From: Topi Miettinen <toiwoton@gmail.com>
To: Florian Weimer <fweimer@redhat.com>,
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>,
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 11:17:19 +0300 [thread overview]
Message-ID: <511318fd-efde-f2fc-9159-9d16ac8d33a7@gmail.com> (raw)
In-Reply-To: <87sga6snjn.fsf@oldenburg2.str.redhat.com>
On 22.10.2020 10.54, Florian Weimer wrote:
> * 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.
What if the loader always enabled BTI for PROT_EXEC pages, but then when
discovering that this was a mistake, mprotect() the pages without BTI?
Then both BTI and MDWX would work and the penalty of not getting MDWX
would fall to non-BTI programs. What's the expected proportion of BTI
enabled code vs. disabled in the future, is it perhaps expected that a
distro would enable the flag globally so eventually only a few legacy
programs might be unprotected?
-Topi
WARNING: multiple messages have this Message-ID (diff)
From: Topi Miettinen <toiwoton@gmail.com>
To: Florian Weimer <fweimer@redhat.com>,
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>,
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 11:17:19 +0300 [thread overview]
Message-ID: <511318fd-efde-f2fc-9159-9d16ac8d33a7@gmail.com> (raw)
In-Reply-To: <87sga6snjn.fsf@oldenburg2.str.redhat.com>
On 22.10.2020 10.54, Florian Weimer wrote:
> * 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.
What if the loader always enabled BTI for PROT_EXEC pages, but then when
discovering that this was a mistake, mprotect() the pages without BTI?
Then both BTI and MDWX would work and the penalty of not getting MDWX
would fall to non-BTI programs. What's the expected proportion of BTI
enabled code vs. disabled in the future, is it perhaps expected that a
distro would enable the flag globally so eventually only a few legacy
programs might be unprotected?
-Topi
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next prev parent reply other threads:[~2020-10-22 8:17 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
2020-10-22 7:54 ` Florian Weimer
2020-10-22 8:17 ` Topi Miettinen [this message]
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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