linux-kernel - [PATCH v4 5/6] powerpc/fault: Avoid heavy search_exception

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Message-Id: <9870f01e293a5a76c4f4e4ddd4a6b0f63038c591.1607491748.git.christophe.leroy@csgroup.eu>
Date:   Wed,  9 Dec 2020 05:29:24 +0000 (UTC)
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, npiggin@...il.com,
        aneesh.kumar@...ux.ibm.com
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH v4 5/6] powerpc/fault: Avoid heavy search_exception_tables()
 verification

search_exception_tables() is an heavy operation, we have to avoid it.
When KUAP is selected, we'll know the fault has been blocked by KUAP.
When it is blocked by KUAP, check whether we are in an expected
userspace access place. If so, emit a warning to spot something is
going work. Otherwise, just remain silent, it will likely Oops soon.

When KUAP is not selected, it behaves just as if the address was
already in the TLBs and no fault was generated.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
Reviewed-by: Nicholas Piggin <npiggin@...il.com>
---
v4: keep the search once we hit kuap_bad_fault() in order to warn or not
v3: rebased
v2: Squashed with the preceeding patch which was re-ordering tests that get removed in this patch.
---
 arch/powerpc/mm/fault.c | 28 +++++++++++++---------------
 1 file changed, 13 insertions(+), 15 deletions(-)

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index 04505f938bbc..389a2a875262 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -210,28 +210,26 @@ static bool bad_kernel_fault(struct pt_regs *regs, unsigned long error_code,
 		return true;
 	}
 
-	if (!is_exec && address < TASK_SIZE && (error_code & (DSISR_PROTFAULT | DSISR_KEYFAULT)) &&
-	    !search_exception_tables(regs->nip)) {
-		pr_crit_ratelimited("Kernel attempted to access user page (%lx) - exploit attempt? (uid: %d)\n",
-				    address,
-				    from_kuid(&init_user_ns, current_uid()));
-	}
-
 	// Kernel fault on kernel address is bad
 	if (address >= TASK_SIZE)
 		return true;
 
-	// Fault on user outside of certain regions (eg. copy_tofrom_user()) is bad
-	if (!search_exception_tables(regs->nip))
-		return true;
+	// Read/write fault blocked by KUAP is bad, it can never succeed.
+	if (bad_kuap_fault(regs, address, is_write)) {
+		pr_crit_ratelimited("Kernel attempted to %s user page (%lx) - exploit attempt? (uid: %d)\n",
+				    is_write ? "write" : "read", address,
+				    from_kuid(&init_user_ns, current_uid()));
+
+		// Fault on user outside of certain regions (eg. copy_tofrom_user()) is bad
+		if (!search_exception_tables(regs->nip))
+			return true;
 
-	// Read/write fault in a valid region (the exception table search passed
-	// above), but blocked by KUAP is bad, it can never succeed.
-	if (bad_kuap_fault(regs, address, is_write))
+		// Read/write fault in a valid region (the exception table search passed
+		// above), but blocked by KUAP is bad, it can never succeed.
 		return WARN(true, "Bug: %s fault blocked by KUAP!", is_write ? "Write" : "Read");
+	}
 
-	// What's left? Kernel fault on user in well defined regions (extable
-	// matched), and allowed by KUAP in the faulting context.
+	// What's left? Kernel fault on user and allowed by KUAP in the faulting context.
 	return false;
 }
 
-- 
2.25.0