linux-kernel - RE: [PATCH v2] ARM: unwind: improve unwinders for noreturn case

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: <401453a216644af98d577f51c12d292b@AcuMS.aculab.com>
Date: Thu, 21 Mar 2024 15:20:57 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Russell King' <linux@...linux.org.uk>
CC: Ard Biesheuvel <ardb@...nel.org>, 'Jiangfeng Xiao'
	<xiaojiangfeng@...wei.com>, "arnd@...db.de" <arnd@...db.de>,
	"keescook@...omium.org" <keescook@...omium.org>, "haibo.li@...iatek.com"
	<haibo.li@...iatek.com>, "angelogioacchino.delregno@...labora.com"
	<angelogioacchino.delregno@...labora.com>, "amergnat@...libre.com"
	<amergnat@...libre.com>, "akpm@...ux-foundation.org"
	<akpm@...ux-foundation.org>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>, "douzhaolei@...wei.com"
	<douzhaolei@...wei.com>, "gustavoars@...nel.org" <gustavoars@...nel.org>,
	"jpoimboe@...nel.org" <jpoimboe@...nel.org>, "kepler.chenxin@...wei.com"
	<kepler.chenxin@...wei.com>, "kirill.shutemov@...ux.intel.com"
	<kirill.shutemov@...ux.intel.com>, "linux-hardening@...r.kernel.org"
	<linux-hardening@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "nixiaoming@...wei.com"
	<nixiaoming@...wei.com>, "peterz@...radead.org" <peterz@...radead.org>,
	"wangbing6@...wei.com" <wangbing6@...wei.com>, "wangfangpeng1@...wei.com"
	<wangfangpeng1@...wei.com>, "jannh@...gle.com" <jannh@...gle.com>,
	"willy@...radead.org" <willy@...radead.org>
Subject: RE: [PATCH v2] ARM: unwind: improve unwinders for noreturn case

From: Russell King
> Sent: 21 March 2024 14:56
> 
> On Thu, Mar 21, 2024 at 02:37:28PM +0000, David Laight wrote:
> > From: Russell King
> > > Sent: 21 March 2024 13:08
> > >
> > > On Thu, Mar 21, 2024 at 12:57:07PM +0000, David Laight wrote:
> > > > From: Russell King
> > > > > Sent: 21 March 2024 12:23
> > > > ...
> > > > > > That might mean you can get the BL in the middle of a function
> > > > > > but where the following instruction is for the 'no stack frame'
> > > > > > side of the branch.
> > > > > > That is very likely to break any stack offset calculations.
> > > > >
> > > > > No it can't. At any one point in the function, the stack has to be in
> > > > > a well defined state, so that access to local variables can work, and
> > > > > also the stack can be correctly unwound. If there exists a point in
> > > > > the function body which can be reached where the stack could be in two
> > > > > different states, then the stack can't be restored to the parent
> > > > > context.
> > > >
> > > > Actually you can get there with a function that has a lot of args.
> > > > So you can have:
> > > > 	if (...) {
> > > > 		push x
> > > > 		bl func
> > > > 		add %sp, #8
> > > > 	}
> > > > 	code;
> > > > which is fine.
> > >
> > > No you can't.... and that isn't even Arm code. Arm doesn't use %sp.
> > > Moreover, that "bl" will stomp over the link register, meaning this
> > > function can not return.
> >
..
> 
> Don't show me Arm64 assembly when we're discussing Arm32.

Oops - I'd assumed no one did 32bit :-)
In any case it is much the same, see https://godbolt.org/z/7dcbKrs76

f4:
        push    {r3, lr}
        subs    r3, r0, #0
        ble     .L2
        mov     r2, r3
        mov     r1, r3
        bl      f
L2:
        pop     {r3, pc}

f5:
        subs    r3, r0, #0
        ble     .L6
        push    {lr}
        sub     sp, sp, #12
        mov     r2, r3
        mov     r1, r3
        str     r3, [sp]
        bl      f
L6:
        bx      lr

That is with -mno-sched-prolog but with 5+ args they spill to stack
and the %sp change is pulled into the conditional.

It does look like %lr is being saved (and for arm64 I think).

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)