linux-kernel - [PATCH v1 1/6] powerpc/64e: Fix early TLB miss with KUAP

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Date:   Tue, 28 Jun 2022 16:48:54 +0200
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>, dja@...ens.net
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        stable@...r.kernel.org
Subject: [PATCH v1 1/6] powerpc/64e: Fix early TLB miss with KUAP

With KUAP, the TLB miss handler bails out when an access to user
memory is performed with a nul TID.

But the normal TLB miss routine which is only used early during boot
does the check regardless for all memory areas, not only user memory.

By chance there is no early IO or vmalloc access, but when KASAN
come we will start having early TLB misses.

Fix it by creating a special branch for user accesses similar to the
one in the 'bolted' TLB miss handlers. Unfortunately SPRN_MAS1 is
now read too early and there are no registers available to preserve
it so it will be read a second time.

Fixes: 57bc963837f5 ("powerpc/kuap: Wire-up KUAP on book3e/64")
Cc: stable@...r.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 arch/powerpc/mm/nohash/tlb_low_64e.S | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/arch/powerpc/mm/nohash/tlb_low_64e.S b/arch/powerpc/mm/nohash/tlb_low_64e.S
index 8b97c4acfebf..9e9ab3803fb2 100644
--- a/arch/powerpc/mm/nohash/tlb_low_64e.S
+++ b/arch/powerpc/mm/nohash/tlb_low_64e.S
@@ -583,7 +583,7 @@ itlb_miss_fault_e6500:
 	 */
 	rlwimi	r11,r14,32-19,27,27
 	rlwimi	r11,r14,32-16,19,19
-	beq	normal_tlb_miss
+	beq	normal_tlb_miss_user
 	/* XXX replace the RMW cycles with immediate loads + writes */
 1:	mfspr	r10,SPRN_MAS1
 	cmpldi	cr0,r15,8		/* Check for vmalloc region */
@@ -626,7 +626,7 @@ itlb_miss_fault_e6500:
 
 	cmpldi	cr0,r15,0			/* Check for user region */
 	std	r14,EX_TLB_ESR(r12)		/* write crazy -1 to frame */
-	beq	normal_tlb_miss
+	beq	normal_tlb_miss_user
 
 	li	r11,_PAGE_PRESENT|_PAGE_BAP_SX	/* Base perm */
 	oris	r11,r11,_PAGE_ACCESSED@h
@@ -653,6 +653,12 @@ itlb_miss_fault_e6500:
  * r11 = PTE permission mask
  * r10 = crap (free to use)
  */
+normal_tlb_miss_user:
+#ifdef CONFIG_PPC_KUAP
+	mfspr	r14,SPRN_MAS1
+	rlwinm.	r14,r14,0,0x3fff0000
+	beq-	normal_tlb_miss_access_fault /* KUAP fault */
+#endif
 normal_tlb_miss:
 	/* So we first construct the page table address. We do that by
 	 * shifting the bottom of the address (not the region ID) by
@@ -683,11 +689,6 @@ finish_normal_tlb_miss:
 	/* Check if required permissions are met */
 	andc.	r15,r11,r14
 	bne-	normal_tlb_miss_access_fault
-#ifdef CONFIG_PPC_KUAP
-	mfspr	r11,SPRN_MAS1
-	rlwinm.	r10,r11,0,0x3fff0000
-	beq-	normal_tlb_miss_access_fault /* KUAP fault */
-#endif
 
 	/* Now we build the MAS:
 	 *
@@ -709,9 +710,7 @@ finish_normal_tlb_miss:
 	rldicl	r10,r14,64-8,64-8
 	cmpldi	cr0,r10,BOOK3E_PAGESZ_4K
 	beq-	1f
-#ifndef CONFIG_PPC_KUAP
 	mfspr	r11,SPRN_MAS1
-#endif
 	rlwimi	r11,r14,31,21,24
 	rlwinm	r11,r11,0,21,19
 	mtspr	SPRN_MAS1,r11
-- 
2.36.1