[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CAJcbSZHY5Et+EqjbQPx4KfgAiaYCo29vFfSbX2Aj=BZBg8i0Gg@mail.gmail.com>
Date: Thu, 9 Mar 2017 07:52:30 -0800
From: Thomas Garnier <thgarnie@...gle.com>
To: Mark Rutland <mark.rutland@....com>
Cc: 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>, Russell King <linux@...linux.org.uk>,
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 <linux-api@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>, "the arch/x86 maintainers" <x86@...nel.org>,
linux-arm-kernel@...ts.infradead.org,
Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2 1/4] syscalls: Restore address limit after a syscall
On Thu, Mar 9, 2017 at 4:09 AM, Mark Rutland <mark.rutland@....com> 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.
>
I was half sure of that so that's why I added the barrier. If it is
not needed then I can remove it. Thanks!
>> + 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
>
>> @@ -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 ...
Not sure I understood that point. The goal is to see if get_fs was
changed, that's why I check before the syscall and I want to ensure
the call is not shuffled after the syscall, therefore the original
barrier.
>
>> __MAP(x,__SC_TEST,__VA_ARGS__); \
>> __PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__)); \
>> return ret; \
>
> [...]
>
>> +/* Called before coming back to user-mode */
>> +asmlinkage void verify_pre_usermode_state(void)
>
> ... and we just prepend a couple of underscores here.
>
>> +{
>> + if (CHECK_DATA_CORRUPTION(!segment_eq(get_fs(), USER_DS),
>> + "incorrect get_fs() on user-mode return"))
>> + set_fs(USER_DS);
>> +}
>
> Thanks,
> Mark.
--
Thomas
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.
