(unsigned long stack_top);
>   unsigned long randomize_page(unsigned long start, unsigned long range);
>   
> -extern unsigned long get_unmapped_area(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
> +unsigned long
> +__get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
> +		    unsigned long pgoff, unsigned long flags, vm_flags_t vm_flags);
> +
> +static inline unsigned long
> +get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
> +		  unsigned long pgoff, unsigned long flags)
> +{
> +	return __get_unmapped_area(file, addr, len, pgoff, flags, 0);
> +}
>   
>   extern unsigned long mmap_region(struct file *file, unsigned long addr,
>   	unsigned long len, vm_flags_t vm_flags, unsigned long pgoff,
> diff --git a/mm/mmap.c b/mm/mmap.c
> index e23ce8ca24c9..a3128ed26676 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1257,18 +1257,6 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>   	if (mm->map_count > sysctl_max_map_count)
>   		return -ENOMEM;
>   
> -	/* Obtain the address to map to. we verify (or select) it and ensure
> -	 * that it represents a valid section of the address space.
> -	 */
> -	addr = get_unmapped_area(file, addr, len, pgoff, flags);
> -	if (IS_ERR_VALUE(addr))
> -		return addr;
> -
> -	if (flags & MAP_FIXED_NOREPLACE) {
> -		if (find_vma_intersection(mm, addr, addr + len))
> -			return -EEXIST;
> -	}
> -
>   	if (prot == PROT_EXEC) {
>   		pkey = execute_only_pkey(mm);
>   		if (pkey < 0)
> @@ -1282,6 +1270,18 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>   	vm_flags |= calc_vm_prot_bits(prot, pkey) | calc_vm_flag_bits(flags) |
>   			mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;
>   
> +	/* Obtain the address to map to. we verify (or select) it and ensure
> +	 * that it represents a valid section of the address space.
> +	 */
> +	addr = __get_unmapped_area(file, addr, len, pgoff, flags, vm_flags);
> +	if (IS_ERR_VALUE(addr))
> +		return addr;
> +
> +	if (flags & MAP_FIXED_NOREPLACE) {
> +		if (find_vma_intersection(mm, addr, addr + len))
> +			return -EEXIST;
> +	}
> +
>   	if (flags & MAP_LOCKED)
>   		if (!can_do_mlock())
>   			return -EPERM;
> @@ -1839,8 +1839,8 @@ unsigned long mm_get_unmapped_area_vmflags(struct mm_struct *mm, struct file *fi
>   }
>   
>   unsigned long
> -get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
> -		unsigned long pgoff, unsigned long flags)
> +__get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
> +		unsigned long pgoff, unsigned long flags, vm_flags_t vm_flags)
>   {
>   	unsigned long (*get_area)(struct file *, unsigned long,
>   				  unsigned long, unsigned long, unsigned long)
> @@ -1875,8 +1875,8 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
>   	if (get_area)
>   		addr = get_area(file, addr, len, pgoff, flags);

What about get_area(), what happens to vm_flags in case that function is 
used ?

>   	else
> -		addr = mm_get_unmapped_area(current->mm, file, addr, len,
> -					    pgoff, flags);
> +		addr = mm_get_unmapped_area_vmflags(current->mm, file, addr, len,
> +						    pgoff, flags, vm_flags);
>   	if (IS_ERR_VALUE(addr))
>   		return addr;
>   
> @@ -1889,8 +1889,6 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
>   	return error ? error : addr;
>   }
>   
> -EXPORT_SYMBOL(get_unmapped_area);
> -

Don't you have to export __get_unmapped_area() so that the static inline 
get_unmapped_area() used in a module have access to __get_unmapped_area() ?

If it was pointless to export get_unmapped_area(), its removal should be 
a separate patch.

>   unsigned long
>   mm_get_unmapped_area(struct mm_struct *mm, struct file *file,
>   		     unsigned long addr, unsigned long len,