linux-kernel - Re: [RFC PATCH 1/3] seccomp: Don't allow tracers to abuse RET

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <acabbed13c4f02e653879844a9cbddef.squirrel@webmail.greenhost.nl>
Date:	Thu, 24 May 2012 19:54:50 +0200
From:	"Indan Zupancic" <indan@....nu>
To:	"Will Drewry" <wad@...omium.org>
Cc:	linux-kernel@...r.kernel.org, mcgrathr@...gle.com, hpa@...or.com,
	netdev@...isplace.org, linux-security-module@...r.kernel.org,
	kernel-hardening@...ts.openwall.com, mingo@...hat.com,
	oleg@...hat.com, peterz@...radead.org, rdunlap@...otime.net,
	tglx@...utronix.de, luto@....edu, serge.hallyn@...onical.com,
	pmoore@...hat.com, akpm@...ux-foundation.org, corbet@....net,
	markus@...omium.org, coreyb@...ux.vnet.ibm.com,
	keescook@...omium.org, viro@...iv.linux.org.uk, jmorris@...ei.org,
	"Will Drewry" <wad@...omium.org>
Subject: Re: [RFC PATCH 1/3] seccomp: Don't allow tracers to abuse RET_TRACE

On Thu, May 24, 2012 18:07, Will Drewry wrote:
> Ensure that consumers of the PTRACE_EVENT_SECCOMP notification
> cannot change the system call number for the traced task
> without it resulting in the system call being skipped.
>
> Traditionally, tracers will set the system call number to
> -1 to skip the system call. This behavior will work as expected
> but the tracer will be unable to remap the system call to a valid
> system call after the seccomp policy has been evaluated.
>
> Signed-off-by: Will Drewry <wad@...omium.org>
> ---
>  kernel/seccomp.c |    4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index ee376be..33f0ad6 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -425,6 +425,10 @@ int __secure_computing(int this_syscall)
>  			 */
>  			if (fatal_signal_pending(current))
>  				break;
> +			/* Skip the system call if the tracer changed it. */
> +			if (this_syscall !=
> +			    syscall_get_nr(current, task_pt_regs(current)))
> +				goto skip;
>  			return 0;
>  		case SECCOMP_RET_ALLOW:
>  			return 0;
> --

This patch doesn't make any sense whatsoever. You can't know why a system
call was blocked by a seccomp filter, assuming it's always because of the
system call number is wrong.

Also, you don't check if an allowed system call is changed into a denied
one, so this doesn't protect against ptracers bypassing seccomp filters.

And one of the main points of PTRACE_EVENT_SECCOMP events was that it's
useful for cases that can't be handled or decided by the seccomp filter.
Then taking away the ability to change the syscall number makes it a lot
less useful.

Either do the seccomp test before or after ptrace, or both, but please
don't introduce ad hoc checks like this.

Greetings,

Indan


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/