linux-kernel - [PATCH] x86: entry: flush the cache if syscall error

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Date:   Thu, 11 Oct 2018 11:54:58 -0700
From:   Kristen Carlson Accardi <kristen@...ux.intel.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     kernel-hardening@...ts.openwall.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>, linux-kernel@...r.kernel.org
Subject: [PATCH] x86: entry: flush the cache if syscall error

This patch aims to make it harder to perform cache timing attacks on data
left behind by system calls. If we have an error returned from a syscall,
flush the L1 cache.

It's important to note that this patch is not addressing any specific
exploit, nor is it intended to be a complete defense against anything.
It is intended to be a low cost way of eliminating some of side effects
of a failed system call.

A performance test using sysbench on one hyperthread and a script which
attempts to repeatedly access files it does not have permission to access
on the other hyperthread found no significant performance impact.

Suggested-by: Alan Cox <alan@...ux.intel.com>
Signed-off-by: Kristen Carlson Accardi <kristen@...ux.intel.com>
---
 arch/x86/Kconfig        |  9 +++++++++
 arch/x86/entry/common.c | 18 ++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1a0be022f91d..bde978eb3b4e 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -445,6 +445,15 @@ config RETPOLINE
 	  code are eliminated. Since this includes the syscall entry path,
 	  it is not entirely pointless.
 
+config SYSCALL_FLUSH
+	bool "Clear L1 Cache on syscall errors"
+	default n
+	help
+	  Selecting 'y' allows the L1 cache to be cleared upon return of
+	  an error code from a syscall if the CPU supports "flush_l1d".
+	  This may reduce the likelyhood of speculative execution style
+	  attacks on syscalls.
+
 config INTEL_RDT
 	bool "Intel Resource Director Technology support"
 	default n
diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 3b2490b81918..26de8ea71293 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -268,6 +268,20 @@ __visible inline void syscall_return_slowpath(struct pt_regs *regs)
 	prepare_exit_to_usermode(regs);
 }
 
+__visible inline void l1_cache_flush(struct pt_regs *regs)
+{
+	if (IS_ENABLED(CONFIG_SYSCALL_FLUSH) &&
+	    static_cpu_has(X86_FEATURE_FLUSH_L1D)) {
+		if (regs->ax == 0 || regs->ax == -EAGAIN ||
+		    regs->ax == -EEXIST || regs->ax == -ENOENT ||
+		    regs->ax == -EXDEV || regs->ax == -ETIMEDOUT ||
+		    regs->ax == -ENOTCONN || regs->ax == -EINPROGRESS)
+			return;
+
+		wrmsrl(MSR_IA32_FLUSH_CMD, L1D_FLUSH);
+	}
+}
+
 #ifdef CONFIG_X86_64
 __visible void do_syscall_64(unsigned long nr, struct pt_regs *regs)
 {
@@ -290,6 +304,8 @@ __visible void do_syscall_64(unsigned long nr, struct pt_regs *regs)
 		regs->ax = sys_call_table[nr](regs);
 	}
 
+	l1_cache_flush(regs);
+
 	syscall_return_slowpath(regs);
 }
 #endif
@@ -338,6 +354,8 @@ static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
 #endif /* CONFIG_IA32_EMULATION */
 	}
 
+	l1_cache_flush(regs);
+
 	syscall_return_slowpath(regs);
 }
 
-- 
2.14.4