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Message-ID: <20170309120955.GA6320@leverpostej>
Date: Thu, 9 Mar 2017 12:09:55 +0000
From: Mark Rutland <mark.rutland@....com>
To: Thomas Garnier <thgarnie@...gle.com>
Cc: David Howells <dhowells@...hat.com>,
Dave Hansen <dave.hansen@...el.com>, Arnd Bergmann <arnd@...db.de>,
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Subject: Re: [PATCH v2 1/4] syscalls: Restore address limit after a syscall
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
> @@ -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 ...
> __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.
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