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Date: Tue, 20 Jun 2017 13:18:02 -0700
From: Kees Cook <keescook@...omium.org>
To: Thomas Garnier <thgarnie@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
"H . Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>,
Rik van Riel <riel@...hat.com>, Oleg Nesterov <oleg@...hat.com>, Josh Poimboeuf <jpoimboe@...hat.com>,
Petr Mladek <pmladek@...e.com>, Miroslav Benes <mbenes@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
Arnd Bergmann <arnd@...db.de>, Dave Hansen <dave.hansen@...el.com>,
David Howells <dhowells@...hat.com>, Russell King <linux@...linux.org.uk>,
Andy Lutomirski <luto@...capital.net>, Will Drewry <wad@...omium.org>,
Will Deacon <will.deacon@....com>, Catalin Marinas <catalin.marinas@....com>,
Mark Rutland <mark.rutland@....com>, Pratyush Anand <panand@...hat.com>,
Chris Metcalf <cmetcalf@...lanox.com>, "x86@...nel.org" <x86@...nel.org>,
LKML <linux-kernel@...r.kernel.org>, Linux API <linux-api@...r.kernel.org>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v10 2/3] arm/syscalls: Check address limit on user-mode return
On Wed, Jun 14, 2017 at 6:12 PM, Thomas Garnier <thgarnie@...gle.com> wrote:
> Ensure the address limit is a user-mode segment before returning to
> user-mode. Otherwise a process can corrupt kernel-mode memory and
> elevate privileges [1].
>
> The set_fs function sets the TIF_SETFS flag to force a slow path on
> return. In the slow path, the address limit is checked to be USER_DS if
> needed.
>
> The TIF_SETFS flag is added to _TIF_WORK_MASK shifting _TIF_SYSCALL_WORK
> for arm instruction immediate support. The global work mask is too big
> to used on a single instruction so adapt ret_fast_syscall.
>
> [1] https://bugs.chromium.org/p/project-zero/issues/detail?id=990
>
> Signed-off-by: Thomas Garnier <thgarnie@...gle.com>
> ---
> v10 redesigns the change to use work flags on set_fs as recommended by
> Linus and agreed by others.
>
> Based on next-20170609
> ---
> arch/arm/include/asm/thread_info.h | 15 +++++++++------
> arch/arm/include/asm/uaccess.h | 2 ++
> arch/arm/kernel/entry-common.S | 9 +++++++--
> arch/arm/kernel/signal.c | 5 +++++
> 4 files changed, 23 insertions(+), 8 deletions(-)
>
> diff --git a/arch/arm/include/asm/thread_info.h b/arch/arm/include/asm/thread_info.h
> index 776757d1604a..1d468b527b7b 100644
> --- a/arch/arm/include/asm/thread_info.h
> +++ b/arch/arm/include/asm/thread_info.h
> @@ -139,10 +139,11 @@ extern int vfp_restore_user_hwstate(struct user_vfp __user *,
> #define TIF_NEED_RESCHED 1 /* rescheduling necessary */
> #define TIF_NOTIFY_RESUME 2 /* callback before returning to user */
> #define TIF_UPROBE 3 /* breakpointed or singlestepping */
> -#define TIF_SYSCALL_TRACE 4 /* syscall trace active */
> -#define TIF_SYSCALL_AUDIT 5 /* syscall auditing active */
> -#define TIF_SYSCALL_TRACEPOINT 6 /* syscall tracepoint instrumentation */
> -#define TIF_SECCOMP 7 /* seccomp syscall filtering active */
> +#define TIF_FSCHECK 4 /* Check FS is USER_DS on return */
> +#define TIF_SYSCALL_TRACE 5 /* syscall trace active */
> +#define TIF_SYSCALL_AUDIT 6 /* syscall auditing active */
> +#define TIF_SYSCALL_TRACEPOINT 7 /* syscall tracepoint instrumentation */
> +#define TIF_SECCOMP 8 /* seccomp syscall filtering active */
>
> #define TIF_NOHZ 12 /* in adaptive nohz mode */
> #define TIF_USING_IWMMXT 17
> @@ -153,6 +154,7 @@ extern int vfp_restore_user_hwstate(struct user_vfp __user *,
> #define _TIF_NEED_RESCHED (1 << TIF_NEED_RESCHED)
> #define _TIF_NOTIFY_RESUME (1 << TIF_NOTIFY_RESUME)
> #define _TIF_UPROBE (1 << TIF_UPROBE)
> +#define _TIF_FSCHECK (1 << TIF_FSCHECK)
> #define _TIF_SYSCALL_TRACE (1 << TIF_SYSCALL_TRACE)
> #define _TIF_SYSCALL_AUDIT (1 << TIF_SYSCALL_AUDIT)
> #define _TIF_SYSCALL_TRACEPOINT (1 << TIF_SYSCALL_TRACEPOINT)
> @@ -166,8 +168,9 @@ extern int vfp_restore_user_hwstate(struct user_vfp __user *,
> /*
> * Change these and you break ASM code in entry-common.S
> */
> -#define _TIF_WORK_MASK (_TIF_NEED_RESCHED | _TIF_SIGPENDING | \
> - _TIF_NOTIFY_RESUME | _TIF_UPROBE)
> +#define _TIF_WORK_MASK (_TIF_NEED_RESCHED | _TIF_SIGPENDING | \
> + _TIF_NOTIFY_RESUME | _TIF_UPROBE | \
> + _TIF_FSCHECK)
>
> #endif /* __KERNEL__ */
> #endif /* __ASM_ARM_THREAD_INFO_H */
> diff --git a/arch/arm/include/asm/uaccess.h b/arch/arm/include/asm/uaccess.h
> index 2577405d082d..6cc882223e34 100644
> --- a/arch/arm/include/asm/uaccess.h
> +++ b/arch/arm/include/asm/uaccess.h
> @@ -77,6 +77,8 @@ static inline void set_fs(mm_segment_t fs)
> {
> current_thread_info()->addr_limit = fs;
> modify_domain(DOMAIN_KERNEL, fs ? DOMAIN_CLIENT : DOMAIN_MANAGER);
> + /* On user-mode return, check fs is correct */
> + set_thread_flag(TIF_FSCHECK);
> }
>
> #define segment_eq(a, b) ((a) == (b))
> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> index eb5cd77bf1d8..e33c32d56193 100644
> --- a/arch/arm/kernel/entry-common.S
> +++ b/arch/arm/kernel/entry-common.S
> @@ -41,7 +41,9 @@ ret_fast_syscall:
> UNWIND(.cantunwind )
> disable_irq_notrace @ disable interrupts
> ldr r1, [tsk, #TI_FLAGS] @ re-check for syscall tracing
> - tst r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
> + tst r1, #_TIF_SYSCALL_WORK
> + bne fast_work_pending
> + tst r1, #_TIF_WORK_MASK
(IIUC) MOV32 is 2 cycles (MOVW, MOVT), and each TST above is 1 cycle
and each BNE is 1 cycle (when not taken). So:
mov32 r2, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
tst r1, r2
bne fast_work_pending
is 4 cycles and tst, bne, tst, bne is also 4 cycles. Would mov32 be
more readable (since it keeps the flags together)?
-Kees
--
Kees Cook
Pixel Security
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