Messages in this thread | | | From | (Eric W. Biederman) | Date | Fri, 12 Jun 2020 15:02:16 -0500 | Subject | Re: [GIT PULL] proc fixes v2 for v5.8-rc1 |
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Linus Torvalds <torvalds@linux-foundation.org> writes:
> On Fri, Jun 12, 2020 at 12:34 PM Eric W. Biederman > <ebiederm@xmission.com> wrote: >> >> ebiederm@xmission.com (Eric W. Biederman) writes: > > What happened to that first version of the email? I never got it..
A little distracted I think. I forgot to edit the above line out, and v2 because it is my second pull request for a proc fix into v5.8-rc1.
>> Looking at the code the fsnotify watch should have been removed by >> fsnotify_sb_delete in generic_shutdown_super. > > Hmm. Correct. The new_inode_pseudo() is for things that don't have > quotas, fsnotify or writeback. > > That was used somewhat intentionally on /proc, though. /proc certainly > doesn't have quotas or writeback.
It also means we break our debugging by not putting inodes on the s_inodes list.
AKA this line in generic_shutdown_super is also depent on that behavior. if (!list_empty(&sb->s_inodes)) { printk("VFS: Busy inodes after unmount of %s. " "Self-destruct in 5 seconds. Have a nice day...\n", sb->s_id); }
> And fsnotify on /proc seems a bit questionably too. Do people actually > _do_ this and depend on it, or is this just about syzbot doing > something odd and thus showing the problem? > > Anyway, I have pulled your fix, because I think it's reasonable and > safe, but I do wonder if we should have kept the new_inode_pseudo(), > and instead just make fsnotify say "you can't notify on an inode that > isn't on the superblock list". Hmm? > > Is fsnotify on /proc really sensible? Do we actually generate any > useful notifications?
I don't know of any proc code that does anything to make fsnotify/inotify work. Since changes to proc are not initialiated through the vfs that probably means fsnotify is pretty much useless.
I have a sense that a use after free that anyone can trigger could be a bit dangerous, and despite not being the only virtual filesystem in the kernel proc is the only virtual filesystem that called new_inode_pseudo.
Eric
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