parisc: Revise gateway LWS calls to probe user read access

We use load and stbys,e instructions to trigger memory reference
interruptions without writing to memory. Because of the way read
access support is implemented, read access interruptions are only
triggered at privilege levels 2 and 3. The kernel and gateway
page execute at privilege level 0, so this code never triggers
a read access interruption. Thus, it is currently possible for
user code to execute a LWS compare and swap operation at an
address that is read protected at privilege level 3 (PRIV_USER).

Fix this by probing read access rights at privilege level 3 and
branching to lws_fault if access isn't allowed.

Signed-off-by: John David Anglin <[email protected]>
Signed-off-by: Helge Deller <[email protected]>
Cc: [email protected] # v5.12+

Author: danglin44

arch/parisc/kernel/syscall.S

@@ -613,6 +613,9 @@ lws_compare_and_swap32:
 lws_compare_and_swap:
 	/* Trigger memory reference interruptions without writing to memory */
 1:	ldw	0(%r26), %r28
+	proberi	(%r26), PRIV_USER, %r28
+	comb,=,n	%r28, %r0, lws_fault /* backwards, likely not taken */
+	nop
 2:	stbys,e	%r0, 0(%r26)
 
 	/* Calculate 8-bit hash index from virtual address */
@@ -767,6 +770,9 @@ cas2_lock_start:
 	copy	%r26, %r28
 	depi_safe	0, 31, 2, %r28
 10:	ldw	0(%r28), %r1
+	proberi	(%r28), PRIV_USER, %r1
+	comb,=,n	%r1, %r0, lws_fault /* backwards, likely not taken */
+	nop
 11:	stbys,e	%r0, 0(%r28)
 
 	/* Calculate 8-bit hash index from virtual address */
@@ -951,41 +957,47 @@ atomic_xchg_begin:
 
 	/* 8-bit exchange */
 1:	ldb	0(%r24), %r20
+	proberi	(%r24), PRIV_USER, %r20
+	comb,=,n	%r20, %r0, lws_fault /* backwards, likely not taken */
+	nop
 	copy	%r23, %r20
 	depi_safe	0, 31, 2, %r20
 	b	atomic_xchg_start
 2:	stbys,e	%r0, 0(%r20)
-	nop
-	nop
-	nop
 
 	/* 16-bit exchange */
 3:	ldh	0(%r24), %r20
+	proberi	(%r24), PRIV_USER, %r20
+	comb,=,n	%r20, %r0, lws_fault /* backwards, likely not taken */
+	nop
 	copy	%r23, %r20
 	depi_safe	0, 31, 2, %r20
 	b	atomic_xchg_start
 4:	stbys,e	%r0, 0(%r20)
-	nop
-	nop
-	nop
 
 	/* 32-bit exchange */
 5:	ldw	0(%r24), %r20
+	proberi	(%r24), PRIV_USER, %r20
+	comb,=,n	%r20, %r0, lws_fault /* backwards, likely not taken */
+	nop
 	b	atomic_xchg_start
 6:	stbys,e	%r0, 0(%r23)
 	nop
 	nop
-	nop
-	nop
-	nop
 
 	/* 64-bit exchange */
 #ifdef CONFIG_64BIT
 7:	ldd	0(%r24), %r20
+	proberi	(%r24), PRIV_USER, %r20
+	comb,=,n	%r20, %r0, lws_fault /* backwards, likely not taken */
+	nop
 8:	stdby,e	%r0, 0(%r23)
 #else
 7:	ldw	0(%r24), %r20
 8:	ldw	4(%r24), %r20
+	proberi	(%r24), PRIV_USER, %r20
+	comb,=,n	%r20, %r0, lws_fault /* backwards, likely not taken */
+	nop
 	copy	%r23, %r20
 	depi_safe	0, 31, 2, %r20
 9:	stbys,e	%r0, 0(%r20)