Re: [PATCH 9/9] overlayfs: Append MAY_READ when diluting write checks
From: Miklos Szeredi <miklos@szeredi.hu>
Date: 2016-07-14 06:49:34
Also in:
linux-fsdevel, lkml
On Wed, Jul 13, 2016 at 4:44 PM, Vivek Goyal [off-list ref] wrote:
quoted hunk ↗ jump to hunk
Right now we remove MAY_WRITE/MAY_APPEND bits from mask if realfile is on lower/. This is done as files on lower will never be written and will be copied up. But to copy up a file, mounter should have MAY_READ permission otherwise copy up will fail. So set MAY_READ in mask when MAY_WRITE is reset. Dan Walsh noticed this when he did access(lowerfile, W_OK) and it returned True (context mounts) but when he tried to actually write to file, it failed as mounter did not have permission on lower file. Reported-by: Dan Walsh <redacted> Signed-off-by: Vivek Goyal <vgoyal@redhat.com> --- fs/overlayfs/inode.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c index 6d9d86e..83b2422 100644 --- a/fs/overlayfs/inode.c +++ b/fs/overlayfs/inode.c@@ -140,8 +140,13 @@ int ovl_permission(struct inode *inode, int mask) return err; old_cred = ovl_override_creds(inode->i_sb); - if (!is_upper && !special_file(realinode->i_mode)) - mask &= ~(MAY_WRITE | MAY_APPEND); + if (!is_upper && !special_file(realinode->i_mode)) { + if (mask & MAY_WRITE || mask & MAY_APPEND) {
Hmm. From what I can tell O_APPEND with O_RDONLY is equivalent to O_RDONLY, hence MAY_APPEND without MAY_WRITE should not behave as if MAY_WRITE was given. I.e. it should not trigger a copy up. Not sure what MAC checks do with MAY_APPEND, though. Pushed amended commit to overlayfs-next. Thanks, Miklos
+ mask &= ~(MAY_WRITE | MAY_APPEND);
+ /* Make sure mounter can read file for copy up later */
+ mask |= MAY_READ;
+ }
+ }
err = inode_permission(realinode, mask);
revert_creds(old_cred);
--
2.7.4