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From: Florian Weimer <fweimer@redhat.com>
To: Lennart Poettering <mzxreary@0pointer.de>
Cc: Jeremy Linton <jeremy.linton@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	systemd-devel@lists.freedesktop.org,
	Kees Cook <keescook@chromium.org>,
	Catalin Marinas <Catalin.Marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Mark Brown <broonie@kernel.org>,
	libc-alpha@sourceware.org, Dave Martin <dave.martin@arm.com>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [systemd-devel] BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures
Date: Thu, 22 Oct 2020 09:54:52 +0200	[thread overview]
Message-ID: <87sga6snjn.fsf@oldenburg2.str.redhat.com> (raw)
In-Reply-To: <20201022071812.GA324655@gardel-login> (Lennart Poettering's message of "Thu, 22 Oct 2020 09:18:12 +0200")

* Lennart Poettering:

> On Mi, 21.10.20 22:44, Jeremy Linton (jeremy.linton@arm.com) wrote:
>
>> Hi,
>>
>> There is a problem with glibc+systemd on BTI enabled systems. Systemd
>> has a service flag "MemoryDenyWriteExecute" which uses seccomp to deny
>> PROT_EXEC changes. Glibc enables BTI only on segments which are marked as
>> being BTI compatible by calling mprotect PROT_EXEC|PROT_BTI. That call is
>> caught by the seccomp filter, resulting in service failures.
>>
>> So, at the moment one has to pick either denying PROT_EXEC changes, or BTI.
>> This is obviously not desirable.
>>
>> Various changes have been suggested, replacing the mprotect with mmap calls
>> having PROT_BTI set on the original mapping, re-mmapping the segments,
>> implying PROT_EXEC on mprotect PROT_BTI calls when VM_EXEC is already set,
>> and various modification to seccomp to allow particular mprotect cases to
>> bypass the filters. In each case there seems to be an undesirable attribute
>> to the solution.
>>
>> So, whats the best solution?
>
> Did you see Topi's comments on the systemd issue?
>
> https://github.com/systemd/systemd/issues/17368#issuecomment-710485532
>
> I think I agree with this: it's a bit weird to alter the bits after
> the fact. Can't glibc set up everything right from the begining? That
> would keep both concepts working.

The dynamic loader has to process the LOAD segments to get to the ELF
note that says to enable BTI.  Maybe we could do a first pass and load
only the segments that cover notes.  But that requires lots of changes
to generic code in the loader.

Thanks,
Florian
-- 
Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn,
Commercial register: Amtsgericht Muenchen, HRB 153243,
Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill


WARNING: multiple messages have this Message-ID (diff)
From: Florian Weimer <fweimer@redhat.com>
To: Lennart Poettering <mzxreary@0pointer.de>
Cc: Mark Rutland <mark.rutland@arm.com>,
	systemd-devel@lists.freedesktop.org,
	Kees Cook <keescook@chromium.org>,
	Catalin Marinas <Catalin.Marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Jeremy Linton <jeremy.linton@arm.com>,
	Mark Brown <broonie@kernel.org>,
	libc-alpha@sourceware.org, Dave Martin <dave.martin@arm.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [systemd-devel] BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures
Date: Thu, 22 Oct 2020 09:54:52 +0200	[thread overview]
Message-ID: <87sga6snjn.fsf@oldenburg2.str.redhat.com> (raw)
In-Reply-To: <20201022071812.GA324655@gardel-login> (Lennart Poettering's message of "Thu, 22 Oct 2020 09:18:12 +0200")

* Lennart Poettering:

> On Mi, 21.10.20 22:44, Jeremy Linton (jeremy.linton@arm.com) wrote:
>
>> Hi,
>>
>> There is a problem with glibc+systemd on BTI enabled systems. Systemd
>> has a service flag "MemoryDenyWriteExecute" which uses seccomp to deny
>> PROT_EXEC changes. Glibc enables BTI only on segments which are marked as
>> being BTI compatible by calling mprotect PROT_EXEC|PROT_BTI. That call is
>> caught by the seccomp filter, resulting in service failures.
>>
>> So, at the moment one has to pick either denying PROT_EXEC changes, or BTI.
>> This is obviously not desirable.
>>
>> Various changes have been suggested, replacing the mprotect with mmap calls
>> having PROT_BTI set on the original mapping, re-mmapping the segments,
>> implying PROT_EXEC on mprotect PROT_BTI calls when VM_EXEC is already set,
>> and various modification to seccomp to allow particular mprotect cases to
>> bypass the filters. In each case there seems to be an undesirable attribute
>> to the solution.
>>
>> So, whats the best solution?
>
> Did you see Topi's comments on the systemd issue?
>
> https://github.com/systemd/systemd/issues/17368#issuecomment-710485532
>
> I think I agree with this: it's a bit weird to alter the bits after
> the fact. Can't glibc set up everything right from the begining? That
> would keep both concepts working.

The dynamic loader has to process the LOAD segments to get to the ELF
note that says to enable BTI.  Maybe we could do a first pass and load
only the segments that cover notes.  But that requires lots of changes
to generic code in the loader.

Thanks,
Florian
-- 
Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn,
Commercial register: Amtsgericht Muenchen, HRB 153243,
Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill


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  reply	other threads:[~2020-10-22  7:55 UTC|newest]

Thread overview: 88+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <8584c14f-5c28-9d70-c054-7c78127d84ea@arm.com>
2020-10-22  7:18 ` [systemd-devel] BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures Lennart Poettering
2020-10-22  7:18   ` Lennart Poettering
2020-10-22  7:54   ` Florian Weimer [this message]
2020-10-22  7:54     ` Florian Weimer
2020-10-22  8:17     ` Topi Miettinen
2020-10-22  8:17       ` Topi Miettinen
2020-10-22  8:25       ` Florian Weimer
2020-10-22  8:25         ` Florian Weimer
2020-10-22  8:29       ` Szabolcs Nagy
2020-10-22  8:29         ` Szabolcs Nagy
2020-10-22  8:38         ` Lennart Poettering
2020-10-22  8:38           ` Lennart Poettering
2020-10-22  9:31           ` Catalin Marinas
2020-10-22  9:31             ` Catalin Marinas
2020-10-22 10:12             ` Topi Miettinen
2020-10-22 10:12               ` Topi Miettinen
2020-10-22 10:27               ` Florian Weimer
2020-10-22 10:27                 ` Florian Weimer
2020-10-23  6:13             ` Szabolcs Nagy
2020-10-23  6:13               ` Szabolcs Nagy
2020-10-23  9:04               ` Catalin Marinas
2020-10-23  9:04                 ` Catalin Marinas
2020-10-22 10:03         ` Topi Miettinen
2020-10-22 10:03           ` Topi Miettinen
2020-10-22  8:05   ` Szabolcs Nagy
2020-10-22  8:05     ` Szabolcs Nagy
2020-10-22  8:31     ` Lennart Poettering
2020-10-22  8:31       ` Lennart Poettering
     [not found] ` <20201022075447.GO3819@arm.com>
2020-10-22 10:39   ` Topi Miettinen
2020-10-22 10:39     ` Topi Miettinen
2020-10-22 20:02     ` Kees Cook
2020-10-22 20:02       ` Kees Cook
2020-10-22 20:02       ` Kees Cook
2020-10-22 22:24       ` Topi Miettinen
2020-10-22 22:24         ` Topi Miettinen
2020-10-22 22:24         ` Topi Miettinen
2020-10-23 17:52         ` Salvatore Mesoraca
2020-10-23 17:52           ` Salvatore Mesoraca
2020-10-23 17:52           ` Salvatore Mesoraca
2020-10-24 11:34           ` Topi Miettinen
2020-10-24 11:34             ` Topi Miettinen
2020-10-24 11:34             ` Topi Miettinen
2020-10-24 14:12             ` Salvatore Mesoraca
2020-10-24 14:12               ` Salvatore Mesoraca
2020-10-24 14:12               ` Salvatore Mesoraca
2020-10-25 13:42               ` Jordan Glover
2020-10-25 13:42                 ` Jordan Glover
2020-10-25 13:42                 ` Jordan Glover
2020-10-23  9:02       ` Catalin Marinas
2020-10-23  9:02         ` Catalin Marinas
2020-10-23  9:02         ` Catalin Marinas
2020-10-24 11:01         ` Topi Miettinen
2020-10-24 11:01           ` Topi Miettinen
2020-10-24 11:01           ` Topi Miettinen
2020-10-26 14:52           ` Catalin Marinas
2020-10-26 14:52             ` Catalin Marinas
2020-10-26 14:52             ` Catalin Marinas
2020-10-26 15:56             ` Dave Martin
2020-10-26 15:56               ` Dave Martin
2020-10-26 15:56               ` Dave Martin
2020-10-26 16:51               ` Mark Brown
2020-10-26 16:51                 ` Mark Brown
2020-10-26 16:51                 ` Mark Brown
2020-10-26 16:31             ` Topi Miettinen
2020-10-26 16:31               ` Topi Miettinen
2020-10-26 16:31               ` Topi Miettinen
2020-10-26 16:24 ` Dave Martin
2020-10-26 16:24   ` Dave Martin
2020-10-26 16:39   ` Topi Miettinen
2020-10-26 16:39     ` Topi Miettinen
2020-10-26 16:45   ` Florian Weimer
2020-10-26 16:45     ` Florian Weimer
2020-10-27 14:22     ` Dave Martin
2020-10-27 14:22       ` Dave Martin
2020-10-27 14:41       ` Florian Weimer
2020-10-27 14:41         ` Florian Weimer
2020-10-26 16:57   ` Szabolcs Nagy
2020-10-26 16:57     ` Szabolcs Nagy
2020-10-26 17:52     ` Dave Martin
2020-10-26 17:52       ` Dave Martin
2020-10-26 22:39       ` Jeremy Linton
2020-10-26 22:39         ` Jeremy Linton
2020-10-27 14:15         ` Dave Martin
2020-10-27 14:15           ` Dave Martin
2020-10-29 11:02           ` Catalin Marinas
2020-10-29 11:02             ` Catalin Marinas
2020-11-04 12:18             ` Dave Martin
2020-11-04 12:18               ` Dave Martin

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