Re: [PATCH 3.16 208/245] namei: allow restricted O_CREAT of FIFOs and regular files

Ben Hutchings
2020-04-24T15:13:36Z

[-- Attachment #1: Type: text/plain, Size: 2230 bytes --]

On Fri, 2020-04-24 at 15:52 +0200, Solar Designer wrote:
> On Fri, Apr 24, 2020 at 12:07:15AM +0100, Ben Hutchings wrote:
> > 3.16.83-rc1 review patch. If anyone has any objections, please let me know.
>
> I do. This patch is currently known-buggy, see this thread:
>
> https://www.openwall.com/lists/oss-security/2020/01/28/2
>
> It is (partially) fixed with these newer commits in 5.5 and 5.5.2:
>
> commit d0cb50185ae942b03c4327be322055d622dc79f6
> Author: Al Viro <viro@zeniv.linux.org.uk>
> Date: Sun Jan 26 09:29:34 2020 -0500
>
> do_last(): fetch directory ->i_mode and ->i_uid before it's too late
>
> may_create_in_sticky() call is done when we already have dropped the
> reference to dir.
>
> Fixes: 30aba6656f61e (namei: allow restricted O_CREAT of FIFOs and regular files)
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
>
> commit d76341d93dedbcf6ed5a08dfc8bce82d3e9a772b
> Author: Al Viro <viro@zeniv.linux.org.uk>
> Date: Sat Feb 1 16:26:45 2020 +0000
>
> vfs: fix do_last() regression
>
> commit 6404674acd596de41fd3ad5f267b4525494a891a upstream.
[...]
> At least inclusion of the above fixes is mandatory for any backports.

I know, and those are the next 2 patches in the series.

> Also, I think no one has fixed the logic of may_create_in_sticky() so
> that it wouldn't unintentionally apply the "protection" when the file
> is neither a FIFO nor a regular file (something I found and mentioned in
> the oss-security posting above).
[...]
> I think the implementation of may_create_in_sticky() should be rewritten
> such that it'd directly correspond to the textual description in the
> comment above. As we've seen, trying to write the code "more optimally"
> resulted in its logic actually being different from the description.
>
> Meanwhile, I think backporting known-so-buggy code is a bad idea.

I can see that it's not quite right, but does it matter in practice?
Directories and symlinks are handled separately; sockets can't be
opened anyway; block and character devices wonn't normally appear in a
sticky directory.

Ben.

--
Ben Hutchings
Knowledge is power. France is bacon.

[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 833 bytes --]