ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

[Marian Marinov <mm@1h.com>]

Hello,
when using user namespaces I found a bug in the capability checks done by ioctl.

If someone tries to use chattr +i while in a different user namespace it will get the following:

ioctl(3, EXT2_IOC_SETFLAGS, 0x7fffa4fedacc) = -1 EPERM (Operation not permitted)

I'm proposing a fix to this, by replacing the capable(CAP_LINUX_IMMUTABLE) check with 
ns_capable(current_cred()->user_ns, CAP_LINUX_IMMUTABLE).

If you agree I can send patches for all filesystems.

I'm proposing the following patch:

---
  fs/ext4/ioctl.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index d011b69..25683d0 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -265,7 +265,7 @@ long ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
                  * This test looks nicer. Thanks to Pauline Middelink
                  */
                 if ((flags ^ oldflags) & (EXT4_APPEND_FL | EXT4_IMMUTABLE_FL)) {
-                       if (!capable(CAP_LINUX_IMMUTABLE))
+                       if (!ns_capable(current_cred()->user_ns, CAP_LINUX_IMMUTABLE))
                                 goto flags_out;
                 }

-- 
1.8.4


-- 
Marian Marinov
Founder & CEO of 1H Ltd.
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