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Date: Tue, 25 Apr 2017 08:33:05 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Thomas Garnier <thgarnie@...gle.com>
Cc: Martin Schwidefsky <schwidefsky@...ibm.com>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Arnd Bergmann <arnd@...db.de>, Dave Hansen <dave.hansen@...el.com>,
Andrew Morton <akpm@...ux-foundation.org>,
David Howells <dhowells@...hat.com>,
René Nyffenegger <mail@...enyffenegger.ch>,
"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
Thomas Gleixner <tglx@...utronix.de>,
Oleg Nesterov <oleg@...hat.com>,
Stephen Smalley <sds@...ho.nsa.gov>,
Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>,
Andy Lutomirski <luto@...nel.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Kees Cook <keescook@...omium.org>, Rik van Riel <riel@...hat.com>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Borislav Petkov <bp@...en8.de>, Brian Gerst <brgerst@...il.com>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
Christian Borntraeger <borntraeger@...ibm.com>,
Russell King <linux@...linux.org.uk>,
Will Deacon <will.deacon@....com>,
Catalin Marinas <catalin.marinas@....com>,
Mark Rutland <mark.rutland@....com>,
James Morse <james.morse@....com>, linux-s390@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v7 1/4] syscalls: Restore address limit after a syscall
* Thomas Garnier <thgarnie@...gle.com> 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@...gle.com>
> Tested-by: Kees Cook <keescook@...omium.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
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