linux-kernel - [PATCH 04/13] x86/fault: Fold smap_violation() into do_user_addr

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Message-Id: <806c366f6ca861152398ce2c01744d59d9aceb6d.1542667307.git.luto@kernel.org>
Date:   Mon, 19 Nov 2018 14:45:28 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH 04/13] x86/fault: Fold smap_violation() into do_user_addr_fault()

smap_violation() has a single caller, and the contents are a bit
nonsensical.  I'm going to fix it, but first let's fold it into its
caller for ease of comprehension.

In this particular case, the user_mode(regs) check is incorrect --
it will cause false positives in the case of a user-initiated
kernel-privileged access.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/mm/fault.c | 23 ++++++-----------------
 1 file changed, 6 insertions(+), 17 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 0597342d4a55..c50cd67521b6 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1148,20 +1148,6 @@ static int fault_in_kernel_space(unsigned long address)
 	return address >= TASK_SIZE_MAX;
 }
 
-static inline bool smap_violation(int error_code, struct pt_regs *regs)
-{
-	if (!cpu_feature_enabled(X86_FEATURE_SMAP))
-		return false;
-
-	if (error_code & X86_PF_USER)
-		return false;
-
-	if (!user_mode(regs) && (regs->flags & X86_EFLAGS_AC))
-		return false;
-
-	return true;
-}
-
 /*
  * Called for all faults where 'address' is part of the kernel address
  * space.  Might get called for faults that originate from *code* that
@@ -1249,10 +1235,13 @@ void do_user_addr_fault(struct pt_regs *regs,
 		pgtable_bad(regs, hw_error_code, address);
 
 	/*
-	 * Check for invalid kernel (supervisor) access to user
-	 * pages in the user address space.
+	 * If SMAP is on, check for invalid kernel (supervisor)
+	 * access to user pages in the user address space.
 	 */
-	if (unlikely(smap_violation(hw_error_code, regs))) {
+	if (unlikely(cpu_feature_enabled(X86_FEATURE_SMAP) &&
+		     !(hw_error_code & X86_PF_USER) &&
+		     (user_mode(regs) || !(regs->flags & X86_EFLAGS_AC))))
+	{
 		bad_area_nosemaphore(regs, hw_error_code, address);
 		return;
 	}
-- 
2.17.2