Re: [PATCH] fs: clear file set[ug]id when writing via mmap

[Willy Tarreau]

Hi Kees,

On Thu, Nov 19, 2015 at 04:10:43PM -0800, Kees Cook wrote:
> Normally, when a user can modify a file that has setuid or setgid bits,
> those bits are cleared when they are not the file owner or a member of the
> group. This is enforced when using write() directly but not when writing
> to a shared mmap on the file. This could allow the file writer to gain
> privileges by changing the binary without losing the setuid/setgid bits.
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  mm/memory.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index deb679c31f2a..4c970a4e0057 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2036,6 +2036,7 @@ static inline int wp_page_reuse(struct mm_struct *mm,
>  
>  		if (!page_mkwrite)
>  			file_update_time(vma->vm_file);
> +		file_remove_privs(vma->vm_file);

I thought you said in one of the early mails of this thread that it
didn't work. Or maybe I misunderstood.

Also, don't you think we should move that into the if (!page_mkwrite)
just like for the time update ?

Willy

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