Re: [PATCH 2/3] mm/mseal: limit scope of mseal address zero to address zero

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/16/26 15:43, Lorenzo Stoakes (ARM) wrote:
> Commit 44f65d900698 ("binfmt_elf: mseal address zero") unconditionally
> provided do_mseal() to any internal kernel caller in order to address a
> corner case slated for possible removal.
> 
> It also incorrectly attempts to mseal() without checking to see whether the
> mapping even succeeded.
> 
> Restrict the scope to the corner case by providing mseal_mmap_page_zero()
> which asserts the MMAP_PAGE_ZERO personality.
> 
> Avoid unnecessary checks in the start, end range by abstracting the actual
> mseal()'ing to mseal() and have mseal_mmap_page_zero() call that instead.
> 
> Only try to seal the VMA if we mapped the VMA.
> 
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> ---
>  fs/binfmt_elf.c    |  7 ++-----
>  include/linux/mm.h |  8 ++------
>  mm/mseal.c         | 48 +++++++++++++++++++++++++++++++++++-------------
>  3 files changed, 39 insertions(+), 24 deletions(-)
> 
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 16a56b6b3f6c..e3131a311995 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -1353,11 +1353,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  		   emulate the SVr4 behavior. Sigh. */
>  		error = vm_mmap(NULL, 0, PAGE_SIZE, PROT_READ | PROT_EXEC,
>  				MAP_FIXED | MAP_PRIVATE, 0);
> -
> -		retval = do_mseal(0, PAGE_SIZE, 0);
> -		if (retval)
> -			pr_warn_ratelimited("pid=%d, couldn't seal address 0, ret=%d.\n",
> -					    task_pid_nr(current), retval);
> +		if (!error)
> +			mseal_mmap_page_zero();
>  	}
>  
>  	regs = current_pt_regs();
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 550fb92957d1..87feaa5a2b78 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -5291,13 +5291,9 @@ int reserve_mem_find_by_name(const char *name, phys_addr_t *start, phys_addr_t *
>  int reserve_mem_release_by_name(const char *name);
>  
>  #ifdef CONFIG_64BIT
> -int do_mseal(unsigned long start, size_t len_in, unsigned long flags);
> +void mseal_mmap_page_zero(void);

do_mseal() gone here, nice.

>  #else
> -static inline int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
> -{
> -	/* noop on 32 bit */
> -	return 0;
> -}
> +static inline void mseal_mmap_page_zero(void) {}
>  #endif
>  
>  /*
> diff --git a/mm/mseal.c b/mm/mseal.c
> index 207fea89c61e..5930551d84f2 100644
> --- a/mm/mseal.c
> +++ b/mm/mseal.c
> @@ -32,7 +32,7 @@ static bool range_contains_unmapped(unsigned long start, unsigned long end)
>  	return prev_end < end;
>  }
>  
> -static int mseal_apply(unsigned long start, unsigned long end)
> +static int __mseal(unsigned long start, unsigned long end)
>  {
>  	struct vm_area_struct *vma, *prev;
>  	VMA_ITERATOR(vmi, current->mm, start);
> @@ -66,6 +66,38 @@ static int mseal_apply(unsigned long start, unsigned long end)
>  	return 0;
>  }
>  
> +static int mseal(unsigned long start, unsigned long end)

mseal_range() could be indeed nicer. But do_mseal() is now a pure helper for
"SYSCALL_DEFINE3(mseal" only ...could we simply inline do_mseal into
SYSCALL_DEFINE3(mseal)?

We could do that separately, ofc.

-- 
Cheers,

David