[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20170309134456.GI21222@n2100.armlinux.org.uk>
Date: Thu, 9 Mar 2017 13:44:56 +0000
From: Russell King - ARM Linux <linux@...linux.org.uk>
To: Mark Rutland <mark.rutland@....com>
Cc: Thomas Garnier <thgarnie@...gle.com>,
David Howells <dhowells@...hat.com>,
Dave Hansen <dave.hansen@...el.com>, Arnd Bergmann <arnd@...db.de>,
Al Viro <viro@...iv.linux.org.uk>,
René Nyffenegger <mail@...enyffenegger.ch>,
Andrew Morton <akpm@...ux-foundation.org>,
Kees Cook <keescook@...omium.org>,
"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
"David S . Miller" <davem@...emloft.net>,
Andy Lutomirski <luto@...nel.org>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Nicolas Pitre <nicolas.pitre@...aro.org>,
Petr Mladek <pmladek@...e.com>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Helge Deller <deller@....de>, Rik van Riel <riel@...hat.com>,
Ingo Molnar <mingo@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
John Stultz <john.stultz@...aro.org>,
Thomas Gleixner <tglx@...utronix.de>,
Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
Frederic Weisbecker <fweisbec@...il.com>,
Stephen Smalley <sds@...ho.nsa.gov>,
Stanislav Kinsburskiy <skinsbursky@...tuozzo.com>,
Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>,
Paolo Bonzini <pbonzini@...hat.com>, Borislav Petkov <bp@...en8.de>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Brian Gerst <brgerst@...il.com>, Jan Beulich <JBeulich@...e.com>,
Christian Borntraeger <borntraeger@...ibm.com>,
"Luis R . Rodriguez" <mcgrof@...nel.org>,
He Chen <he.chen@...ux.intel.com>,
Will Deacon <will.deacon@....com>,
Catalin Marinas <catalin.marinas@....com>,
James Morse <james.morse@....com>,
Pratyush Anand <panand@...hat.com>,
Vladimir Murzin <vladimir.murzin@....com>,
Chris Metcalf <cmetcalf@...lanox.com>,
Andre Przywara <andre.przywara@....com>, linux-api@...r.kernel.org,
linux-kernel@...r.kernel.org, x86@...nel.org,
linux-arm-kernel@...ts.infradead.org,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v2 1/4] syscalls: Restore address limit after a syscall
On Thu, Mar 09, 2017 at 12:09:55PM +0000, Mark Rutland wrote:
> Hi,
>
> On Wed, Mar 08, 2017 at 05:24:53PM -0800, Thomas Garnier wrote:
> > This patch ensures a syscall does not return to user-mode with a kernel
> > address limit. If that happened, a process can corrupt kernel-mode
> > memory and elevate privileges.
> >
> > For example, it would mitigation this bug:
> >
> > - https://bugs.chromium.org/p/project-zero/issues/detail?id=990
> >
> > If the CONFIG_BUG_ON_DATA_CORRUPTION option is enabled, an incorrect
> > state will result in a BUG_ON.
> >
> > The CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE option is also
> > added so each architecture can optimize this change.
>
> > +#ifndef CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
> > +static inline bool has_user_ds(void) {
> > + bool ret = segment_eq(get_fs(), USER_DS);
> > + // Prevent re-ordering the call
> > + barrier();
>
> What ordering are we trying to ensure, that isn't otherwise given?
>
> We expect get_fs() and set_fs() to be ordered w.r.t. each other and
> w.r.t. uaccess uses, or we'd need barriers all over the place.
>
> Given that, I can't see why we need a barrier here. So this needs a
> better comment, at least.
>
> > + return ret;
> > +}
> > +#else
> > +static inline bool has_user_ds(void) {
> > + return false;
> > +}
> > +#endif
>
> It would be simpler to wrap the call entirely, e.g. have:
>
> #ifdef CONFIG_WHATEVER
> static inline void verify_pre_usermode_state(void)
> {
> if (segment_eq(get_fs(), USER_DS))
> __verify_pre_usermode_state();
> }
> #else
> static inline void verify_pre_usermode_state(void) { }
> #endif
That's utterly pointless - you've missed a detail.
> > @@ -199,7 +215,10 @@ extern struct trace_event_functions exit_syscall_print_funcs;
> > asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__)); \
> > asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__)) \
> > { \
> > + bool user_caller = has_user_ds(); \
> > long ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__)); \
> > + if (user_caller) \
> > + verify_pre_usermode_state(); \
>
> ... then we can unconditionally use verify_pre_usermode_state() here ...
Look at this closely. has_user_ds() is called _before_ the syscall code
is invoked. It's checking what conditions the syscall was entered from.
If the syscall was entered with the user segment selected, then we run
a check on the system state _after_ the syscall code has returned.
Putting both after the syscall code has returned is completely pointless -
it turns it into this code:
if (segment_eq(get_fs(), USER_DS))
if (CHECK_DATA_CORRUPTION(!segment_eq(get_fs(), USER_DS),
"incorrect get_fs() on user-mode return"))
set_fs(USER_DS);
which is obviously bogus (it'll never fire.)
--
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
