On Tue, Oct 02, 2018 at 02:04:31AM +1000, Aleksa Sarai wrote: > On 2018-10-01, Christian Brauner <christian@xxxxxxxxxx> wrote: > > On Mon, Oct 01, 2018 at 02:28:03PM +0200, Jann Horn wrote: > > > On Sat, Sep 29, 2018 at 4:28 PM Aleksa Sarai <cyphar@xxxxxxxxxx> wrote: > > > > * AT_BENEATH: Disallow ".." or absolute paths (either in the path or > > > > found during symlink resolution) to escape the starting point of name > > > > resolution, though ".." is permitted in cases like "foo/../bar". > > > > Relative symlinks are still allowed (as long as they don't escape the > > > > starting point). > > > > > > As I said on the other thread, I would strongly prefer an API that > > > behaves along the lines of David Drysdale's old patch > > > https://lore.kernel.org/lkml/1439458366-8223-2-git-send-email-drysdale@xxxxxxxxxx/ > > > : Forbid any use of "..". This would also be more straightforward to > > > implement safely. If that doesn't work for you, I would like it if you > > > could at least make that an option. I would like it if this API could > > > mitigate straightforward directory traversal bugs such as > > > https://bugs.chromium.org/p/project-zero/issues/detail?id=1583, where > > > a confused deputy attempts to access a path like > > > "/mnt/media_rw/../../data" while intending to access a directory under > > > "/mnt/media_rw". > > > > Oh, the semantics for this changed in this patchset, hah. I was still on > > vacation so didn't get to look at it before it was sent out. From prior > > discussion I remember that the original intention actual was what you > > argue for. And the patchset should be as tight as possible. Having > > special cases where ".." is allowed just sounds like an invitation for > > userspace to get it wrong. > > Aleksa, did you have a specific use-case in mind that made you change > > this or was it already present in an earlier iteration of the patchset > > by someone else? > > Al's original patchset allowed "..". A quick survey of my machine shows > that there are 100k symlinks that contain ".." (~37% of all symlinks on > my machine). This indicates to me that you would be restricting a large > amount of reasonable resolutions because of this restriction. > > I posted a proposed way to protect against ".." shenanigans. If it's > turns out this is not possible, I'm okay with disallowing ".." (assuming > Al is also okay with that). Sounds acceptable to me.