[PATCH v2] powerpc: pseries/hvconsole: fix stack overread via udbg

[Daniel Axtens]

While developing kasan for 64-bit book3s, I hit the following stack
over-read.

It occurs because the hypercall to put characters onto the terminal
takes 2 longs (128 bits/16 bytes) of characters at a time, and so
hvc_put_chars would unconditionally copy 16 bytes from the argument
buffer, regardless of supplied length. However, udbg_hvc_putc can
call hvc_put_chars with a single-byte buffer, leading to the error.

[    0.001931] ==================================================================                                                  [150/819]
[    0.001933] BUG: KASAN: stack-out-of-bounds in hvc_put_chars+0xdc/0x110
[    0.001934] Read of size 8 at addr c0000000023e7a90 by task swapper/0
[    0.001934]
[    0.001935] CPU: 0 PID: 0 Comm: swapper Not tainted 5.2.0-rc2-next-20190528-02824-g048a6ab4835b #113
[    0.001935] Call Trace:
[    0.001936] [c0000000023e7790] [c000000001b4a450] dump_stack+0x104/0x154 (unreliable)
[    0.001937] [c0000000023e77f0] [c0000000006d3524] print_address_description+0xa0/0x30c
[    0.001938] [c0000000023e7880] [c0000000006d318c] __kasan_report+0x20c/0x224
[    0.001939] [c0000000023e7950] [c0000000006d19d8] kasan_report+0x18/0x30
[    0.001940] [c0000000023e7970] [c0000000006d4854] __asan_report_load8_noabort+0x24/0x40
[    0.001941] [c0000000023e7990] [c0000000001511ac] hvc_put_chars+0xdc/0x110
[    0.001942] [c0000000023e7a10] [c000000000f81cfc] hvterm_raw_put_chars+0x9c/0x110
[    0.001943] [c0000000023e7a50] [c000000000f82634] udbg_hvc_putc+0x154/0x200
[    0.001944] [c0000000023e7b10] [c000000000049c90] udbg_write+0xf0/0x240
[    0.001945] [c0000000023e7b70] [c0000000002e5d88] console_unlock+0x868/0xd30
[    0.001946] [c0000000023e7ca0] [c0000000002e6e00] register_console+0x970/0xe90
[    0.001947] [c0000000023e7d80] [c000000001ff1928] register_early_udbg_console+0xf8/0x114
[    0.001948] [c0000000023e7df0] [c000000001ff1174] setup_arch+0x108/0x790
[    0.001948] [c0000000023e7e90] [c000000001fe41c8] start_kernel+0x104/0x784
[    0.001949] [c0000000023e7f90] [c00000000000b368] start_here_common+0x1c/0x534
[    0.001950]
[    0.001950]
[    0.001951] Memory state around the buggy address:
[    0.001952]  c0000000023e7980: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    0.001952]  c0000000023e7a00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1
[    0.001953] >c0000000023e7a80: f1 f1 01 f2 f2 f2 00 00 00 00 00 00 00 00 00 00
[    0.001953]                          ^
[    0.001954]  c0000000023e7b00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    0.001954]  c0000000023e7b80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    0.001955] ==================================================================

Document that a 16-byte buffer is requred, and provide it in udbg.

CC: Dmitry Vyukov <dvyukov at google.com>
Signed-off-by: Daniel Axtens <dja at axtens.net>

---

v2: avoid memcpy, push responsibility to caller.
    Solution suggested by mpe.
---
 arch/powerpc/platforms/pseries/hvconsole.c |  2 +-
 drivers/tty/hvc/hvc_vio.c                  | 16 +++++++++++++++-
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/hvconsole.c b/arch/powerpc/platforms/pseries/hvconsole.c
index 74da18de853a..73ec15cd2708 100644
--- a/arch/powerpc/platforms/pseries/hvconsole.c
+++ b/arch/powerpc/platforms/pseries/hvconsole.c
@@ -62,7 +62,7 @@ EXPORT_SYMBOL(hvc_get_chars);
  * @vtermno: The vtermno or unit_address of the adapter from which the data
  *	originated.
  * @buf: The character buffer that contains the character data to send to
- *	firmware.
+ *	firmware. Must be at least 16 bytes, even if count is less than 16.
  * @count: Send this number of characters.
  */
 int hvc_put_chars(uint32_t vtermno, const char *buf, int count)
diff --git a/drivers/tty/hvc/hvc_vio.c b/drivers/tty/hvc/hvc_vio.c
index 6de6d4a1a221..7af54d6ed5b8 100644
--- a/drivers/tty/hvc/hvc_vio.c
+++ b/drivers/tty/hvc/hvc_vio.c
@@ -107,6 +107,14 @@ static int hvterm_raw_get_chars(uint32_t vtermno, char *buf, int count)
 	return got;
 }
 
+/**
+ * hvterm_raw_put_chars: send characters to firmware for given vterm adapter
+ * @vtermno: The virtual terminal number.
+ * @buf: The characters to send. Because of the underlying hypercall in
+ *       hvc_put_chars(), this buffer must be at least 16 bytes long, even if
+ *       you are sending fewer chars.
+ * @count: number of chars to send.
+ */
 static int hvterm_raw_put_chars(uint32_t vtermno, const char *buf, int count)
 {
 	struct hvterm_priv *pv = hvterm_privs[vtermno];
@@ -219,6 +227,7 @@ static const struct hv_ops hvterm_hvsi_ops = {
 static void udbg_hvc_putc(char c)
 {
 	int count = -1;
+	unsigned char bounce_buffer[16];
 
 	if (!hvterm_privs[0])
 		return;
@@ -229,7 +238,12 @@ static void udbg_hvc_putc(char c)
 	do {
 		switch(hvterm_privs[0]->proto) {
 		case HV_PROTOCOL_RAW:
-			count = hvterm_raw_put_chars(0, &c, 1);
+			/*
+			 * hvterm_raw_put_chars requires at least a 16-byte
+			 * buffer, so go via the bounce buffer
+			 */
+			bounce_buffer[0] = c;
+			count = hvterm_raw_put_chars(0, bounce_buffer, 1);
 			break;
 		case HV_PROTOCOL_HVSI:
 			count = hvterm_hvsi_put_chars(0, &c, 1);
-- 
2.19.1