tty: evh_bytechan: Fix out of bounds accesses

ev_byte_channel_send() assumes that its third argument is a 16 byte
array. Some places where it is called it may not be (or we can't
easily tell if it is). Newer compilers have started producing warnings
about this, so make sure we actually pass a 16 byte array.

There may be more elegant solutions to this, but the driver is quite
old and hasn't been updated in many years.

The warnings (from a powerpc allyesconfig build) are:

  In file included from include/linux/byteorder/big_endian.h:5,
                   from arch/powerpc/include/uapi/asm/byteorder.h:14,
                   from include/asm-generic/bitops/le.h:6,
                   from arch/powerpc/include/asm/bitops.h:250,
                   from include/linux/bitops.h:29,
                   from include/linux/kernel.h:12,
                   from include/asm-generic/bug.h:19,
                   from arch/powerpc/include/asm/bug.h:109,
                   from include/linux/bug.h:5,
                   from include/linux/mmdebug.h:5,
                   from include/linux/gfp.h:5,
                   from include/linux/slab.h:15,
                   from drivers/tty/ehv_bytechan.c:24:
  drivers/tty/ehv_bytechan.c: In function ‘ehv_bc_udbg_putc’:
  arch/powerpc/include/asm/epapr_hcalls.h:298:20: warning: array subscript 1 is outside array bounds of ‘const char[1]’ [-Warray-bounds]
    298 |  r6 = be32_to_cpu(p[1]);
  include/uapi/linux/byteorder/big_endian.h:40:51: note: in definition of macro ‘__be32_to_cpu’
     40 | #define __be32_to_cpu(x) ((__force __u32)(__be32)(x))
        |                                                   ^
  arch/powerpc/include/asm/epapr_hcalls.h:298:7: note: in expansion of macro ‘be32_to_cpu’
    298 |  r6 = be32_to_cpu(p[1]);
        |       ^~~~~~~~~~~
  drivers/tty/ehv_bytechan.c:166:13: note: while referencing ‘data’
    166 | static void ehv_bc_udbg_putc(char c)
        |             ^~~~~~~~~~~~~~~~

Fixes: dcd83aaff1 ("tty/powerpc: introduce the ePAPR embedded hypervisor byte channel driver")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Tested-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
[mpe: Trim warnings from change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200109183912.5fcb52aa@canb.auug.org.au

drivers/tty/ehv_bytechan.c

@@ -136,6 +136,21 @@ static int find_console_handle(void)
 	return 1;
 }
 
+static unsigned int local_ev_byte_channel_send(unsigned int handle,
+					       unsigned int *count,
+					       const char *p)
+{
+	char buffer[EV_BYTE_CHANNEL_MAX_BYTES];
+	unsigned int c = *count;
+
+	if (c < sizeof(buffer)) {
+		memcpy(buffer, p, c);
+		memset(&buffer[c], 0, sizeof(buffer) - c);
+		p = buffer;
+	}
+	return ev_byte_channel_send(handle, count, p);
+}
+
 /*************************** EARLY CONSOLE DRIVER ***************************/
 
 #ifdef CONFIG_PPC_EARLY_DEBUG_EHV_BC
@@ -154,7 +169,7 @@ static void byte_channel_spin_send(const char data)
 
 	do {
 		count = 1;
-		ret = ev_byte_channel_send(CONFIG_PPC_EARLY_DEBUG_EHV_BC_HANDLE,
+		ret = local_ev_byte_channel_send(CONFIG_PPC_EARLY_DEBUG_EHV_BC_HANDLE,
 					   &count, &data);
 	} while (ret == EV_EAGAIN);
 }
@@ -221,7 +236,7 @@ static int ehv_bc_console_byte_channel_send(unsigned int handle, const char *s,
 	while (count) {
 		len = min_t(unsigned int, count, EV_BYTE_CHANNEL_MAX_BYTES);
 		do {
-			ret = ev_byte_channel_send(handle, &len, s);
+			ret = local_ev_byte_channel_send(handle, &len, s);
 		} while (ret == EV_EAGAIN);
 		count -= len;
 		s += len;
@@ -401,7 +416,7 @@ static void ehv_bc_tx_dequeue(struct ehv_bc_data *bc)
 			    CIRC_CNT_TO_END(bc->head, bc->tail, BUF_SIZE),
 			    EV_BYTE_CHANNEL_MAX_BYTES);
 
-		ret = ev_byte_channel_send(bc->handle, &len, bc->buf + bc->tail);
+		ret = local_ev_byte_channel_send(bc->handle, &len, bc->buf + bc->tail);
 
 		/* 'len' is valid only if the return code is 0 or EV_EAGAIN */
 		if (!ret || (ret == EV_EAGAIN))