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-rw-r--r--fs/namei.c122
1 files changed, 122 insertions, 0 deletions
diff --git a/fs/namei.c b/fs/namei.c
index afa087649dd..3861d85f848 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -650,6 +650,119 @@ static inline void put_link(struct nameidata *nd, struct path *link, void *cooki
path_put(link);
}
+int sysctl_protected_symlinks __read_mostly = 1;
+int sysctl_protected_hardlinks __read_mostly = 1;
+
+/**
+ * may_follow_link - Check symlink following for unsafe situations
+ * @link: The path of the symlink
+ *
+ * In the case of the sysctl_protected_symlinks sysctl being enabled,
+ * CAP_DAC_OVERRIDE needs to be specifically ignored if the symlink is
+ * in a sticky world-writable directory. This is to protect privileged
+ * processes from failing races against path names that may change out
+ * from under them by way of other users creating malicious symlinks.
+ * It will permit symlinks to be followed only when outside a sticky
+ * world-writable directory, or when the uid of the symlink and follower
+ * match, or when the directory owner matches the symlink's owner.
+ *
+ * Returns 0 if following the symlink is allowed, -ve on error.
+ */
+static inline int may_follow_link(struct path *link, struct nameidata *nd)
+{
+ const struct inode *inode;
+ const struct inode *parent;
+
+ if (!sysctl_protected_symlinks)
+ return 0;
+
+ /* Allowed if owner and follower match. */
+ inode = link->dentry->d_inode;
+ if (current_cred()->fsuid == inode->i_uid)
+ return 0;
+
+ /* Allowed if parent directory not sticky and world-writable. */
+ parent = nd->path.dentry->d_inode;
+ if ((parent->i_mode & (S_ISVTX|S_IWOTH)) != (S_ISVTX|S_IWOTH))
+ return 0;
+
+ /* Allowed if parent directory and link owner match. */
+ if (parent->i_uid == inode->i_uid)
+ return 0;
+
+ path_put_conditional(link, nd);
+ path_put(&nd->path);
+ return -EACCES;
+}
+
+/**
+ * safe_hardlink_source - Check for safe hardlink conditions
+ * @inode: the source inode to hardlink from
+ *
+ * Return false if at least one of the following conditions:
+ * - inode is not a regular file
+ * - inode is setuid
+ * - inode is setgid and group-exec
+ * - access failure for read and write
+ *
+ * Otherwise returns true.
+ */
+static bool safe_hardlink_source(struct inode *inode)
+{
+ umode_t mode = inode->i_mode;
+
+ /* Special files should not get pinned to the filesystem. */
+ if (!S_ISREG(mode))
+ return false;
+
+ /* Setuid files should not get pinned to the filesystem. */
+ if (mode & S_ISUID)
+ return false;
+
+ /* Executable setgid files should not get pinned to the filesystem. */
+ if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP))
+ return false;
+
+ /* Hardlinking to unreadable or unwritable sources is dangerous. */
+ if (inode_permission(inode, MAY_READ | MAY_WRITE))
+ return false;
+
+ return true;
+}
+
+/**
+ * may_linkat - Check permissions for creating a hardlink
+ * @link: the source to hardlink from
+ *
+ * Block hardlink when all of:
+ * - sysctl_protected_hardlinks enabled
+ * - fsuid does not match inode
+ * - hardlink source is unsafe (see safe_hardlink_source() above)
+ * - not CAP_FOWNER
+ *
+ * Returns 0 if successful, -ve on error.
+ */
+static int may_linkat(struct path *link)
+{
+ const struct cred *cred;
+ struct inode *inode;
+
+ if (!sysctl_protected_hardlinks)
+ return 0;
+
+ cred = current_cred();
+ inode = link->dentry->d_inode;
+
+ /* Source inode owner (or CAP_FOWNER) can hardlink all they like,
+ * otherwise, it must be a safe source.
+ */
+ if (cred->fsuid == inode->i_uid || safe_hardlink_source(inode) ||
+ capable(CAP_FOWNER))
+ return 0;
+
+ return -EPERM;
+}
+
static __always_inline int
follow_link(struct path *link, struct nameidata *nd, void **p)
{
@@ -1818,6 +1931,9 @@ static int path_lookupat(int dfd, const char *name,
while (err > 0) {
void *cookie;
struct path link = path;
+ err = may_follow_link(&link, nd);
+ if (unlikely(err))
+ break;
nd->flags |= LOOKUP_PARENT;
err = follow_link(&link, nd, &cookie);
if (err)
@@ -2778,6 +2894,9 @@ static struct file *path_openat(int dfd, const char *pathname,
error = -ELOOP;
break;
}
+ error = may_follow_link(&link, nd);
+ if (unlikely(error))
+ break;
nd->flags |= LOOKUP_PARENT;
nd->flags &= ~(LOOKUP_OPEN|LOOKUP_CREATE|LOOKUP_EXCL);
error = follow_link(&link, nd, &cookie);
@@ -3421,6 +3540,9 @@ SYSCALL_DEFINE5(linkat, int, olddfd, const char __user *, oldname,
error = -EXDEV;
if (old_path.mnt != new_path.mnt)
goto out_dput;
+ error = may_linkat(&old_path);
+ if (unlikely(error))
+ goto out_dput;
error = security_path_link(old_path.dentry, &new_path, new_dentry);
if (error)
goto out_dput;