[systemd-devel] BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures

[Florian Weimer]

* Lennart Poettering:

> On Mi, 21.10.20 22:44, Jeremy Linton (jeremy.linton at 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
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