Re: [PATCH 00/25] move handling of setuid/gid bits from VFS into individual setattr functions (RESEND)

[Jeff Layton <jlayton@redhat.com>]

On Sat, 11 Aug 2007 03:57:39 +0100
Christoph Hellwig <hch@infradead.org> wrote:
> 
> I like the idea of checking ia_valid after return a lot.  But instead of
> going BUG() it should just do the default action, that we can avoid
> touching all the filesystem and only need to change those that need
> special care.  I also have plans to add some new AT_ flags for implementing
> some filesystem ioctl in generic code that would benefit greatly from
> the ia_valid checkin after return to return ENOTTY fr filesystems not
> implementing those ioctls.

That sounds good (if I follow your meaning correctly). How about
something like the patch below? If either ATTR_KILL bit is set after
the setattr, try to handle them in the "standard" way with a second
setattr call. It also does a printk in this situation to alert
filesystem developers that they should convert to the "new" scheme,
so they can avoid this second setattr call.

If this idea seems sound then I'll start the grunt work to fix up the
in-tree filesystems so that they don't need the second setattr call.

Signed-off-by: Jeff Layton <jlayton@redhat.com>

commit 521ada37ab165d9d0a12dfb59a631a2ec58a8f84
Author: Jeff Layton <jlayton@redhat.com>
Date:   Mon Aug 13 07:43:56 2007 -0400

    VFS: move ATTR_KILL handling from notify_change into helper function
    
    Separate the handling of the local ia_valid bitmask from the one in
    attr->ia_valid. This allows us to hand off the actual handling of the
    ATTR_KILL_* flags to the .setattr i_op when one is defined.
    
    notify_change still needs to process those flags for the local ia_valid
    variable, since it uses that to decide whether to return early, and to pass
    a (hopefully) appropriate bitmask to fsnotify_change.
    
    Also, check the ia_valid after the setattr op returns and see if either
    ATTR_KILL_* bit is set. If so, then throw a warning and try to clear the
    bits in the "standard" way. This should help us to catch filesystems that
    don't handle these bits correctly without breaking them outright.

diff --git a/fs/attr.c b/fs/attr.c
index f8dfc22..7cbb883 100644
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -100,15 +100,53 @@ int inode_setattr(struct inode * inode, struct iattr * attr)
 }
 EXPORT_SYMBOL(inode_setattr);
 
+/**
+ * generic_attrkill - helper to convert ATTR_KILL_* bits into mode change
+ * @mode: current mode of inode
+ * @attr: inode attribute changes requested by VFS
+ * Context: anywhere
+ *
+ * This is a helper function to convert ATTR_KILL_SUID and ATTR_KILL_SGID
+ * into a mode change. Filesystems should call this from their setattr
+ * inode op when they want "conventional" setuid-clearing behavior.
+ *
+ * Filesystems that declare a setattr inode operation are now expected to
+ * handle the ATTR_KILL_SUID and ATTR_KILL_SGID bits appropriately. The VFS
+ * no longer automatically converts these bits to a mode change for
+ * inodes that have their own setattr operation.
+ **/
+void generic_attrkill(mode_t mode, struct iattr *attr)
+{
+	if (attr->ia_valid & ATTR_KILL_SUID) {
+		attr->ia_valid &= ~ATTR_KILL_SUID;
+		if (mode & S_ISUID) {
+			if (!(attr->ia_valid & ATTR_MODE)) {
+				attr->ia_valid |= ATTR_MODE;
+				attr->ia_mode = mode;
+			}
+			attr->ia_mode &= ~S_ISUID;
+		}
+	}
+	if (attr->ia_valid & ATTR_KILL_SGID) {
+		attr->ia_valid &= ~ATTR_KILL_SGID;
+		if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
+			if (!(attr->ia_valid & ATTR_MODE)) {
+				attr->ia_valid |= ATTR_MODE;
+				attr->ia_mode = mode;
+			}
+			attr->ia_mode &= ~S_ISGID;
+		}
+	}
+}
+EXPORT_SYMBOL(generic_attrkill);
+
 int notify_change(struct dentry * dentry, struct iattr * attr)
 {
 	struct inode *inode = dentry->d_inode;
-	mode_t mode;
-	int error;
+	int error, once = 0;
 	struct timespec now;
 	unsigned int ia_valid = attr->ia_valid;
 
-	mode = inode->i_mode;
 	now = current_fs_time(inode->i_sb);
 
 	attr->ia_ctime = now;
@@ -117,36 +155,48 @@ int notify_change(struct dentry * dentry, struct iattr * attr)
 	if (!(ia_valid & ATTR_MTIME_SET))
 		attr->ia_mtime = now;
 	if (ia_valid & ATTR_KILL_SUID) {
-		attr->ia_valid &= ~ATTR_KILL_SUID;
-		if (mode & S_ISUID) {
-			if (!(ia_valid & ATTR_MODE)) {
-				ia_valid = attr->ia_valid |= ATTR_MODE;
-				attr->ia_mode = inode->i_mode;
-			}
-			attr->ia_mode &= ~S_ISUID;
-		}
+		ia_valid &= ~ATTR_KILL_SUID;
+		if (inode->i_mode & S_ISUID)
+			ia_valid |= ATTR_MODE;
 	}
 	if (ia_valid & ATTR_KILL_SGID) {
-		attr->ia_valid &= ~ ATTR_KILL_SGID;
-		if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
-			if (!(ia_valid & ATTR_MODE)) {
-				ia_valid = attr->ia_valid |= ATTR_MODE;
-				attr->ia_mode = inode->i_mode;
-			}
-			attr->ia_mode &= ~S_ISGID;
-		}
+		ia_valid &= ~ATTR_KILL_SGID;
+		if ((inode->i_mode & (S_ISGID | S_IXGRP)) ==
+		    (S_ISGID | S_IXGRP))
+			ia_valid |= ATTR_MODE;
 	}
-	if (!attr->ia_valid)
+	if (!ia_valid)
 		return 0;
 
 	if (ia_valid & ATTR_SIZE)
 		down_write(&dentry->d_inode->i_alloc_sem);
 
 	if (inode->i_op && inode->i_op->setattr) {
+try_again:
 		error = security_inode_setattr(dentry, attr);
 		if (!error)
 			error = inode->i_op->setattr(dentry, attr);
+		/*
+		 * if ATTR_KILL_SUID or ATTR_KILL_SGID is still set, then
+		 * assume that the setattr inode op didn't handle them
+		 * correctly. Try to clear these bits the standard way
+		 * and throw a warning.
+		 */
+		if (!error && !once && unlikely(attr->ia_valid &
+					(ATTR_KILL_SUID|ATTR_KILL_SGID))) {
+			attr->ia_valid &=
+				(ATTR_MODE|ATTR_KILL_SUID|ATTR_KILL_SGID);
+			generic_attrkill(inode->i_mode, attr);
+			once = 1;
+			if (printk_ratelimit())
+				printk(KERN_ERR "%s: %s doesn't seem to "
+					"handle setuid/setgid bit clearing "
+					"correctly.\n"
+					__func__, inode->i_sb->s_type->name);
+			goto try_again;
+		}
 	} else {
+		generic_attrkill(inode->i_mode, attr);
 		error = inode_change_ok(inode, attr);
 		if (!error)
 			error = security_inode_setattr(dentry, attr);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 6bf1395..daac0e5 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1561,6 +1561,7 @@ extern int do_remount_sb(struct super_block *sb, int flags,
 #ifdef CONFIG_BLOCK
 extern sector_t bmap(struct inode *, sector_t);
 #endif
+extern void generic_attrkill(mode_t mode, struct iattr *attr);
 extern int notify_change(struct dentry *, struct iattr *);
 extern int permission(struct inode *, int, struct nameidata *);
 extern int generic_permission(struct inode *, int,
-
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