acct: block access to kernel internal filesystems

There's no point in allowing anything kernel internal nor procfs or
sysfs.

Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/r/[email protected]
Fixes: 1da177e ("Linux-2.6.12-rc2")
Reviewed-by: Amir Goldstein <[email protected]>
Reported-by: Zicheng Qu <[email protected]>
Cc: [email protected]
Signed-off-by: Christian Brauner <[email protected]>

Author: brauner

Diff for: kernel/acct.c

@@ -243,6 +243,20 @@ static int acct_on(struct filename *pathname)
 		return -EACCES;
 	}
 
+	/* Exclude kernel kernel internal filesystems. */
+	if (file_inode(file)->i_sb->s_flags & (SB_NOUSER | SB_KERNMOUNT)) {
+		kfree(acct);
+		filp_close(file, NULL);
+		return -EINVAL;
+	}
+
+	/* Exclude procfs and sysfs. */
+	if (file_inode(file)->i_sb->s_iflags & SB_I_USERNS_VISIBLE) {
+		kfree(acct);
+		filp_close(file, NULL);
+		return -EINVAL;
+	}
+
 	if (!(file->f_mode & FMODE_CAN_WRITE)) {
 		kfree(acct);
 		filp_close(file, NULL);