Re: [PATCH] selinux: fix mprotect PROT_EXEC regression caused by mm change

[Paul Moore <paul@paul-moore.com>]

On Fri, Jul 10, 2015 at 9:40 AM, Stephen Smalley <sds@tycho.nsa.gov> wrote:
> commit 66fc13039422ba7df2d01a8ee0873e4ef965b50b ("mm: shmem_zero_setup skip
> security check and lockdep conflict with XFS") caused a regression for
> SELinux by disabling any SELinux checking of mprotect PROT_EXEC on
> shared anonymous mappings.  However, even before that regression, the
> checking on such mprotect PROT_EXEC calls was inconsistent with the
> checking on a mmap PROT_EXEC call for a shared anonymous mapping.  On a
> mmap, the security hook is passed a NULL file and knows it is dealing with
> an anonymous mapping and therefore applies an execmem check and no file
> checks.  On a mprotect, the security hook is passed a vma with a
> non-NULL vm_file (as this was set from the internally-created shmem
> file during mmap) and therefore applies the file-based execute check and
> no execmem check.  Since the aforementioned commit now marks the shmem
> zero inode with the S_PRIVATE flag, the file checks are disabled and
> we have no checking at all on mprotect PROT_EXEC.  Add a test to
> the mprotect hook logic for such private inodes, and apply an execmem
> check in that case.  This makes the mmap and mprotect checking consistent
> for shared anonymous mappings, as well as for /dev/zero and ashmem.
>
> Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
> ---
>  security/selinux/hooks.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

Thanks for the discussion, and the patch.  I'll send this up to James
for 4.2 and mark it for stable.

> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 6231081..564079c 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -3283,7 +3283,8 @@ static int file_map_prot_check(struct file *file, unsigned long prot, int shared
>         int rc = 0;
>
>         if (default_noexec &&
> -           (prot & PROT_EXEC) && (!file || (!shared && (prot & PROT_WRITE)))) {
> +           (prot & PROT_EXEC) && (!file || IS_PRIVATE(file_inode(file)) ||
> +                                  (!shared && (prot & PROT_WRITE)))) {
>                 /*
>                  * We are making executable an anonymous mapping or a
>                  * private file mapping that will also be writable.
> --
> 2.1.0
>

-- 
paul moore
www.paul-moore.com

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