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Date: Thu, 9 Feb 2017 15:05:14 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Kees Cook <keescook@...omium.org>
Cc: Thomas Garnier <thgarnie@...gle.com>, Dave Hansen <dave.hansen@...el.com>,
Arnd Bergmann <arnd@...db.de>, René Nyffenegger <mail@...enyffenegger.ch>,
Stephen Bates <stephen.bates@...s.com>, Jeff Moyer <jmoyer@...hat.com>,
Milosz Tanski <milosz@...in.com>, Linux API <linux-api@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC] syscalls: Restore address limit after a syscall
On Thu, Feb 9, 2017 at 11:31 AM, Kees Cook <keescook@...omium.org> wrote:
> On Thu, Feb 9, 2017 at 10:33 AM, Thomas Garnier <thgarnie@...gle.com> wrote:
>> This patch prevents a syscall to modify the address limit of the
>> caller. The address limit is kept by the syscall wrapper and restored
>> just after the syscall ends.
>>
>> For example, it would mitigation this bug:
>>
>> - https://bugs.chromium.org/p/project-zero/issues/detail?id=990
>>
>> Signed-off-by: Thomas Garnier <thgarnie@...gle.com>
>> ---
>> Based on next-20170209
>> ---
>> include/linux/syscalls.h | 5 ++++-
>> 1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
>> index 91a740f6b884..a1b6a62a9849 100644
>> --- a/include/linux/syscalls.h
>> +++ b/include/linux/syscalls.h
>> @@ -198,7 +198,10 @@ 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__)) \
>> { \
>> - long ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__)); \
>> + long ret; \
>> + mm_segment_t fs = get_fs(); \
>> + ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__)); \
>> + set_fs(fs); \
>> __MAP(x,__SC_TEST,__VA_ARGS__); \
>> __PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__)); \
>> return ret; \
>> --
>> 2.11.0.483.g087da7b7c-goog
>>
>
> I have a memory of Andy looking at this before, and there was some
> problem with how a bunch of compat code would set fs and then re-call
> the syscall... but I can't quite find the conversation. Andy, do you
> remember the details?
>
> This seems like an entirely reasonable thing to enforce for syscalls,
> though I'm sure there's a gotcha somewhere. :)
This sounds vaguely familiar, but that's about all.
Anyway, it seems reasonable that the SyS_foobar wrappers are genuinely
only used for syscalls and not for other things, so the code should
*work*. That being said, I think there's room for several
improvements.
1. Why save the old "fs" value? For that matter, why restore it?
IOW, I'd rather see BUG_ON(get_fs() != USER_DS) at the end.
2. I'd rather see the mechanism be more general. If we had, effectively:
asmlinkage long SyS_foo(...) {
sys_foo();
verify_pre_usermode_state();
}
and let verify_pre_usermode_state() potentially do more things, we'd
get a more flexible mechanism. On arches like x86_32, we could save a
decent amount of code size by moving verify_pre_usermode_state() into
prepare_exit_to_usermode(), but that would have to be a per-arch
opt-in. x86_64 probably would *not* select this due to the fast path
(or it would do it in asm. hmm.).
3. If this thing gets factored out, then arch code can call it for
non-syscall entries, too.
4. Can we make this configurable?
For x86, a nice implementation might be:
select ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
... in prepare_exit_to_usermode():
verify_pre_usermode_state(); // right at the beginning
... in the asm syscall fast path:
#ifdef CONFIG_VERIFY_PRE_USERMODE_STATE
call verify_pre_usermode_staet
#endif
(or just inline the interesting bit)
--Andy
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