Re: [PATCH v3] btrfs: preserve i_mode if __btrfs_set_acl() fails

[David Sterba]

On Wed, Aug 02, 2017 at 03:18:27AM -0300, Ernesto A. Fernández wrote:
> When changing a file's acl mask, btrfs_set_acl() will first set the
> group bits of i_mode to the value of the mask, and only then set the
> actual extended attribute representing the new acl.
> 
> If the second part fails (due to lack of space, for example) and the
> file had no acl attribute to begin with, the system will from now on
> assume that the mask permission bits are actual group permission bits,
> potentially granting access to the wrong users.
> 
> Prevent this by restoring the original mode bits if __btrfs_set_acl
> fails.
> 
> Signed-off-by: Ernesto A. Fernández 
> ---
> Please ignore the two previous versions, this is far simpler and has the
> same effect. To Josef Bacik: thank you for your review, I'm sorry I
> wasted your time.

This version is much better, starting the transaction would add some
overhead and would need to be measured so we have an idea about the
impact.

Reviewed-by: David Sterba 
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[PATCH v3] btrfs: preserve i_mode if __btrfs_set_acl() fails

[Ernesto A . Fernández]

When changing a file's acl mask, btrfs_set_acl() will first set the
group bits of i_mode to the value of the mask, and only then set the
actual extended attribute representing the new acl.

If the second part fails (due to lack of space, for example) and the
file had no acl attribute to begin with, the system will from now on
assume that the mask permission bits are actual group permission bits,
potentially granting access to the wrong users.

Prevent this by restoring the original mode bits if __btrfs_set_acl
fails.

Signed-off-by: Ernesto A. Fernández 
---
Please ignore the two previous versions, this is far simpler and has the
same effect. To Josef Bacik: thank you for your review, I'm sorry I
wasted your time.

 fs/btrfs/acl.c | 6 +-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/acl.c b/fs/btrfs/acl.c
index 8d8370d..1ba49eb 100644
--- a/fs/btrfs/acl.c
+++ b/fs/btrfs/acl.c
@@ -114,13 +114,17 @@ static int __btrfs_set_acl(struct btrfs_trans_handle 
*trans,
 int btrfs_set_acl(struct inode *inode, struct posix_acl *acl, int type)
 {
int ret;
+   umode_t old_mode = inode->i_mode;
 
if (type == ACL_TYPE_ACCESS && acl) {
ret = posix_acl_update_mode(inode, >i_mode, );
if (ret)
return ret;
}
-   return __btrfs_set_acl(NULL, inode, acl, type);
+   ret = __btrfs_set_acl(NULL, inode, acl, type);
+   if (ret)
+   inode->i_mode = old_mode;
+   return ret;
 }
 
 /*
-- 
2.1.4


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