path: root/security
diff options
authorJeff Layton <jlayton@redhat.com>2013-04-01 08:14:24 -0400
committerJames Morris <james.l.morris@oracle.com>2013-04-02 11:30:13 +1100
commit094f7b69ea738d7d619cba449d2af97159949459 (patch)
treec9ee28631d0c0c72487ba45113cda2793417c4eb /security
parent9b0271d8ca4d99becbf9ca1871c0d0faa5fad676 (diff)
selinux: make security_sb_clone_mnt_opts return an error on context mismatch
I had the following problem reported a while back. If you mount the same filesystem twice using NFSv4 with different contexts, then the second context= option is ignored. For instance: # mount server:/export /mnt/test1 # mount server:/export /mnt/test2 -o context=system_u:object_r:tmp_t:s0 # ls -dZ /mnt/test1 drwxrwxrwt. root root system_u:object_r:nfs_t:s0 /mnt/test1 # ls -dZ /mnt/test2 drwxrwxrwt. root root system_u:object_r:nfs_t:s0 /mnt/test2 When we call into SELinux to set the context of a "cloned" superblock, it will currently just bail out when it notices that we're reusing an existing superblock. Since the existing superblock is already set up and presumably in use, we can't go overwriting its context with the one from the "original" sb. Because of this, the second context= option in this case cannot take effect. This patch fixes this by turning security_sb_clone_mnt_opts into an int return operation. When it finds that the "new" superblock that it has been handed is already set up, it checks to see whether the contexts on the old superblock match it. If it does, then it will just return success, otherwise it'll return -EBUSY and emit a printk to tell the admin why the second mount failed. Note that this patch may cause casualties. The NFSv4 code relies on being able to walk down to an export from the pseudoroot. If you mount filesystems that are nested within one another with different contexts, then this patch will make those mounts fail in new and "exciting" ways. For instance, suppose that /export is a separate filesystem on the server: # mount server:/ /mnt/test1 # mount salusa:/export /mnt/test2 -o context=system_u:object_r:tmp_t:s0 mount.nfs: an incorrect mount option was specified ...with the printk in the ring buffer. Because we *might* eventually walk down to /mnt/test1/export, the mount is denied due to this patch. The second mount needs the pseudoroot superblock, but that's already present with the wrong context. OTOH, if we mount these in the reverse order, then both mounts work, because the pseudoroot superblock created when mounting /export is discarded once that mount is done. If we then however try to walk into that directory, the automount fails for the similar reasons: # cd /mnt/test1/scratch/ -bash: cd: /mnt/test1/scratch: Device or resource busy The story I've gotten from the SELinux folks that I've talked to is that this is desirable behavior. In SELinux-land, mounting the same data under different contexts is wrong -- there can be only one. Cc: Steve Dickson <steved@redhat.com> Cc: Stephen Smalley <sds@tycho.nsa.gov> Signed-off-by: Jeff Layton <jlayton@redhat.com> Acked-by: Eric Paris <eparis@redhat.com> Signed-off-by: James Morris <james.l.morris@oracle.com>
Diffstat (limited to 'security')
3 files changed, 39 insertions, 7 deletions
diff --git a/security/capability.c b/security/capability.c
index 57977508896..a6290b625be 100644
--- a/security/capability.c
+++ b/security/capability.c
@@ -98,9 +98,10 @@ static int cap_sb_set_mnt_opts(struct super_block *sb,
return 0;
-static void cap_sb_clone_mnt_opts(const struct super_block *oldsb,
+static int cap_sb_clone_mnt_opts(const struct super_block *oldsb,
struct super_block *newsb)
+ return 0;
static int cap_sb_parse_opts_str(char *options, struct security_mnt_opts *opts)
diff --git a/security/security.c b/security/security.c
index 7b88c6aeaed..108281d2307 100644
--- a/security/security.c
+++ b/security/security.c
@@ -299,10 +299,10 @@ int security_sb_set_mnt_opts(struct super_block *sb,
-void security_sb_clone_mnt_opts(const struct super_block *oldsb,
+int security_sb_clone_mnt_opts(const struct super_block *oldsb,
struct super_block *newsb)
- security_ops->sb_clone_mnt_opts(oldsb, newsb);
+ return security_ops->sb_clone_mnt_opts(oldsb, newsb);
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 2fa28c88900..3c02be3f673 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -750,7 +750,37 @@ out_double_mount:
goto out;
-static void selinux_sb_clone_mnt_opts(const struct super_block *oldsb,
+static int selinux_cmp_sb_context(const struct super_block *oldsb,
+ const struct super_block *newsb)
+ struct superblock_security_struct *old = oldsb->s_security;
+ struct superblock_security_struct *new = newsb->s_security;
+ char oldflags = old->flags & SE_MNTMASK;
+ char newflags = new->flags & SE_MNTMASK;
+ if (oldflags != newflags)
+ goto mismatch;
+ if ((oldflags & FSCONTEXT_MNT) && old->sid != new->sid)
+ goto mismatch;
+ if ((oldflags & CONTEXT_MNT) && old->mntpoint_sid != new->mntpoint_sid)
+ goto mismatch;
+ if ((oldflags & DEFCONTEXT_MNT) && old->def_sid != new->def_sid)
+ goto mismatch;
+ if (oldflags & ROOTCONTEXT_MNT) {
+ struct inode_security_struct *oldroot = oldsb->s_root->d_inode->i_security;
+ struct inode_security_struct *newroot = newsb->s_root->d_inode->i_security;
+ if (oldroot->sid != newroot->sid)
+ goto mismatch;
+ }
+ return 0;
+ printk(KERN_WARNING "SELinux: mount invalid. Same superblock, "
+ "different security settings for (dev %s, "
+ "type %s)\n", newsb->s_id, newsb->s_type->name);
+ return -EBUSY;
+static int selinux_sb_clone_mnt_opts(const struct super_block *oldsb,
struct super_block *newsb)
const struct superblock_security_struct *oldsbsec = oldsb->s_security;
@@ -765,14 +795,14 @@ static void selinux_sb_clone_mnt_opts(const struct super_block *oldsb,
* mount options. thus we can safely deal with this superblock later
if (!ss_initialized)
- return;
+ return 0;
/* how can we clone if the old one wasn't set up?? */
BUG_ON(!(oldsbsec->flags & SE_SBINITIALIZED));
- /* if fs is reusing a sb, just let its options stand... */
+ /* if fs is reusing a sb, make sure that the contexts match */
if (newsbsec->flags & SE_SBINITIALIZED)
- return;
+ return selinux_cmp_sb_context(oldsb, newsb);
@@ -805,6 +835,7 @@ static void selinux_sb_clone_mnt_opts(const struct super_block *oldsb,
+ return 0;
static int selinux_parse_opts_str(char *options,