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From: Jeremy Linton <jeremy.linton@arm.com>
To: Dave Martin <Dave.Martin@arm.com>, Szabolcs Nagy <szabolcs.nagy@arm.com>
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>,
	toiwoton@gmail.com, libc-alpha@sourceware.org,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures
Date: Mon, 26 Oct 2020 17:39:42 -0500	[thread overview]
Message-ID: <45c64b49-a38b-4b0c-d9cf-6c586dacbcc9@arm.com> (raw)
In-Reply-To: <20201026175230.GC27285@arm.com>

Hi,

On 10/26/20 12:52 PM, Dave Martin wrote:
> On Mon, Oct 26, 2020 at 04:57:55PM +0000, Szabolcs Nagy via Libc-alpha wrote:
>> The 10/26/2020 16:24, Dave Martin via Libc-alpha wrote:
>>> Unrolling this discussion a bit, this problem comes from a few sources:
>>>
>>> 1) systemd is trying to implement a policy that doesn't fit SECCOMP
>>> syscall filtering very well.
>>>
>>> 2) The program is trying to do something not expressible through the
>>> syscall interface: really the intent is to set PROT_BTI on the page,
>>> with no intent to set PROT_EXEC on any page that didn't already have it
>>> set.
>>>
>>>
>>> This limitation of mprotect() was known when I originally added PROT_BTI,
>>> but at that time we weren't aware of a clear use case that would fail.
>>>
>>>
>>> Would it now help to add something like:
>>>
>>> int mchangeprot(void *addr, size_t len, int old_flags, int new_flags)
>>> {
>>> 	int ret = -EINVAL;
>>> 	mmap_write_lock(current->mm);
>>> 	if (all vmas in [addr .. addr + len) have
>>> 			their mprotect flags set to old_flags) {
>>>
>>> 		ret = mprotect(addr, len, new_flags);
>>> 	}
>>> 	
>>> 	mmap_write_unlock(current->mm);
>>> 	return ret;
>>> }
>>
>> if more prot flags are introduced then the exact
>> match for old_flags may be restrictive and currently
>> there is no way to query these flags to figure out
>> how to toggle one prot flag in a future proof way,
>> so i don't think this solves the issue completely.
> 
> Ack -- I illustrated this model because it makes the seccomp filter's
> job easy, but it does have limitations.
> 
>> i think we might need a new api, given that aarch64
>> now has PROT_BTI and PROT_MTE while existing code
>> expects RWX only, but i don't know what api is best.
> 
> An alternative option would be a call that sets / clears chosen
> flags and leaves others unchanged.

I tend to favor a set/clear API, but that could also just be done by 
creating a new PROT_BTI_IF_X which enables BTI for areas already set to 
_EXEC. That goes right by the seccomp filters too, and actually is 
closer to what glibc wants to do anyway.


> 
> The trouble with that is that the MDWX policy then becomes hard to
> implement again.
> 
> 
> But policies might be best set via another route, such as a prctl,
> rather than being implemented completely in a seccomp filter.
> 
> Cheers
> ---Dave
> 


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  reply	other threads:[~2020-10-26 22:41 UTC|newest]

Thread overview: 38+ 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  7:54   ` Florian Weimer
2020-10-22  8:17     ` Topi Miettinen
2020-10-22  8:25       ` Florian Weimer
2020-10-22  8:29       ` Szabolcs Nagy
2020-10-22  8:38         ` Lennart Poettering
2020-10-22  9:31           ` Catalin Marinas
2020-10-22 10:12             ` Topi Miettinen
2020-10-22 10:27               ` Florian Weimer
2020-10-23  6:13             ` Szabolcs Nagy
2020-10-23  9:04               ` Catalin Marinas
2020-10-22 10:03         ` Topi Miettinen
2020-10-22  8:05   ` Szabolcs Nagy
2020-10-22  8:31     ` Lennart Poettering
     [not found] ` <20201022075447.GO3819@arm.com>
2020-10-22 10:39   ` Topi Miettinen
2020-10-22 20:02     ` Kees Cook
2020-10-22 22:24       ` Topi Miettinen
2020-10-23 17:52         ` Salvatore Mesoraca
2020-10-24 11:34           ` Topi Miettinen
2020-10-24 14:12             ` Salvatore Mesoraca
2020-10-25 13:42               ` Jordan Glover
2020-10-23  9:02       ` Catalin Marinas
2020-10-24 11:01         ` Topi Miettinen
2020-10-26 14:52           ` Catalin Marinas
2020-10-26 15:56             ` Dave Martin
2020-10-26 16:51               ` Mark Brown
2020-10-26 16:31             ` Topi Miettinen
2020-10-26 16:24 ` Dave Martin
2020-10-26 16:39   ` Topi Miettinen
2020-10-26 16:45   ` Florian Weimer
2020-10-27 14:22     ` Dave Martin
2020-10-27 14:41       ` Florian Weimer
2020-10-26 16:57   ` Szabolcs Nagy
2020-10-26 17:52     ` Dave Martin
2020-10-26 22:39       ` Jeremy Linton [this message]
2020-10-27 14:15         ` Dave Martin
2020-10-29 11:02           ` Catalin Marinas
2020-11-04 12:18             ` Dave Martin

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