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From: Nikolay Borisov <n.borisov.lkml@gmail.com>
To: Lai Jiangshan <jiangshanlai@gmail.com>, linux-kernel@vger.kernel.org
Cc: Lai Jiangshan <laijs@linux.alibaba.com>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 24/24] x86/syscall/64: Move the checking for sysret to C code
Date: Fri, 10 Sep 2021 10:20:02 +0300	[thread overview]
Message-ID: <0801d530-9a86-fbc8-9c27-6beae1f114f3@gmail.com> (raw)
In-Reply-To: <20210831175025.27570-25-jiangshanlai@gmail.com>



On 31.08.21 г. 20:50, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
> 
> Like do_fast_syscall_32() which checks whether it can return to userspace
> via fast instructions before the function returns, do_syscall_64()
> also checks whether it can use sysret to return to userspace before
> do_syscall_64() returns via C code.  And a bunch of ASM code can be removed.
> 
> No functional change intended.
> 
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>

<snip>

> +/*
> + * Check if it can use SYSRET.
> + *
> + * Try to use SYSRET instead of IRET if we're returning to
> + * a completely clean 64-bit userspace context.
> + *
> + * Returns 0 to return using IRET or 1 to return using SYSRET.
> + */
> +static __always_inline int can_sysret(struct pt_regs *regs)

nit: Since this is a predicate function why not simply return bool ?

> +{
> +	/* In the Xen PV case we must use iret anyway. */
> +	if (static_cpu_has(X86_FEATURE_XENPV))
> +		return 0;
> +
> +	/* SYSRET requires RCX == RIP && R11 == RFLAGS */
> +	if (regs->ip != regs->cx || regs->flags != regs->r11)
> +		return 0;
> +
> +	/* CS and SS must match SYSRET */
> +	if (regs->cs != __USER_CS || regs->ss != __USER_DS)
> +		return 0;
> +
> +	/*
> +	 * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
> +	 * in kernel space.  This essentially lets the user take over
> +	 * the kernel, since userspace controls RSP.
> +	 */
> +	if (regs->cx != canonical_address(regs->cx))
> +		return 0;
> +
> +	/*
> +	 * SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot
> +	 * restore RF properly. If the slowpath sets it for whatever reason, we
> +	 * need to restore it correctly.
> +	 *
> +	 * SYSRET can restore TF, but unlike IRET, restoring TF results in a
> +	 * trap from userspace immediately after SYSRET.  This would cause an
> +	 * infinite loop whenever #DB happens with register state that satisfies
> +	 * the opportunistic SYSRET conditions.  For example, single-stepping
> +	 * this user code:
> +	 *
> +	 *           movq	$stuck_here, %rcx
> +	 *           pushfq
> +	 *           popq %r11
> +	 *   stuck_here:
> +	 *
> +	 * would never get past 'stuck_here'.
> +	 */
> +	if (regs->r11 & (X86_EFLAGS_RF | X86_EFLAGS_TF))
> +		return 0;
> +
> +	return 1;
> +}
> +
> +/* Returns 0 to return using IRET or 1 to return using SYSRET. */
> +__visible noinstr int do_syscall_64(struct pt_regs *regs, int nr)

nit: Ditto about bool

>  {
>  	add_random_kstack_offset();
>  	nr = syscall_enter_from_user_mode(regs, nr);
> @@ -84,6 +154,7 @@ __visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
>  
>  	instrumentation_end();
>  	syscall_exit_to_user_mode(regs);
> +	return can_sysret(regs);
>  }
>  #endif
>  

<snip>

  reply	other threads:[~2021-09-10  7:20 UTC|newest]

Thread overview: 72+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-31 17:50 [PATCH 00/24] x86/entry/64: Convert a bunch of ASM entry code into " Lai Jiangshan
2021-08-31 17:50 ` [PATCH 01/24] x86/traps: Remove stack-protector from traps.c Lai Jiangshan
2021-08-31 17:50 ` [PATCH 02/24] x86/traps: Move arch/x86/kernel/traps.c to arch/x86/entry/ Lai Jiangshan
2021-09-02  8:09   ` Joerg Roedel
2021-09-02  9:21     ` Lai Jiangshan
2021-09-02 10:50       ` Peter Zijlstra
2021-09-02 11:54         ` Lai Jiangshan
2021-09-02 12:05           ` Peter Zijlstra
2021-09-02 13:34             ` Peter Zijlstra
2021-09-02 17:05               ` Nick Desaulniers
2021-09-02 17:19                 ` Miguel Ojeda
2021-09-02 17:23                   ` Nick Desaulniers
2021-09-03  7:36                 ` Martin Liška
2021-09-07 21:12                   ` Nick Desaulniers
2021-09-08  7:33                     ` Martin Liška
2021-08-31 17:50 ` [PATCH 03/24] x86/traps: Move declaration of native_irq_return_iret up Lai Jiangshan
2021-08-31 17:50 ` [PATCH 04/24] x86/entry: Expose the address of .Lgs_change to traps.c Lai Jiangshan
2021-09-02  9:14   ` Peter Zijlstra
2021-09-02  9:20     ` Lai Jiangshan
2021-08-31 17:50 ` [PATCH 05/24] x86/entry: Introduce __entry_text for entry code written in C Lai Jiangshan
2021-08-31 19:34   ` Peter Zijlstra
2021-09-01  0:23     ` Lai Jiangshan
2021-08-31 17:50 ` [PATCH 06/24] x86/entry: Move PTI_USER_* to arch/x86/include/asm/processor-flags.h Lai Jiangshan
2021-08-31 17:50 ` [PATCH 07/24] x86: Mark __native_read_cr3() & native_write_cr3() as __always_inline Lai Jiangshan
2021-08-31 17:50 ` [PATCH 08/24] x86/traps: Add C verion of SWITCH_TO_KERNEL_CR3 as switch_to_kernel_cr3() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 09/24] x86/traps: Add fence_swapgs_{user,kernel}_entry() Lai Jiangshan
2021-09-02  9:25   ` Peter Zijlstra
2021-08-31 17:50 ` [PATCH 10/24] x86/traps: Move pt_regs only in fixup_bad_iret() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 11/24] x86/entry: Replace the most of asm code of error_entry to C code Lai Jiangshan
2021-09-02 10:16   ` Peter Zijlstra
2021-09-02 12:08     ` Lai Jiangshan
2021-08-31 17:50 ` [PATCH 12/24] x86/traps: Reconstruct pt_regs on task stack directly in fixup_bad_iret() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 13/24] x86/traps: Mark sync_regs() and fixup_bad_iret() as static __always_inline Lai Jiangshan
2021-08-31 17:50 ` [PATCH 14/24] x86/entry: Make paranoid_exit() callable Lai Jiangshan
2021-08-31 17:50 ` [PATCH 15/24] x86/entry: Call paranoid_exit() in asm_exc_nmi() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 16/24] x86/entry: Use skip_rdi instead of save_ret for PUSH_AND_CLEAR_REGS Lai Jiangshan
2021-08-31 17:50 ` [PATCH 17/24] x86/entry: Introduce struct ist_regs Lai Jiangshan
2021-09-10  0:18   ` Lai Jiangshan
2021-09-10  0:36     ` Lai Jiangshan
2021-09-10  4:31     ` H. Peter Anvin
2021-09-10  7:13       ` Lai Jiangshan
2021-09-10  7:14         ` H. Peter Anvin
2021-09-10  4:50     ` H. Peter Anvin
2021-09-10  4:51       ` H. Peter Anvin
2021-08-31 17:50 ` [PATCH 18/24] x86/entry: Add the C version ist_switch_to_kernel_cr3() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 19/24] x86/entry: Add the C version ist_restore_cr3() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 20/24] x86/entry: Add the C version get_percpu_base() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 21/24] x86/entry: Add the C version ist_switch_to_kernel_gsbase() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 22/24] x86/entry: Implement and use do_paranoid_entry() and paranoid_exit() Lai Jiangshan
2021-09-02 10:33   ` Peter Zijlstra
2021-09-02 10:42     ` Lai Jiangshan
2021-09-02 12:02       ` Peter Zijlstra
2021-09-02 11:58     ` Lai Jiangshan
2021-09-02 12:29       ` Joerg Roedel
2021-08-31 17:50 ` [PATCH 23/24] x86/entry: Remove the unused ASM macros Lai Jiangshan
2021-08-31 17:50 ` [PATCH 24/24] x86/syscall/64: Move the checking for sysret to C code Lai Jiangshan
2021-09-10  7:20   ` Nikolay Borisov [this message]
2021-09-10  7:30     ` Lai Jiangshan
2021-08-31 20:44 ` [PATCH 00/24] x86/entry/64: Convert a bunch of ASM entry code into " Peter Zijlstra
2021-09-02  6:28   ` Lai Jiangshan
2021-09-02  7:44     ` Peter Zijlstra
2021-09-02 10:50 ` [PATCH 25/24] x86/traps: Rewrite native_load_gs_index in " Lai Jiangshan
2021-09-08  1:38   ` H. Peter Anvin
2021-09-08  4:42     ` H. Peter Anvin
2021-09-08  5:00       ` H. Peter Anvin
2021-09-08  7:12         ` Lai Jiangshan
2021-09-09 23:16           ` H. Peter Anvin
2021-09-13 20:01   ` Andy Lutomirski
2021-09-14  2:04     ` Lai Jiangshan
2021-09-14  8:14       ` Peter Zijlstra
2021-09-14  8:17         ` Borislav Petkov
2021-09-14  8:40         ` Lai Jiangshan

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