Re: [PATCH mm-hotfixes] mm/mseal: update VMA end correctly on merge

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

On 3/27/26 10:06, Lorenzo Stoakes (Oracle) wrote:
> Previously we stored the end of the current VMA in curr_end, and then upon
> iterating to the next VMA updated curr_start to curr_end to advance to the
> next VMA.
> 
> However, this doesn't take into account the fact that a VMA might be
> updated due to a merge by vma_modify_flags(), which can result in curr_end
> being stale and thus, upon setting curr_start to curr_end, ending up with
> an incorrect curr_start on the next iteration.
> 
> Resolve the issue by setting curr_end to vma->vm_end unconditionally to
> ensure this value remains updated should this occur.
> 
> Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> Fixes: 6c2da14ae1e0 ("mm/mseal: rework mseal apply logic")
> Cc: <stable@vger.kernel.org>
> Reported-by: Antonius <antonius@bluedragonsec.com>
> Closes: https://lore.kernel.org/linux-mm/CAK8a0jyHXqBpt8Xe8v9SNDbnRiwz7OthA8SKY=NLRY7smPEP3Q@mail.gmail.com/
> ---
>  mm/mseal.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/mseal.c b/mm/mseal.c
> index 316b5e1dec78..2d72a15d8ea1 100644
> --- a/mm/mseal.c
> +++ b/mm/mseal.c
> @@ -66,7 +66,7 @@ static int mseal_apply(struct mm_struct *mm,
>  		prev = vma;
> 
>  	for_each_vma_range(vmi, vma, end) {
> -		const unsigned long curr_end = MIN(vma->vm_end, end);
> +		unsigned long curr_end = MIN(vma->vm_end, end);
> 
>  		if (!(vma->vm_flags & VM_SEALED)) {
>  			vm_flags_t vm_flags = vma->vm_flags | VM_SEALED;
> @@ -76,6 +76,7 @@ static int mseal_apply(struct mm_struct *mm,
>  			if (IS_ERR(vma))
>  				return PTR_ERR(vma);
>  			vm_flags_set(vma, VM_SEALED);
> +			curr_end = vma->vm_end; /* Merge may have updated. */
>  		}


I was a bit confused why curr_start is allowed to not start within the VMA,
but before it. Then I recalled that range_contains_unmapped() checks for no holes.


Would the following also sort out the problem and even simplify the code?

diff --git a/mm/mseal.c b/mm/mseal.c
index 603df53ad267..e2093ae3d25c 100644
--- a/mm/mseal.c
+++ b/mm/mseal.c
@@ -56,7 +56,6 @@ static int mseal_apply(struct mm_struct *mm,
                unsigned long start, unsigned long end)
 {
        struct vm_area_struct *vma, *prev;
-       unsigned long curr_start = start;
        VMA_ITERATOR(vmi, mm, start);
 
        /* We know there are no gaps so this will be non-NULL. */
@@ -66,6 +65,7 @@ static int mseal_apply(struct mm_struct *mm,
                prev = vma;
 
        for_each_vma_range(vmi, vma, end) {
+               const unsigned long curr_start = MAX(vma->vm_start, start);
                const unsigned long curr_end = MIN(vma->vm_end, end);
 
                if (!vma_test(vma, VMA_SEALED_BIT)) {
@@ -82,7 +82,6 @@ static int mseal_apply(struct mm_struct *mm,
                }
 
                prev = vma;
-               curr_start = curr_end;
        }
 
        return 0;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c


I might be wrong about that, I've been staring at the screen for too long today.


-- 
Cheers,

David