From: Ingo Molnar <mingo-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
To: Thomas Garnier <thgarnie-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Cc: "Martin Schwidefsky"
<schwidefsky-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>,
"Heiko Carstens"
<heiko.carstens-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>,
"Arnd Bergmann" <arnd-r2nGTMty4D4@public.gmane.org>,
"Dave Hansen"
<dave.hansen-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>,
"Andrew Morton"
<akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>,
"David Howells"
<dhowells-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
"René Nyffenegger"
<mail-gLCNRsNSrVdVZEhyV+6z5nIPMjoJpjVV@public.gmane.org>,
"Paul E . McKenney"
<paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>,
"Thomas Gleixner" <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>,
"Oleg Nesterov" <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
"Stephen Smalley" <sds-+05T5uksL2qpZYMLLGbcSA@public.gmane.org>,
"Pavel Tikhomirov"
<ptikhomirov-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>,
"Ingo Molnar" <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
"H . Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>,
"Andy Lutomirski" <luto-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
"Paolo Bonzini"
<pbonzini-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
"Kees Cook" <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>,
"Rik van Riel" <riel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
"Josh Poimboeuf"
<jpoimboe-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Subject: Re: [PATCH v7 1/4] syscalls: Restore address limit after a syscall
Date: Tue, 25 Apr 2017 08:33:05 +0200 [thread overview]
Message-ID: <20170425063305.hwjuxupa37rwe6zj@gmail.com> (raw)
In-Reply-To: <20170410164420.64003-1-thgarnie-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
* Thomas Garnier <thgarnie-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 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.
Don't start changelogs with 'This patch' - it's obvious that we are talking about
this patch. Writing:
Ensure that a syscall does not return to user-mode with a kernel address limit.
If that happens, a process can corrupt kernel-mode memory and elevate
privileges.
also note the spelling fix I did. (There's another spelling error elsewhere in
this changelog as well.)
Please read changelogs!
> For example, it would mitigation this bug:
>
> - https://bugs.chromium.org/p/project-zero/issues/detail?id=990
>
> The CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE option is also
> added so each architecture can optimize this change.
As I pointed it out in my previous reply this Kconfig name is awfully long - but
it should have been obvious when this changelog was written ...
> Signed-off-by: Thomas Garnier <thgarnie-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> Tested-by: Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
> ---
> Based on next-20170410
> ---
> arch/s390/Kconfig | 1 +
> include/linux/syscalls.h | 26 +++++++++++++++++++++++++-
> init/Kconfig | 6 ++++++
> kernel/sys.c | 13 +++++++++++++
> 4 files changed, 45 insertions(+), 1 deletion(-)
>
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index d25435d94b6e..489a0cc6e46b 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -103,6 +103,7 @@ config S390
> select ARCH_INLINE_WRITE_UNLOCK_BH
> select ARCH_INLINE_WRITE_UNLOCK_IRQ
> select ARCH_INLINE_WRITE_UNLOCK_IRQRESTORE
> + select ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
> select ARCH_SAVE_PAGE_KEYS if HIBERNATION
> select ARCH_SUPPORTS_ATOMIC_RMW
> select ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INIT
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 980c3c9b06f8..801a7a74fe28 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -191,6 +191,27 @@ extern struct trace_event_functions exit_syscall_print_funcs;
> SYSCALL_METADATA(sname, x, __VA_ARGS__) \
> __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)
>
> +
> +/*
> + * Called before coming back to user-mode. Returning to user-mode with an
> + * address limit different than USER_DS can allow to overwrite kernel memory.
> + */
> +static inline void verify_pre_usermode_state(void) {
> + BUG_ON(!segment_eq(get_fs(), USER_DS));
> +}
Non-standard coding style.
> +
> +#ifndef CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
> +#define __CHECK_USER_CALLER() \
> + bool user_caller = segment_eq(get_fs(), USER_DS)
> +#define __VERIFY_PRE_USERMODE_STATE() \
> + if (user_caller) verify_pre_usermode_state()
> +#else
> +#define __CHECK_USER_CALLER()
> +#define __VERIFY_PRE_USERMODE_STATE()
> +asmlinkage void address_limit_check_failed(void);
> +#endif
> +
> +
> #define __PROTECT(...) asmlinkage_protect(__VA_ARGS__)
> #define __SYSCALL_DEFINEx(x, name, ...) \
> asmlinkage long sys##name(__MAP(x,__SC_DECL,__VA_ARGS__)) \
> @@ -199,7 +220,10 @@ 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; \
> + __CHECK_USER_CALLER(); \
> + ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__)); \
> + __VERIFY_PRE_USERMODE_STATE(); \
> __MAP(x,__SC_TEST,__VA_ARGS__); \
> __PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__)); \
> return ret; \
BTW., the '__VERIFY_PRE_USERMODE_STATE()' name is highly misleading: the 'pre'
prefix suggests that this is done before a system call - while it's done
afterwards.
The solution is to not try to specify the exact call placement in the name, just
describe the functionality (and harmonize along the common prefix).
> +config ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
> + bool
> + help
> + Disable the generic pre-usermode state verification. Allow each
> + architecture to optimize how and when the verification is done.
> +
Please name the Kconfig symbols something like this:
CONFIG_ADDR_LIMIT_CHECK
CONFIG_ADDR_LIMIT_CHECK_ARCH
or so, which tells us whether the check is done by the architecture code, without
breaking the col80 limit with a single Kconfig name.
BTW:
> +#ifdef CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
> +/*
> + * This function is called when an architecture specific implementation detected
> + * an invalid address limit. The generic user-mode state checker will finish on
> + * the appropriate BUG_ON.
> + */
> +asmlinkage void address_limit_check_failed(void)
> +{
> + verify_pre_usermode_state();
> + panic("address_limit_check_failed called with a valid user-mode state");
It's very unconstructive to unconditionally panic the system, just because some
kernel code leaked the address limit! Do a warn-once printout and kill the current
task (i.e. don't continue execution), but don't crash everything else!
Thanks,
Ingo
next prev parent reply other threads:[~2017-04-25 6:33 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-10 16:44 [PATCH v7 1/4] syscalls: Restore address limit after a syscall Thomas Garnier
2017-04-10 16:44 ` [PATCH v7 3/4] arm/syscalls: Architecture specific pre-usermode check Thomas Garnier
2017-04-10 16:44 ` [PATCH v7 4/4] arm64/syscalls: " Thomas Garnier
2017-04-10 17:12 ` Catalin Marinas
[not found] ` <20170410171240.GE30647-M2fw3Uu6cmfZROr8t4l/smS4ubULX0JqMm0uRHvK7Nw@public.gmane.org>
2017-04-10 20:06 ` Thomas Garnier
[not found] ` <20170410200622.8654-1-thgarnie-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
2017-04-10 20:09 ` Thomas Garnier
2017-04-10 20:07 ` Thomas Garnier
[not found] ` <20170410164420.64003-1-thgarnie-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
2017-04-10 16:44 ` [PATCH v7 2/4] x86/syscalls: " Thomas Garnier
2017-04-24 23:57 ` [PATCH v7 1/4] syscalls: Restore address limit after a syscall Kees Cook
[not found] ` <CAGXu5j+J67zC9tcncacHTDvXKMF3HZuPCivk-Uz4J6h-cEv-qQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-04-25 6:23 ` Ingo Molnar
[not found] ` <20170425062324.pdpi5v7ypobw74ki-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2017-04-25 14:12 ` Thomas Garnier
2017-04-25 6:33 ` Ingo Molnar [this message]
[not found] ` <20170425063305.hwjuxupa37rwe6zj-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2017-04-25 14:18 ` Thomas Garnier
2017-04-26 8:12 ` Ingo Molnar
[not found] ` <20170426081229.6wnugrs7w3at4xry-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2017-04-26 14:09 ` Thomas Garnier
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