FAILED: patch "[PATCH] ovl: make ioctl() safe" failed to apply to 4.19-stable tree

FAILED: patch "[PATCH] ovl: make ioctl() safe" failed to apply to 4.19-stable tree

[gregkh]

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 89bdfaf93d9157499c3a0d61f489df66f2dead7f Mon Sep 17 00:00:00 2001
From: Miklos Szeredi <mszeredi@redhat.com>
Date: Mon, 14 Dec 2020 15:26:14 +0100
Subject: [PATCH] ovl: make ioctl() safe

ovl_ioctl_set_flags() does a capability check using flags, but then the
real ioctl double-fetches flags and uses potentially different value.

The "Check the capability before cred override" comment misleading: user
can skip this check by presenting benign flags first and then overwriting
them to non-benign flags.

Just remove the cred override for now, hoping this doesn't cause a
regression.

The proper solution is to create a new setxflags i_op (patches are in the
works).

Xfstests don't show a regression.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Fixes: dab5ca8fd9dd ("ovl: add lsattr/chattr support")
Cc: <stable@vger.kernel.org> # v4.19

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index efccb7c1f9bc..a1f72ac053e5 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -541,46 +541,31 @@ static long ovl_real_ioctl(struct file *file, unsigned int cmd,
 			   unsigned long arg)
 {
 	struct fd real;
-	const struct cred *old_cred;
 	long ret;
 
 	ret = ovl_real_fdget(file, &real);
 	if (ret)
 		return ret;
 
-	old_cred = ovl_override_creds(file_inode(file)->i_sb);
 	ret = security_file_ioctl(real.file, cmd, arg);
-	if (!ret)
+	if (!ret) {
+		/*
+		 * Don't override creds, since we currently can't safely check
+		 * permissions before doing so.
+		 */
 		ret = vfs_ioctl(real.file, cmd, arg);
-	revert_creds(old_cred);
+	}
 
 	fdput(real);
 
 	return ret;
 }
 
-static unsigned int ovl_iflags_to_fsflags(unsigned int iflags)
-{
-	unsigned int flags = 0;
-
-	if (iflags & S_SYNC)
-		flags |= FS_SYNC_FL;
-	if (iflags & S_APPEND)
-		flags |= FS_APPEND_FL;
-	if (iflags & S_IMMUTABLE)
-		flags |= FS_IMMUTABLE_FL;
-	if (iflags & S_NOATIME)
-		flags |= FS_NOATIME_FL;
-
-	return flags;
-}
-
 static long ovl_ioctl_set_flags(struct file *file, unsigned int cmd,
-				unsigned long arg, unsigned int flags)
+				unsigned long arg)
 {
 	long ret;
 	struct inode *inode = file_inode(file);
-	unsigned int oldflags;
 
 	if (!inode_owner_or_capable(inode))
 		return -EACCES;
@@ -591,10 +576,13 @@ static long ovl_ioctl_set_flags(struct file *file, unsigned int cmd,
 
 	inode_lock(inode);
 
-	/* Check the capability before cred override */
-	oldflags = ovl_iflags_to_fsflags(READ_ONCE(inode->i_flags));
-	ret = vfs_ioc_setflags_prepare(inode, oldflags, flags);
-	if (ret)
+	/*
+	 * Prevent copy up if immutable and has no CAP_LINUX_IMMUTABLE
+	 * capability.
+	 */
+	ret = -EPERM;
+	if (!ovl_has_upperdata(inode) && IS_IMMUTABLE(inode) &&
+	    !capable(CAP_LINUX_IMMUTABLE))
 		goto unlock;
 
 	ret = ovl_maybe_copy_up(file_dentry(file), O_WRONLY);
@@ -613,46 +601,6 @@ static long ovl_ioctl_set_flags(struct file *file, unsigned int cmd,
 
 }
 
-static long ovl_ioctl_set_fsflags(struct file *file, unsigned int cmd,
-				  unsigned long arg)
-{
-	unsigned int flags;
-
-	if (get_user(flags, (int __user *) arg))
-		return -EFAULT;
-
-	return ovl_ioctl_set_flags(file, cmd, arg, flags);
-}
-
-static unsigned int ovl_fsxflags_to_fsflags(unsigned int xflags)
-{
-	unsigned int flags = 0;
-
-	if (xflags & FS_XFLAG_SYNC)
-		flags |= FS_SYNC_FL;
-	if (xflags & FS_XFLAG_APPEND)
-		flags |= FS_APPEND_FL;
-	if (xflags & FS_XFLAG_IMMUTABLE)
-		flags |= FS_IMMUTABLE_FL;
-	if (xflags & FS_XFLAG_NOATIME)
-		flags |= FS_NOATIME_FL;
-
-	return flags;
-}
-
-static long ovl_ioctl_set_fsxflags(struct file *file, unsigned int cmd,
-				   unsigned long arg)
-{
-	struct fsxattr fa;
-
-	memset(&fa, 0, sizeof(fa));
-	if (copy_from_user(&fa, (void __user *) arg, sizeof(fa)))
-		return -EFAULT;
-
-	return ovl_ioctl_set_flags(file, cmd, arg,
-				   ovl_fsxflags_to_fsflags(fa.fsx_xflags));
-}
-
 long ovl_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
 	long ret;
@@ -663,12 +611,9 @@ long ovl_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 		ret = ovl_real_ioctl(file, cmd, arg);
 		break;
 
-	case FS_IOC_SETFLAGS:
-		ret = ovl_ioctl_set_fsflags(file, cmd, arg);
-		break;
-
 	case FS_IOC_FSSETXATTR:
-		ret = ovl_ioctl_set_fsxflags(file, cmd, arg);
+	case FS_IOC_SETFLAGS:
+		ret = ovl_ioctl_set_flags(file, cmd, arg);
 		break;
 
 	default:

Re: FAILED: patch "[PATCH] ovl: make ioctl() safe" failed to apply to 4.19-stable tree

[Amir Goldstein]

On Mon, Dec 28, 2020 at 11:26 AM <gregkh@linuxfoundation.org> wrote:
>
>
> The patch below does not apply to the 4.19-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.
>
> thanks,
>
> greg k-h
>
> ------------------ original commit in Linus's tree ------------------
>
> From 89bdfaf93d9157499c3a0d61f489df66f2dead7f Mon Sep 17 00:00:00 2001
> From: Miklos Szeredi <mszeredi@redhat.com>
> Date: Mon, 14 Dec 2020 15:26:14 +0100
> Subject: [PATCH] ovl: make ioctl() safe
>
> ovl_ioctl_set_flags() does a capability check using flags, but then the
> real ioctl double-fetches flags and uses potentially different value.
>
> The "Check the capability before cred override" comment misleading: user
> can skip this check by presenting benign flags first and then overwriting
> them to non-benign flags.
>
> Just remove the cred override for now,
> hoping this doesn't cause a regression.

Above is a sentence you don't want to see in stable patches.
At least it should indicate that a longer period is needed before backporting.

> The proper solution is to create a new setxflags i_op (patches are in the
works).

I looked into this and I am not sure it is worth backporting this temporary fix.
One observation is that the theoretic security flaw is arguably very hard to
exploit in practice.

The solution can be described as the lesser evil, but it is far from
perfect as it regresses some other functional test cases.

If anyone would like to backport this patch, one might also consider
backporting:
  292f902a40c1 ovl: call secutiry hook in ovl_real_ioctl()
* be4df0cea08a ovl: use generic vfs_ioc_setflags_prepare() helper

[*] $SUBJECT patch replaces this code, so applying this commit eases the
     backport. $SUBJECT patch won't apply cleanly, but the merge conflicts
     are trivial.

Thanks,
Amir.