linux-kernel - Re: [PATCH] compat: Fix endian issue in union sigval

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Message-ID: <20150214112220.GB10246@MBP.local>
Date:	Sat, 14 Feb 2015 11:22:23 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	Bamvor Jian Zhang <bamvor.zhangjian@...wei.com>,
	linux-arch@...r.kernel.org, mpe@...erman.id.au,
	benh@...nel.crashing.org, deller@....de,
	"dingtianhong@...wei.com" <dingtianhong@...wei.com>,
	heiko.carstens@...ibm.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Will Deacon <Will.Deacon@....com>, jejb@...isc-linux.org,
	"lizefan@...wei.com" <lizefan@...wei.com>, paulus@...ba.org,
	ralf@...ux-mips.org, hpa@...or.com, schwidefsky@...ibm.com,
	tglx@...utronix.de, mingo@...hat.com, davem@...emloft.net,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] compat: Fix endian issue in union sigval

On Fri, Feb 13, 2015 at 04:56:29PM -0500, Chris Metcalf wrote:
> On 2/13/2015 5:44 AM, Catalin Marinas wrote:
> >On Fri, Feb 13, 2015 at 04:00:43PM +0800, Bamvor Jian Zhang wrote:
> >>diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c
> >>index e299de396e9b..32601939a3c8 100644
> >>--- a/arch/arm64/kernel/signal32.c
> >>+++ b/arch/arm64/kernel/signal32.c
> >>@@ -154,8 +154,7 @@ int copy_siginfo_to_user32(compat_siginfo_t __user *to, const siginfo_t *from)
> >>  	case __SI_TIMER:
> >>  		 err |= __put_user(from->si_tid, &to->si_tid);
> >>  		 err |= __put_user(from->si_overrun, &to->si_overrun);
> >>-		 err |= __put_user((compat_uptr_t)(unsigned long)from->si_ptr,
> >>-				   &to->si_ptr);
> >>+		 err |= __put_user(from->si_int, &to->si_int);
> >>  		break;
> >>  	case __SI_POLL:
> >>  		err |= __put_user(from->si_band, &to->si_band);
> >>@@ -184,7 +183,7 @@ int copy_siginfo_to_user32(compat_siginfo_t __user *to, const siginfo_t *from)
> >>  	case __SI_MESGQ: /* But this is */
> >>  		err |= __put_user(from->si_pid, &to->si_pid);
> >>  		err |= __put_user(from->si_uid, &to->si_uid);
> >>-		err |= __put_user((compat_uptr_t)(unsigned long)from->si_ptr, &to->si_ptr);
> >>+		err |= __put_user(from->si_int, &to->si_int);
> >>  		break;
> >>  	case __SI_SYS:
> >>  		err |= __put_user((compat_uptr_t)(unsigned long)
> 
> I must be confused here, but I don't see that these do anything different.
> 
> If we are writing 32 bits to to->si_ptr or to->si_int, either way the high 32 bits
> are irrelevant.  So whether we read it from from->si_ptr and massage the high bits,
> or just read it from from->si_int as a straight-up 32-bit quantity, either way it
> seems we should end up writing the same bits to userspace.
> 
> I would understand the argument if we were overlaying the si_ptr/si_int union
> from a kernel-side siginfo_t where si_ptr and si_int are different sizes
> onto userspace, but it doesn't seem we ever do that.

That's the problem, "from" above is a kernel siginfo_t while "to" is a
compat_siginfo_t. The call paths go something like:

1. user populates sival_int compat_sigevent and invokes
   compat_sys_mq_notify()
2. kernel get_compat_sigevent() copies compat_sigevent into the native
   sigevent. compat and native sival_int are the same, no problem so
   far. The other half of 64-bit sival_ptr is zeroed by a memset in this
   function (this other half can be top or bottom, depending on
   endianness)
3. signal is about to be delivered to user via arch code. The
   compat_ptr(from->si_ptr) conversion always takes the least
   significant part of the native si_ptr. On big endian 64-bit, this is
   zero because get_compat_sigevent() populated the top part of si_ptr
   with si_int.

So delivering such signals to compat user always sets si_int to 0.
Little endian is fine.

A similar example is sys_timer_create() which takes a sigevent argument.
Maybe Bamvor has a test case.

(btw, I'm off for a week, I'll follow up when I get back)

-- 
Catalin
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