attr: add in_group_or_capable()

In setattr_{copy,prepare}() we need to perform the same permission
checks to determine whether we need to drop the setgid bit or not.
Instead of open-coding it twice add a simple helper the encapsulates the
logic. We will reuse this helpers to make dropping the setgid bit during
write operations more consistent in a follow up patch.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>

diff --git a/fs/attr.c b/fs/attr.c
index 1552a5f..b1162fc 100644 (file)
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -18,6 +18,8 @@
 #include <linux/evm.h>
 #include <linux/ima.h>
 
+#include "internal.h"
+
 /**
  * chown_ok - verify permissions to chown inode
  * @mnt_userns:        user namespace of the mount @inode was found from
@@ -140,8 +142,7 @@ int setattr_prepare(struct user_namespace *mnt_userns, struct dentry *dentry,
                        vfsgid = i_gid_into_vfsgid(mnt_userns, inode);
 
                /* Also check the setgid bit! */
-               if (!vfsgid_in_group_p(vfsgid) &&
-                   !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
+               if (!in_group_or_capable(mnt_userns, inode, vfsgid))
                        attr->ia_mode &= ~S_ISGID;
        }
 
@@ -251,9 +252,8 @@ void setattr_copy(struct user_namespace *mnt_userns, struct inode *inode,
                inode->i_ctime = attr->ia_ctime;
        if (ia_valid & ATTR_MODE) {
                umode_t mode = attr->ia_mode;
-               vfsgid_t vfsgid = i_gid_into_vfsgid(mnt_userns, inode);
-               if (!vfsgid_in_group_p(vfsgid) &&
-                   !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
+               if (!in_group_or_capable(mnt_userns, inode,
+                                        i_gid_into_vfsgid(mnt_userns, inode)))
                        mode &= ~S_ISGID;
                inode->i_mode = mode;
        }

diff --git a/fs/inode.c b/fs/inode.c
index b608528..55299b7 100644 (file)
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2488,6 +2488,28 @@ struct timespec64 current_time(struct inode *inode)
 EXPORT_SYMBOL(current_time);
 
 /**
+ * in_group_or_capable - check whether caller is CAP_FSETID privileged
+ * @mnt_userns: user namespace of the mount @inode was found from
+ * @inode:     inode to check
+ * @vfsgid:    the new/current vfsgid of @inode
+ *
+ * Check wether @vfsgid is in the caller's group list or if the caller is
+ * privileged with CAP_FSETID over @inode. This can be used to determine
+ * whether the setgid bit can be kept or must be dropped.
+ *
+ * Return: true if the caller is sufficiently privileged, false if not.
+ */
+bool in_group_or_capable(struct user_namespace *mnt_userns,
+                        const struct inode *inode, vfsgid_t vfsgid)
+{
+       if (vfsgid_in_group_p(vfsgid))
+               return true;
+       if (capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
+               return true;
+       return false;
+}
+
+/**
  * mode_strip_sgid - handle the sgid bit for non-directories
  * @mnt_userns: User namespace of the mount the inode was created from
  * @dir: parent directory inode
@@ -2508,11 +2530,9 @@ umode_t mode_strip_sgid(struct user_namespace *mnt_userns,
                return mode;
        if (S_ISDIR(mode) || !dir || !(dir->i_mode & S_ISGID))
                return mode;
-       if (in_group_p(i_gid_into_mnt(mnt_userns, dir)))
-               return mode;
-       if (capable_wrt_inode_uidgid(mnt_userns, dir, CAP_FSETID))
+       if (in_group_or_capable(mnt_userns, dir,
+                               i_gid_into_vfsgid(mnt_userns, dir)))
                return mode;
-
        return mode & ~S_ISGID;
 }
 EXPORT_SYMBOL(mode_strip_sgid);

diff --git a/fs/internal.h b/fs/internal.h
index 6f0386b..1de39bb 100644 (file)
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -151,6 +151,8 @@ extern int vfs_open(const struct path *, struct file *);
  */
 extern long prune_icache_sb(struct super_block *sb, struct shrink_control *sc);
 extern int dentry_needs_remove_privs(struct dentry *dentry);
+bool in_group_or_capable(struct user_namespace *mnt_userns,
+                        const struct inode *inode, vfsgid_t vfsgid);
 
 /*
  * fs-writeback.c