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    SubjectRe: [Patch v3] vfs: allow file truncations when both suid and write permissions set
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    On Fri, 2009-08-07 at 16:38 -0400, Eric Paris wrote:
    > On Sat, 2009-08-08 at 05:23 +0900, OGAWA Hirofumi wrote:
    > > Eric Paris <eparis@redhat.com> writes:
    > >
    > > > On Fri, 2009-08-07 at 06:05 -0400, Amerigo Wang wrote:
    > > >> V2 -> V3:
    > > >> Call notify_change() before clearing suid/sgid.
    > > >> Thanks to OGAWA Hirofumi.
    > > >>
    > > >> V1 -> V2:
    > > >> Introduce dentry_remove_suid(), and use it in do_truncate().
    > > >> Thanks to Eric Paris.
    > > >>
    > > >>
    > > >> When suid is set and the non-owner user has write permission,
    > > >> any writing into this file should be allowed and suid should be
    > > >> removed after that.
    > > >>
    > > >> However, current kernel only allows writing without truncations,
    > > >> when we do truncations on that file, we get EPERM. This is a bug.
    > > >>
    > > >> Steps to reproduce this bug:
    > > >>
    > > >> % ls -l rootdir/file1
    > > >> -rwsrwsrwx 1 root root 3 Jun 25 15:42 rootdir/file1
    > > >> % echo h > rootdir/file1
    > > >> zsh: operation not permitted: rootdir/file1
    > > >> % ls -l rootdir/file1
    > > >> -rwsrwsrwx 1 root root 3 Jun 25 15:42 rootdir/file1
    > > >> % echo h >> rootdir/file1
    > > >> % ls -l rootdir/file1
    > > >> -rwxrwxrwx 1 root root 5 Jun 25 16:34 rootdir/file1
    > > >>
    > > >> This patch fixes it.
    > > >>
    > > >> Signed-off-by: WANG Cong <amwang@redhat.com>
    > > >> Cc: Eric Sandeen <esandeen@redhat.com>
    > > >> Cc: Eric Paris <eparis@redhat.com>
    > > >> Cc: Eugene Teo <eteo@redhat.com>
    > > >> Cc: Al Viro <viro@zeniv.linux.org.uk>
    > > >> Cc: hirofumi@mail.parknet.co.jp
    > > >
    > > > I was thinking about this and kept telling myself I was going to test v2
    > > > before I ack/nak. Clearly we shouldn't for the dropping of SUID if the
    > > > process didn't have permission to change the ATTR_SIZE.
    > > >
    > > > Acked-by: Eric Paris <eparis@redhat.com>
    > >
    > > BTW, Do you know why doesn't security modules fix the handling of
    > > do_truncate() (i.e. ATTR_MODE | ATTR_SIZE). And why doesn't it allow to
    > > pass ATTR_FORCE for it?
    >
    > I'm not sure what you mean. I understood ATTR_FORCE to mean 'I am magic
    > and get to override all security checks." Which is why nothing should
    > ever be using ATTR_FORCE with things other than SUID.
    >
    > I guess we could somehow force logic into the LSM to make it only apply
    > to SUID and friends but I'm not sure it buys us anything.

    SELinux shouldn't apply a permission check for the clearing of the suid
    bit on write or truncate. It should only apply a permission check for
    the actual truncate or write operation, and then the clearing of the
    suid bit should always be forced if that check passed.

    --
    Stephen Smalley
    National Security Agency



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