Re: [PATCH v2] fs: clear file privilege bits when mmap writing

[Andrew Morton]

On Wed, 2 Dec 2015 16:03:42 -0800 Kees Cook <keescook@xxxxxxxxxxxx> 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 and truncate but not
> when writing to a shared mmap on the file. This could allow the file
> writer to gain privileges by changing a binary without losing the
> setuid/setgid/caps bits.
> 
> Changing the bits requires holding inode->i_mutex, so it cannot be done
> during the page fault (due to mmap_sem being held during the fault).
> Instead, clear the bits if PROT_WRITE is being used at mmap time.
> 
> ...
>
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1340,6 +1340,17 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>  			if (locks_verify_locked(file))
>  				return -EAGAIN;
>  
> +			/*
> +			 * If we must remove privs, we do it here since
> +			 * doing it during page COW is expensive and
> +			 * cannot hold inode->i_mutex.
> +			 */
> +			if (prot & PROT_WRITE && !IS_NOSEC(inode)) {
> +				mutex_lock(&inode->i_mutex);
> +				file_remove_privs(file);
> +				mutex_unlock(&inode->i_mutex);
> +			}
> +

Still ignoring the file_remove_privs() return value.  If this is
deliberate then a description of the reasons should be included?

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