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

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Message-ID: <20201023061316.GR3819@arm.com>
Date:   Fri, 23 Oct 2020 07:13:17 +0100
From:   Szabolcs Nagy <szabolcs.nagy@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Lennart Poettering <mzxreary@...inter.de>,
        Topi Miettinen <toiwoton@...il.com>,
        Florian Weimer <fweimer@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        systemd-devel@...ts.freedesktop.org,
        Kees Cook <keescook@...omium.org>,
        Will Deacon <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>, libc-alpha@...rceware.org,
        Dave Martin <dave.martin@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [systemd-devel] BTI interaction between seccomp filters in
 systemd and glibc mprotect calls, causing service failures

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

--