[PATCH v3 0/2] vfs: better dedupe permission check

[PATCH v3 0/2] vfs: better dedupe permission check

[Mark Fasheh]

Hi Al,

The following patches fix a couple of issues with the permission check
we do in vfs_dedupe_file_range(). I sent them out for review twice, a
changelog is attached. If they look ok to you, I'd appreciate them
being pushed upstream.

You can get them from git if you like:

git pull https://github.com/markfasheh/linux dedupe-perms


The first patch expands our check to allow dedupe of a file if the
user owns it or otherwise would be allowed to write to it.

Current behavior is that we'll allow dedupe only if:

- the user is an admin (root)
- the user has the file open for write

This makes it impossible for a user to dedupe their own file set
unless they do it as root, or ensure that all files have write
permission. There's a couple of duperemove bugs open for this:

https://github.com/markfasheh/duperemove/issues/129
https://github.com/markfasheh/duperemove/issues/86

The other problem we have is also related to forcing the user to open
target files for write - A process trying to exec a file currently
being deduped gets ETXTBUSY. The answer (as above) is to allow them to
open the targets ro - root can already do this. There was a patch from
Adam Borowski to fix this back in 2016:

https://lkml.org/lkml/2016/7/17/130

which I have incorporated into my changes.


The 2nd patch fixes our return code for permission denied to be
EPERM. For some reason we're returning EINVAL - I think that's
probably my fault. At any rate, we need to be returning something
descriptive of the actual problem, otherwise callers see EINVAL and
can't really make a valid determination of what's gone wrong.

This has also popped up in duperemove, mostly in the form of cryptic
error messages. Because this is a code returned to userspace, I did
check the other users of extent-same that I could find. Both 'bees'
and 'rust-btrfs' do the same as duperemove and simply report the error
(as they should).

Please apply.

Thanks,
  --Mark

Changes from V2 to V3:
- Return bool from allow_file_dedupe
- V2 discussion: https://www.spinics.net/lists/linux-btrfs/msg78421.html

Changes from V1 to V2:
- Add inode_permission check as suggested by Adam Borowski
- V1 discussion: https://marc.info/?l=linux-xfs&m=152606684017965&w=2

[PATCH v3 1/2] vfs: allow dedupe of user owned read-only files

[Mark Fasheh]

The permission check in vfs_dedupe_file_range() is too coarse - We
only allow dedupe of the destination file if the user is root, or
they have the file open for write.

This effectively limits a non-root user from deduping their own read-only
files. In addition, the write file descriptor that the user is forced to
hold open can prevent execution of files. As file data during a dedupe
does not change, the behavior is unexpected and this has caused a number of
issue reports. For an example, see:

https://github.com/markfasheh/duperemove/issues/129

So change the check so we allow dedupe on the target if:

- the root or admin is asking for it
- the process has write access
- the owner of the file is asking for the dedupe
- the process could get write access

That way users can open read-only and still get dedupe.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
---
 fs/read_write.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index e83bd9744b5d..71e9077f8bc1 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1964,6 +1964,20 @@ int vfs_dedupe_file_range_compare(struct inode *src, loff_t srcoff,
 }
 EXPORT_SYMBOL(vfs_dedupe_file_range_compare);
 
+/* Check whether we are allowed to dedupe the destination file */
+static bool allow_file_dedupe(struct file *file)
+{
+	if (capable(CAP_SYS_ADMIN))
+		return true;
+	if (file->f_mode & FMODE_WRITE)
+		return true;
+	if (uid_eq(current_fsuid(), file_inode(file)->i_uid))
+		return true;
+	if (!inode_permission(file_inode(file), MAY_WRITE))
+		return true;
+	return false;
+}
+
 int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
 {
 	struct file_dedupe_range_info *info;
@@ -1972,7 +1986,6 @@ int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
 	u64 len;
 	int i;
 	int ret;
-	bool is_admin = capable(CAP_SYS_ADMIN);
 	u16 count = same->dest_count;
 	struct file *dst_file;
 	loff_t dst_off;
@@ -2036,7 +2049,7 @@ int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
 
 		if (info->reserved) {
 			info->status = -EINVAL;
-		} else if (!(is_admin || (dst_file->f_mode & FMODE_WRITE))) {
+		} else if (!allow_file_dedupe(dst_file)) {
 			info->status = -EINVAL;
 		} else if (file->f_path.mnt != dst_file->f_path.mnt) {
 			info->status = -EXDEV;
-- 
2.15.1

Re: [PATCH v3 1/2] vfs: allow dedupe of user owned read-only files

[Darrick J. Wong]

On Thu, Jun 07, 2018 at 10:38:53AM -0700, Mark Fasheh wrote:
> The permission check in vfs_dedupe_file_range() is too coarse - We
> only allow dedupe of the destination file if the user is root, or
> they have the file open for write.
> 
> This effectively limits a non-root user from deduping their own read-only
> files. In addition, the write file descriptor that the user is forced to
> hold open can prevent execution of files. As file data during a dedupe
> does not change, the behavior is unexpected and this has caused a number of
> issue reports. For an example, see:
> 
> https://github.com/markfasheh/duperemove/issues/129
> 
> So change the check so we allow dedupe on the target if:
> 
> - the root or admin is asking for it
> - the process has write access
> - the owner of the file is asking for the dedupe
> - the process could get write access
> 
> That way users can open read-only and still get dedupe.
> 
> Signed-off-by: Mark Fasheh <mfasheh@suse.de>

Looks ok, but could you please update the manpage for
ioctl_fideduperange to elaborate on when userspace can expect EPERM?

Acked-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
>  fs/read_write.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/read_write.c b/fs/read_write.c
> index e83bd9744b5d..71e9077f8bc1 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1964,6 +1964,20 @@ int vfs_dedupe_file_range_compare(struct inode *src, loff_t srcoff,
>  }
>  EXPORT_SYMBOL(vfs_dedupe_file_range_compare);
>  
> +/* Check whether we are allowed to dedupe the destination file */
> +static bool allow_file_dedupe(struct file *file)
> +{
> +	if (capable(CAP_SYS_ADMIN))
> +		return true;
> +	if (file->f_mode & FMODE_WRITE)
> +		return true;
> +	if (uid_eq(current_fsuid(), file_inode(file)->i_uid))
> +		return true;
> +	if (!inode_permission(file_inode(file), MAY_WRITE))
> +		return true;
> +	return false;
> +}
> +
>  int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
>  {
>  	struct file_dedupe_range_info *info;
> @@ -1972,7 +1986,6 @@ int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
>  	u64 len;
>  	int i;
>  	int ret;
> -	bool is_admin = capable(CAP_SYS_ADMIN);
>  	u16 count = same->dest_count;
>  	struct file *dst_file;
>  	loff_t dst_off;
> @@ -2036,7 +2049,7 @@ int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
>  
>  		if (info->reserved) {
>  			info->status = -EINVAL;
> -		} else if (!(is_admin || (dst_file->f_mode & FMODE_WRITE))) {
> +		} else if (!allow_file_dedupe(dst_file)) {
>  			info->status = -EINVAL;
>  		} else if (file->f_path.mnt != dst_file->f_path.mnt) {
>  			info->status = -EXDEV;
> -- 
> 2.15.1
> 
> --
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Re: [PATCH v3 1/2] vfs: allow dedupe of user owned read-only files

[Mark Fasheh]

Hi Darrick,

On Thu, Jun 07, 2018 at 11:17:51AM -0700, Darrick J. Wong wrote:
> Looks ok, but could you please update the manpage for
> ioctl_fideduperange to elaborate on when userspace can expect EPERM?
> 
> Acked-by: Darrick J. Wong <darrick.wong@oracle.com>

Yes, good idea. I can handle that.

Thanks,
	--Mark

[PATCH v3 2/2] vfs: dedupe should return EPERM if permission is not granted

[Mark Fasheh]

Right now we return EINVAL if a process does not have permission to dedupe a
file. This was an oversight on my part. EPERM gives a true description of
the nature of our error, and EINVAL is already used for the case that the
filesystem does not support dedupe.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Acked-by: David Sterba <dsterba@suse.com>
---
 fs/read_write.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index 71e9077f8bc1..7188982e2733 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -2050,7 +2050,7 @@ int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
 		if (info->reserved) {
 			info->status = -EINVAL;
 		} else if (!allow_file_dedupe(dst_file)) {
-			info->status = -EINVAL;
+			info->status = -EPERM;
 		} else if (file->f_path.mnt != dst_file->f_path.mnt) {
 			info->status = -EXDEV;
 		} else if (S_ISDIR(dst->i_mode)) {
-- 
2.15.1