linux-kernel - Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird crap with vdso on uml/i386)

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]

Date:	Mon, 22 Aug 2011 18:59:48 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Andrew Lutomirski <luto@....edu>,
	Borislav Petkov <bp@...64.org>, Ingo Molnar <mingo@...nel.org>,
	"user-mode-linux-devel@...ts.sourceforge.net" 
	<user-mode-linux-devel@...ts.sourceforge.net>,
	Richard Weinberger <richard@....at>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>
Subject: Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re:
 [RFC] weird crap with vdso on uml/i386)

On Mon, Aug 22, 2011 at 6:13 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> *UGH*.  OK,
>        1) I'm an idiot; int_ret_from_sys_call does *not* usually step on
> rbp (it's callee-saved).  So normally ebp is left as is on the way out,
> which is why we don't see stuff getting buggered left, right and center.

Check.

And the system call restart should actually work fine too, because at
syscall entry we save %ebp *both* in the slot for ebp and ecx when we
enter the first time. So the second time, we'll re-load the third
argument from ebp again, but that's fine - it's still going to be the
right value. Yes? No?

However, I note that the cstar entrypont has a comment about not saving ebp:

 * %ebp Arg2    [note: not saved in the stack frame, should not be touched]

which sounds odd. Why don't we save it? If we take a signal handler
there, don't we want %ebp on the kernel stack in pt_regs, in order to
do everything right?

Now I'm *really* confused.

                       Linus
--
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/