From: Florian Weimer <fweimer@redhat.com> To: Andy Lutomirski <luto@kernel.org> Cc: Dave Hansen <dave.hansen@intel.com>, Linux-MM <linux-mm@kvack.org>, Linux API <linux-api@vger.kernel.org>, linux-x86_64@vger.kernel.org, linux-arch <linux-arch@vger.kernel.org>, X86 ML <x86@kernel.org>, linuxram@us.ibm.com Subject: Re: [PATCH] pkeys: Introduce PKEY_ALLOC_SIGNALINHERIT and change signal semantics Date: Wed, 2 May 2018 23:06:20 +0200 [thread overview] Message-ID: <b2df1386-9df9-2db8-0a25-51bf5ff63592@redhat.com> (raw) In-Reply-To: <CALCETrVrm6yGiv6_z7RqdeB-324RoeMmjpf1EHsrGOh+iKb7+A@mail.gmail.com> On 05/02/2018 10:41 PM, Andy Lutomirski wrote: >> See above. The signal handler will crash if it calls any non-local >> function through the GOT because with the default access rights, it's >> not readable in the signal handler. > >> Any use of memory protection keys for basic infrastructure will run into >> this problem, so I think the current kernel behavior is not very useful. >> It's also x86-specific. > >> From a security perspective, the atomic behavior is not very useful >> because you generally want to modify PKRU *before* computing the details >> of the memory access, so that you don't have a general “poke anywhere >> with this access right” primitive in the text segment. (I called this >> the “suffix problem” in another context.) > > > Ugh, right. It's been long enough that I forgot about the underlying > issue. A big part of the problem here is that pkey_alloc() should set the > initial value of the key across all threads, but it *can't*. There is > literally no way to do it in a multithreaded program that uses RDPKRU and > WRPKRU. The kernel could do *something*, probably along the membarrier system call. I mean, I could implement a reasonable close approximation in userspace, via the setxid mechanism in glibc (but I really don't want to). > But I think the right fix, at least for your use case, is to have a per-mm > init_pkru variable that starts as "deny all". We'd add a new pkey_alloc() > flag like PKEY_ALLOC_UPDATE_INITIAL_STATE that causes the specified mode to > update init_pkru. New threads and delivered signals would get the > init_pkru state instead of the hardcoded default. I implemented this for signal handlers: https://marc.info/?l=linux-api&m=151285420302698&w=2 This does not alter the thread inheritance behavior yet. I would have to investigate how to implement that. Feedback led to the current patch, though. I'm not sure what has changed since then. If I recall correctly, the POWER maintainer did express a strong desire back then for (what is, I believe) their current semantics, which my PKEY_ALLOC_SIGNALINHERIT patch implements for x86, too. Thanks, Florian
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From: Florian Weimer <fweimer@redhat.com> To: Andy Lutomirski <luto@kernel.org> Cc: Dave Hansen <dave.hansen@intel.com>, Linux-MM <linux-mm@kvack.org>, Linux API <linux-api@vger.kernel.org>, linux-x86_64@vger.kernel.org, linux-arch <linux-arch@vger.kernel.org>, X86 ML <x86@kernel.org>, linuxram@us.ibm.com Subject: Re: [PATCH] pkeys: Introduce PKEY_ALLOC_SIGNALINHERIT and change signal semantics Date: Wed, 2 May 2018 23:06:20 +0200 [thread overview] Message-ID: <b2df1386-9df9-2db8-0a25-51bf5ff63592@redhat.com> (raw) In-Reply-To: <CALCETrVrm6yGiv6_z7RqdeB-324RoeMmjpf1EHsrGOh+iKb7+A@mail.gmail.com> On 05/02/2018 10:41 PM, Andy Lutomirski wrote: >> See above. The signal handler will crash if it calls any non-local >> function through the GOT because with the default access rights, it's >> not readable in the signal handler. > >> Any use of memory protection keys for basic infrastructure will run into >> this problem, so I think the current kernel behavior is not very useful. >> It's also x86-specific. > >> From a security perspective, the atomic behavior is not very useful >> because you generally want to modify PKRU *before* computing the details >> of the memory access, so that you don't have a general a??poke anywhere >> with this access righta?? primitive in the text segment. (I called this >> the a??suffix problema?? in another context.) > > > Ugh, right. It's been long enough that I forgot about the underlying > issue. A big part of the problem here is that pkey_alloc() should set the > initial value of the key across all threads, but it *can't*. There is > literally no way to do it in a multithreaded program that uses RDPKRU and > WRPKRU. The kernel could do *something*, probably along the membarrier system call. I mean, I could implement a reasonable close approximation in userspace, via the setxid mechanism in glibc (but I really don't want to). > But I think the right fix, at least for your use case, is to have a per-mm > init_pkru variable that starts as "deny all". We'd add a new pkey_alloc() > flag like PKEY_ALLOC_UPDATE_INITIAL_STATE that causes the specified mode to > update init_pkru. New threads and delivered signals would get the > init_pkru state instead of the hardcoded default. I implemented this for signal handlers: https://marc.info/?l=linux-api&m=151285420302698&w=2 This does not alter the thread inheritance behavior yet. I would have to investigate how to implement that. Feedback led to the current patch, though. I'm not sure what has changed since then. If I recall correctly, the POWER maintainer did express a strong desire back then for (what is, I believe) their current semantics, which my PKEY_ALLOC_SIGNALINHERIT patch implements for x86, too. Thanks, Florian
next prev parent reply other threads:[~2018-05-02 21:06 UTC|newest] Thread overview: 72+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-05-02 13:26 [PATCH] pkeys: Introduce PKEY_ALLOC_SIGNALINHERIT and change signal semantics Florian Weimer 2018-05-02 14:30 ` Dave Hansen 2018-05-02 15:12 ` Florian Weimer 2018-05-02 15:12 ` Florian Weimer 2018-05-02 15:28 ` Dave Hansen 2018-05-02 15:28 ` Dave Hansen 2018-05-02 21:08 ` Florian Weimer 2018-05-02 22:03 ` Dave Hansen 2018-05-02 22:03 ` Dave Hansen 2018-05-07 9:47 ` Florian Weimer 2018-05-07 9:47 ` Florian Weimer 2018-05-02 17:09 ` Andy Lutomirski 2018-05-02 17:17 ` Florian Weimer 2018-05-02 17:17 ` Florian Weimer 2018-05-02 20:41 ` Andy Lutomirski 2018-05-02 21:06 ` Florian Weimer [this message] 2018-05-02 21:06 ` Florian Weimer 2018-05-02 21:23 ` Andy Lutomirski 2018-05-02 22:08 ` Dave Hansen 2018-05-02 22:22 ` Andy Lutomirski 2018-05-02 22:32 ` Dave Hansen 2018-05-02 23:32 ` Andy Lutomirski 2018-05-02 23:58 ` Dave Hansen 2018-05-02 23:58 ` Dave Hansen 2018-05-03 1:14 ` Andy Lutomirski 2018-05-03 14:42 ` Dave Hansen 2018-05-03 14:42 ` Dave Hansen 2018-05-03 14:42 ` Florian Weimer 2018-05-03 14:42 ` Florian Weimer 2018-05-03 2:10 ` Ram Pai 2018-05-03 4:05 ` Andy Lutomirski 2018-05-07 9:48 ` Florian Weimer 2018-05-08 2:49 ` Andy Lutomirski 2018-05-08 2:49 ` Andy Lutomirski 2018-05-08 12:40 ` Florian Weimer 2018-05-08 12:40 ` Florian Weimer 2018-05-09 14:41 ` Andy Lutomirski 2018-05-09 14:41 ` Andy Lutomirski 2018-05-14 12:01 ` Florian Weimer 2018-05-14 12:01 ` Florian Weimer 2018-05-14 15:32 ` Andy Lutomirski 2018-05-14 15:32 ` Andy Lutomirski 2018-05-14 15:32 ` Andy Lutomirski 2018-05-14 15:34 ` Florian Weimer 2018-05-14 15:34 ` Florian Weimer 2018-05-14 15:34 ` Florian Weimer 2018-05-16 17:01 ` Dave Hansen 2018-05-16 17:01 ` Dave Hansen 2018-05-16 17:01 ` Dave Hansen 2018-05-16 20:52 ` Ram Pai 2018-05-16 20:52 ` Ram Pai 2018-05-16 20:54 ` Andy Lutomirski 2018-05-16 20:54 ` Andy Lutomirski 2018-05-16 20:35 ` Ram Pai 2018-05-16 20:35 ` Ram Pai 2018-05-16 20:37 ` Andy Lutomirski 2018-05-16 20:37 ` Andy Lutomirski 2018-05-16 21:07 ` Ram Pai 2018-05-16 21:07 ` Ram Pai 2018-05-17 10:09 ` Florian Weimer 2018-05-17 10:09 ` Florian Weimer 2018-05-17 10:11 ` Florian Weimer 2018-05-17 10:11 ` Florian Weimer 2018-05-03 14:37 ` Florian Weimer 2018-05-02 21:12 ` Ram Pai 2018-05-02 21:12 ` Ram Pai 2018-05-02 21:18 ` Andy Lutomirski 2018-05-02 23:38 ` Ram Pai 2018-05-02 23:38 ` Ram Pai 2018-05-07 9:47 ` Florian Weimer 2018-05-07 9:47 ` Florian Weimer 2018-05-07 9:43 ` Florian Weimer
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