On Sun, Jun 14, 2015 at 12:48 PM, Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> It appears that, at some point last year, XFS made directory handling
> changes which bring it into lockdep conflict with shmem_zero_setup():
> it is surprising that mmap() can clone an inode while holding mmap_sem,
> but that has been so for many years.
>
> Since those few lockdep traces that I've seen all implicated selinux,
> I'm hoping that we can use the __shmem_file_setup(,,,S_PRIVATE) which
> v3.13's commit c7277090927a ("security: shmem: implement kernel private
> shmem inodes") introduced to avoid LSM checks on kernel-internal inodes:
> the mmap("/dev/zero") cloned inode is indeed a kernel-internal detail.
>
> This also covers the !CONFIG_SHMEM use of ramfs to support /dev/zero
> (and MAP_SHARED|MAP_ANONYMOUS). I thought there were also drivers
> which cloned inode in mmap(), but if so, I cannot locate them now.
This causes a regression for SELinux (please, in the future, cc
selinux list and Paul Moore on SELinux-related changes). In
particular, this change disables SELinux checking of mprotect
PROT_EXEC on shared anonymous mappings, so we lose the ability to
control executable mappings. That said, we are only getting that
check today as a side effect of our file execute check on the tmpfs
inode, whereas it would be better (and more consistent with the
mmap-time checks) to apply an execmem check in that case, in which
case we wouldn't care about the inode-based check. However, I am
unclear on how to correctly detect that situation from
selinux_file_mprotect() -> file_map_prot_check(), because we do have a
non-NULL vma->vm_file so we treat it as a file execute check. In
contrast, if directly creating an anonymous shared mapping with
PROT_EXEC via mmap(...PROT_EXEC...), selinux_mmap_file is called with
a NULL file and therefore we end up applying an execmem check.
>
> Reported-and-tested-by: Prarit Bhargava <prarit@xxxxxxxxxx>
> Reported-by: Daniel Wagner <wagi@xxxxxxxxx>
> Reported-by: Morten Stevens <mstevens@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---
>
> mm/shmem.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> --- 4.1-rc7/mm/shmem.c 2015-04-26 19:16:31.352191298 -0700
> +++ linux/mm/shmem.c 2015-06-14 09:26:49.461120166 -0700
> @@ -3401,7 +3401,13 @@ int shmem_zero_setup(struct vm_area_stru
> struct file *file;
> loff_t size = vma->vm_end - vma->vm_start;
>
> - file = shmem_file_setup("dev/zero", size, vma->vm_flags);
> + /*
> + * Cloning a new file under mmap_sem leads to a lock ordering conflict
> + * between XFS directory reading and selinux: since this file is only
> + * accessible to the user through its mapping, use S_PRIVATE flag to
> + * bypass file security, in the same way as shmem_kernel_file_setup().
> + */
> + file = __shmem_file_setup("dev/zero", size, vma->vm_flags, S_PRIVATE);
> if (IS_ERR(file))
> return PTR_ERR(file);
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>