Messages in this thread | | | Date | Mon, 13 Sep 2021 12:01:40 +0200 | From | Christian Brauner <> | Subject | Re: [PATCH] set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds |
| |
On Wed, Sep 08, 2021 at 12:24:00PM +0200, Solar Designer wrote: > Here's a further observation: > > On Tue, Sep 07, 2021 at 11:30:42PM +0200, Solar Designer wrote: > > As I understand, the resulting commit: > > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2863643fb8b92291a7e97ba46e342f1163595fa8 > > > > broke RLIMIT_NPROC support for Apache httpd suexec and likely similar. > > The commit above tries to make things consistent by duplicating the > check from copy_process() also in set_user(). However, the check isn't > actually the same because set_user(new) is called _before_ > security_task_fix_setuid(new, ...), whereas in the described detour via > fork() its check would be reached already as the new user. So those > capable() calls just look the same, but are actually very different, and > that's the problem. My current understanding is the commit actually > increases inconsistency. > > The commit message starts with: > > "in copy_process(): non root users but with capability CAP_SYS_RESOURCE > or CAP_SYS_ADMIN will clean PF_NPROC_EXCEEDED flag even > rlimit(RLIMIT_NPROC) exceeds. Add the same capability check logic here." > > It talks about the obscure case of "non root users but with capability". > However, the capable() calls added by the commit actually also apply to > root, such as in suexec. > > > Anyway, now I suggest that 2863643fb8b92291a7e97ba46e342f1163595fa8 be > > reverted, and if there's any reason to make any change (what reason? > > mere consistency or any real issue?) then I suggest that the flag > > resetting on fork() be made conditional. Something like this: > > > > if (atomic_read(&p->real_cred->user->processes) >= > > task_rlimit(p, RLIMIT_NPROC)) { > > if (p->real_cred->user != INIT_USER && > > !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) > > goto bad_fork_free; > > - } > > - current->flags &= ~PF_NPROC_EXCEEDED; > > + } else > > + current->flags &= ~PF_NPROC_EXCEEDED; > > Alternatively, we could postpone the set_user() calls until we're > running with the new user's capabilities, but that's an invasive change > that's likely to create its own issues. So my suggestion above holds.
Thanks for taking a look at this. We can surely revert this. Fwiw, given how non-obvious this whole thing turned out to be a few comments in the code would've been helpful. I'll try to send a revert by the end of this week with your explanations added in the revert message.
Christian
| |