* Lennart Poettering:
> On Mi, 21.10.20 22:44, Jeremy Linton ([email protected]) 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
On 22.10.2020 10.54, Florian Weimer wrote:
> * Lennart Poettering:
>
>> On Mi, 21.10.20 22:44, Jeremy Linton ([email protected]) 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
* Topi Miettinen:
>> 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?
Is that architecturally supported? How costly is the mprotect change if
the pages have not been faulted in yet?
> 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?
Eventually, I expect that mainstream distributions build everything for
BTI, so yes, the PROT_BTI removal would only be needed for legacy
programs.
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
The 10/22/2020 11:17, Topi Miettinen via Libc-alpha wrote:
> On 22.10.2020 10.54, Florian Weimer wrote:
> > * Lennart Poettering:
> > > 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?
i thought mprotect(PROT_EXEC) would get filtered
with or without bti, is that not the case?
then i guess we can do the protection that way
around, but then i don't see why the filter cannot
treat PROT_EXEC|PROT_BTI the same as PROT_EXEC.
On Do, 22.10.20 09:29, Szabolcs Nagy ([email protected]) wrote:
> > > 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?
>
> i thought mprotect(PROT_EXEC) would get filtered
> with or without bti, is that not the case?
We can adjust the filter in systemd to match any combination of
flags to allow and to deny.
Lennart
--
Lennart Poettering, Berlin
On Thu, Oct 22, 2020 at 10:38:23AM +0200, Lennart Poettering wrote:
> On Do, 22.10.20 09:29, Szabolcs Nagy ([email protected]) wrote:
> > > > 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?
> >
> > i thought mprotect(PROT_EXEC) would get filtered
> > with or without bti, is that not the case?
>
> We can adjust the filter in systemd to match any combination of
> flags to allow and to deny.
Yes but Szabolcs' point to Topi was that if we can adjust the filters to
allow mprotect(PROT_EXEC), why not allow mprotect(PROT_EXEC|PROT_BTI)
instead? Anyway, I see the MDWX and BTI as complementary policies so
ideally we shouldn't have to choose between one or the other. If we
allow mprotect(PROT_EXEC), that would override MDWX and also disable
BTI.
IIUC, the problem is with the main executable which is mapped by the
kernel without PROT_BTI. The dynamic loader wants to set PROT_BTI but
does not have the original file descriptor to be able to remap. Its only
choice is mprotect() and this fails because of the MDWX policy.
Not sure whether the kernel has the right information but could it map
the main executable with PROT_BTI if the corresponding PT_GNU_PROPERTY
is found? The current ABI states it only sets PROT_BTI for the
interpreter who'd be responsible for setting the PROT_BTI on the main
executable. I can't tell whether it would break anything but it's worth
a try:
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 4784011cecac..0a08fb9133e8 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -730,14 +730,6 @@ asmlinkage void __sched arm64_preempt_schedule_irq(void)
int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
bool has_interp, bool is_interp)
{
- /*
- * For dynamically linked executables the interpreter is
- * responsible for setting PROT_BTI on everything except
- * itself.
- */
- if (is_interp != has_interp)
- return prot;
-
if (!(state->flags & ARM64_ELF_BTI))
return prot;
--
Catalin
On 22.10.2020 11.29, Szabolcs Nagy wrote:
> The 10/22/2020 11:17, Topi Miettinen via Libc-alpha wrote:
>> On 22.10.2020 10.54, Florian Weimer wrote:
>>> * Lennart Poettering:
>>>> 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?
>
> i thought mprotect(PROT_EXEC) would get filtered
> with or without bti, is that not the case?
It would be filtered, but the idea is that with modern binaries this
would not happen since the pages would be mapped with mmap(,, PROT_EXEC
| PROT_BTI,,) which is OK for purposes MDWX. The loader would have to
use mprotect(PROT_EXEC) to get rid of PROT_BTI only for the legacy binaries.
-Topi
The 10/22/2020 10:31, Catalin Marinas wrote:
> IIUC, the problem is with the main executable which is mapped by the
> kernel without PROT_BTI. The dynamic loader wants to set PROT_BTI but
> does not have the original file descriptor to be able to remap. Its only
> choice is mprotect() and this fails because of the MDWX policy.
>
> Not sure whether the kernel has the right information but could it map
> the main executable with PROT_BTI if the corresponding PT_GNU_PROPERTY
> is found? The current ABI states it only sets PROT_BTI for the
> interpreter who'd be responsible for setting the PROT_BTI on the main
> executable. I can't tell whether it would break anything but it's worth
> a try:
i think it would work, but now i can't easily
tell from the libc if i have to do the mprotect
on the main exe or not.
i guess i can just always mprotect and ignore
the failure?
>
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 4784011cecac..0a08fb9133e8 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -730,14 +730,6 @@ asmlinkage void __sched arm64_preempt_schedule_irq(void)
> int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
> bool has_interp, bool is_interp)
> {
> - /*
> - * For dynamically linked executables the interpreter is
> - * responsible for setting PROT_BTI on everything except
> - * itself.
> - */
> - if (is_interp != has_interp)
> - return prot;
> -
> if (!(state->flags & ARM64_ELF_BTI))
> return prot;
>
>
> --
> Catalin
--
On Fri, Oct 23, 2020 at 07:13:17AM +0100, Szabolcs Nagy wrote:
> The 10/22/2020 10:31, Catalin Marinas wrote:
> > IIUC, the problem is with the main executable which is mapped by the
> > kernel without PROT_BTI. The dynamic loader wants to set PROT_BTI but
> > does not have the original file descriptor to be able to remap. Its only
> > choice is mprotect() and this fails because of the MDWX policy.
> >
> > Not sure whether the kernel has the right information but could it map
> > the main executable with PROT_BTI if the corresponding PT_GNU_PROPERTY
> > is found? The current ABI states it only sets PROT_BTI for the
> > interpreter who'd be responsible for setting the PROT_BTI on the main
> > executable. I can't tell whether it would break anything but it's worth
> > a try:
>
> i think it would work, but now i can't easily
> tell from the libc if i have to do the mprotect
> on the main exe or not.
>
> i guess i can just always mprotect and ignore
> the failure?
I replied to Keys before reading your email. So yeah, still issue
mprotect() but ignore the failure.
--
Catalin