Re: [PATCH] Fix do_pages_stat to use compat_uptr_t

[David Hildenbrand]

On 25.06.25 17:24, Christoph Berg wrote:

Subject should start with "mm/migrate:"

> For arrays with more than 16 entries, the old code would incorrectly
> advance the pages pointer by 16 words instead of 16 compat_uptr_t.
>
> Signed-off-by: Christoph Berg <myon@xxxxxxxxxx>
> Suggested-by: Bertrand Drouvot <bertranddrouvot.pg@xxxxxxxxx>

Likely we want a

Fixes:

and then this is probably "Reported-by:" paired with a "Closes:" link
to any such report.

But I'm wondering how long this has already been like that. :)

> ---
>   mm/migrate.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 8cf0f9c9599..542c81ec3ed 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2444,7 +2444,13 @@ static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages,
>   		if (copy_to_user(status, chunk_status, chunk_nr * sizeof(*status)))
>   			break;
>   
> -		pages += chunk_nr;
> +		if (in_compat_syscall()) {
> +			compat_uptr_t __user *pages32 = (compat_uptr_t __user *)pages;
> +
> +			pages32 += chunk_nr;
> +			pages = (const void __user * __user *) pages32;
> +		} else
> +			pages += chunk_nr;
>   		status += chunk_nr;
>   		nr_pages -= chunk_nr;
>   	}

Something a bit more elegant might be:

diff --git a/mm/migrate.c b/mm/migrate.c
index ea8c74d996592..fc99448771041 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2398,6 +2398,7 @@ static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages,
 
 static int get_compat_pages_array(const void __user *chunk_pages[],
                                  const void __user * __user *pages,
+                                 unsigned long chunk_offs,
                                  unsigned long chunk_nr)
 {
        compat_uptr_t __user *pages32 = (compat_uptr_t __user *)pages;
@@ -2405,7 +2406,7 @@ static int get_compat_pages_array(const void __user *chunk_pages[],
        int i;
 
        for (i = 0; i < chunk_nr; i++) {
-               if (get_user(p, pages32 + i))
+               if (get_user(p, pages32 + i + chunk_offs))
                        return -EFAULT;
                chunk_pages[i] = compat_ptr(p);
        }
@@ -2424,13 +2425,14 @@ static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages,
 #define DO_PAGES_STAT_CHUNK_NR 16UL
        const void __user *chunk_pages[DO_PAGES_STAT_CHUNK_NR];
        int chunk_status[DO_PAGES_STAT_CHUNK_NR];
+       unsigned long chunk_offs = 0;
 
        while (nr_pages) {
                unsigned long chunk_nr = min(nr_pages, DO_PAGES_STAT_CHUNK_NR);
 
                if (in_compat_syscall()) {
                        if (get_compat_pages_array(chunk_pages, pages,
-                                                  chunk_nr))
+                                                  chunk_offs, chunk_nr))
                                break;
                } else {
                        if (copy_from_user(chunk_pages, pages,
@@ -2440,11 +2442,11 @@ static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages,
 
                do_pages_stat_array(mm, chunk_nr, chunk_pages, chunk_status);
 
-               if (copy_to_user(status, chunk_status, chunk_nr * sizeof(*status)))
+               if (copy_to_user(status + chunk_offs, chunk_status,
+                                chunk_nr * sizeof(*status)))
                        break;
 
-               pages += chunk_nr;
-               status += chunk_nr;
+               chunk_offs += chunk_nr;
                nr_pages -= chunk_nr;
        }
        return nr_pages ? -EFAULT : 0;

(untested, of course)


--
Cheers,

David / dhildenb