On 2019-11-14, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
On Thu, Nov 14, 2019 at 03:49:45PM +1100, Aleksa Sarai wrote:On 2019-11-13, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:On Tue, Nov 05, 2019 at 08:05:47PM +1100, Aleksa Sarai wrote:@@ -862,6 +870,8 @@ static int nd_jump_root(struct nameidata *nd) void nd_jump_link(struct path *path) { struct nameidata *nd = current->nameidata; + + nd->last_magiclink.same_mnt = (nd->path.mnt == path->mnt); path_put(&nd->path); nd->path = *path; @@ -1082,6 +1092,10 @@ const char *get_link(struct nameidata *nd) if (nd->flags & LOOKUP_MAGICLINK_JUMPED) { if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS)) return ERR_PTR(-ELOOP); + if (unlikely(nd->flags & LOOKUP_NO_XDEV)) { + if (!nd->last_magiclink.same_mnt) + return ERR_PTR(-EXDEV); + } }Ugh... Wouldn't it be better to take that logics (some equivalent thereof) into nd_jump_link()? Or just have nd_jump_link() return an error...This could be done, but the reason for stashing it away in last_magiclink is because of the future magic-link re-opening patches which can't be implemented like that without putting the open_flags inside nameidata (which was decided to be too ugly a while ago). My point being that I could implement it this way for this series, but I'd have to implement something like last_magiclink when I end up re-posting the magic-link stuff in a few weeks. Looking at all the nd_jump_link() users, the other option is to just disallow magic-link crossings entirely for LOOKUP_NO_XDEV. The only thing allowing them permits is to resolve file descriptors that are pointing to the same procfs mount -- and it's unclear to me how useful that really is (apparmorfs and nsfs will always give -EXDEV because aafs_mnt and nsfs_mnt are internal kernel vfsmounts).I would rather keep the entire if (nd->flags & LOOKUP_MAGICLINK_JUMPED) out of the get_link(). If you want to generate some error if nd_jump_link() has been called, just do it right there. The fewer pieces of state need to be carried around, the better...
Sure, I can make nd_jump_link() give -ELOOP and drop the current need for LOOKUP_MAGICLINK_JUMPED -- if necessary we can re-add it for the magic-link reopening patches.
And as for opening them... Why would you need full open_flags in there? Details, please...
I was referring to [1] which has been dropped from this series. I misspoke -- you don't need the full open_flags, you just need acc_mode in nameidata -- but from memory you (understandably) weren't in favour of that either because it further muddled the open semantics with namei. So the solution I went with was to stash away the i_mode of the magiclink in nd->last_magiclink.mode (though to avoid a race which Jann found, you actually need to recalculate it when you call nd_jump_link() but that's a different topic) and then check it in trailing_magiclink(). However, I've since figured out that we need to restrict things like bind-mounts and truncate() because they can be used to get around the restrictions. I dropped that patch from this series so that I could work on implementing the restrictions for the other relevant VFS syscalls separately from openat2 (upgrade_mask will be re-added to open_how with those patches). My point was that AFAICS we will either have to have nd->acc_mode (or something similar) or have nd->last_magiclink in order to implement the magic-link reopening hardening. [1]: https://lore.kernel.org/lkml/20190930183316.10190-2-cyphar@xxxxxxxxxx/ -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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