Re: [RFC PATCH v3 2/6] [PATCH 2/6] mm: add merging after mremap resize

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: "Jakub Matěna" <matenajakub@gmail.com>
Cc: linux-mm@kvack.org, patches@lists.linux.dev,
	linux-kernel@vger.kernel.org, vbabka@suse.cz, mhocko@kernel.org,
	mgorman@techsingularity.net, willy@infradead.org,
	liam.howlett@oracle.com, hughd@google.com, riel@surriel.com,
	rostedt@goodmis.org, peterz@infradead.org, david@redhat.com
Subject: Re: [RFC PATCH v3 2/6] [PATCH 2/6] mm: add merging after mremap resize
Date: Fri, 20 May 2022 16:41:24 +0300	[thread overview]
Message-ID: <20220520134124.6glbfzhrgzutfor6@box.shutemov.name> (raw)
In-Reply-To: <20220516125405.1675-3-matenajakub@gmail.com>

On Mon, May 16, 2022 at 02:54:01PM +0200, Jakub Matěna wrote:
> When mremap call results in expansion, it might be possible to merge the
> VMA with the next VMA which might become adjacent. This patch adds
> vma_merge call after the expansion is done to try and merge.
> 
> Signed-off-by: Jakub Matěna <matenajakub@gmail.com>
> ---
>  mm/mremap.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 303d3290b938..75cda854ec58 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -9,6 +9,7 @@
>   */
>  
>  #include <linux/mm.h>
> +#include <linux/mm_inline.h>
>  #include <linux/hugetlb.h>
>  #include <linux/shm.h>
>  #include <linux/ksm.h>
> @@ -1022,8 +1023,10 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
>  				}
>  			}
>  
> -			if (vma_adjust(vma, vma->vm_start, addr + new_len,
> -				       vma->vm_pgoff, NULL)) {
> +			if (!vma_merge(mm, vma, addr + old_len, addr + new_len,
> +					vma->vm_flags, vma->anon_vma, vma->vm_file,
> +					vma->vm_pgoff + (old_len >> PAGE_SHIFT), vma_policy(vma),
> +					vma->vm_userfaultfd_ctx, anon_vma_name(vma))) {

Hm. Don't you need to update 'vma' with result of vma_merge()?

'vma' is used below the point and IIUC it can be use-after-free.

-- 
 Kirill A. Shutemov