Re: [PATCH 1/1] mm: Fix UAF when anon vma name is used after vma is freed

[Andrew Morton <akpm@linux-foundation.org>]

On Wed,  9 Feb 2022 16:18:01 -0800 Suren Baghdasaryan <surenb@google.com> wrote:

> When adjacent vmas are being merged it can result in the vma that was
> originally passed to madvise_update_vma being destroyed. In the current
> implementation, the name parameter passed to madvise_update_vma points
> directly to vma->anon_name->name and it is used after the call to
> vma_merge. In the cases when vma_merge merges the original vma and
> destroys it, this will result in use-after-free bug as shown below:
> 
> madvise_vma_behavior << passes vma->anon_name->name as name param
>   madvise_update_vma(name)
>     vma_merge
>       __vma_adjust
>         vm_area_free <-- frees the vma
>     replace_vma_anon_name(name) <-- UAF
> 
> Fix this by passing madvise_update_vma a copy of the name.
> 
> ...
>
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2263,7 +2263,6 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
>  
>  #ifdef CONFIG_ANON_VMA_NAME
>  
> -#define ANON_VMA_NAME_MAX_LEN		80
>  #define ANON_VMA_NAME_INVALID_CHARS	"\\`$[]"
>  
>  static inline bool is_valid_name_char(char ch)
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 5604064df464..f36a5a9942d8 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -976,6 +976,8 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
>  {
>  	int error;
>  	unsigned long new_flags = vma->vm_flags;
> +	char name_buf[ANON_VMA_NAME_MAX_LEN];
> +	const char *anon_name;
>  
>  	switch (behavior) {
>  	case MADV_REMOVE:
> @@ -1040,8 +1042,18 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
>  		break;
>  	}
>  
> +	anon_name = vma_anon_name(vma);
> +	if (anon_name) {
> +		/*
> +		 * Make a copy of the name because vma might be destroyed when
> +		 * merged with another one and the name parameter might be used
> +		 * after that.
> +		 */
> +		strcpy(name_buf, anon_name);
> +		anon_name = name_buf;
> +	}
>  	error = madvise_update_vma(vma, prev, start, end, new_flags,
> -				   vma_anon_name(vma));
> +				   anon_name);

anon_name is refcounted.  Why not use kref_get()/kref_put() instead of
taking a copy?