Re: mm: shmem_zero_setup skip security check and lockdep conflict with XFS
From: Stephen Smalley <hidden>
Date: 2015-07-08 13:13:14
Also in:
lkml, selinux
On Sun, Jun 14, 2015 at 12:48 PM, Hugh Dickins [off-list ref] 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.
quoted hunk ↗ jump to hunk
Reported-and-tested-by: Prarit Bhargava <redacted> Reported-by: Daniel Wagner <redacted> Reported-by: Morten Stevens <redacted> Signed-off-by: Hugh Dickins <hughd@google.com> --- 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@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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