BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures

[Topi Miettinen]

On 23.10.2020 12.02, Catalin Marinas wrote:
> On Thu, Oct 22, 2020 at 01:02:18PM -0700, Kees Cook wrote:
>> Regardless, it makes sense to me to have the kernel load the executable
>> itself with BTI enabled by default. I prefer gaining Catalin's suggested
>> patch[2]. :)
> [...]
>> [2] https://lore.kernel.org/linux-arm-kernel/20201022093104.GB1229@gaia/
> 
> I think I first heard the idea at Mark R ;).
> 
> It still needs glibc changes to avoid the mprotect(), or at least ignore
> the error. Since this is an ABI change and we don't know which kernels
> would have it backported, maybe better to still issue the mprotect() but
> ignore the failure.

What about kernel adding an auxiliary vector as a flag to indicate that 
BTI is supported and recommended by the kernel? Then dynamic loader 
could use that to detect that a) the main executable is BTI protected 
and there's no need to mprotect() it and b) PROT_BTI flag should be 
added to all PROT_EXEC pages.

In absence of the vector, the dynamic loader might choose to skip doing 
PROT_BTI at all (since the main executable isn't protected anyway 
either, or maybe even the kernel is up-to-date but it knows that it's 
not recommended for some reason, or maybe the kernel is so ancient that 
it doesn't know about BTI). Optionally it could still read the flag from 
ELF later (for compatibility with old kernels) and then do the 
mprotect() dance, which may trip seccomp filters, possibly fatally.

-Topi