linux-kernel - [PATCH v1 4/5] powerpc/fault: Avoid heavy search_exception

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Message-Id: <b07bac7a882c69deb9e6c8f234a68b3022f29072.1596734105.git.christophe.leroy@csgroup.eu>
Date:   Thu,  6 Aug 2020 17:15:31 +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
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH v1 4/5] 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.
Otherwise, 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>
---
 arch/powerpc/mm/fault.c | 20 +++++---------------
 1 file changed, 5 insertions(+), 15 deletions(-)

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index 525e0c2b5406..edde169ba3a6 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -214,24 +214,14 @@ static bool bad_kernel_fault(struct pt_regs *regs, unsigned long error_code,
 	if (address >= TASK_SIZE)
 		return true;
 
-	if (!is_exec && (error_code & DSISR_PROTFAULT) &&
-	    !search_exception_tables(regs->nip)) {
+	// 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 access user page (%lx) - exploit attempt? (uid: %d)\n",
-				    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))
+				    address, from_kuid(&init_user_ns, current_uid()));
 		return true;
+	}
 
-	// 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