[PATCH] tracing: Prevent trace_marker being bigger than unsigned short

[Steven Rostedt <rostedt@goodmis.org>]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

The trace_marker write goes into the ring buffer. A test was added to
write a string as big as the sub-buffer of the ring buffer to see if it
would work. A sub-buffer is typically PAGE_SIZE in length.

On PowerPC architecture, the ftrace selftest for trace_marker started to
fail. This was due to PowerPC having a PAGE_SIZE of 65536 and not 4096. It
would try to write a string that was around 63000 bytes in size. This gave
the following warning:

------------[ cut here ]------------
precision 63492 too large
WARNING: CPU: 15 PID: 2538829 at lib/vsprintf.c:2721 set_precision+0x68/0xa4
Modules linked in:
CPU: 15 PID: 2538829 Comm: awk Tainted: G M O K 6.8.0-rc5-gfca7526b7d89 #1
Hardware name: IBM,9080-HEX POWER10 (raw) 0x800200 0xf000006 of:IBM,FW1060.00 (NH1060_018) hv:phyp pSeries
NIP: c000000000f57c34 LR: c000000000f57c30 CTR: c000000000f5cdf0
REGS: c000000a58e4f5f0 TRAP: 0700 Tainted: G M O K (6.8.0-rc5-gfca7526b7d89)
MSR: 8000000002029033 <SF,VEC,EE,ME,IR,DR,RI,LE> CR: 48000824 XER: 00000005
CFAR: c00000000016154c IRQMASK: 0
GPR00: c000000000f57c30 c000000a58e4f890 c000000001482800 0000000000000019
GPR04: 0000000100011559 c000000a58e4f660 c000000a58e4f658 0000000000000027
GPR08: c000000e84e37c10 0000000000000001 0000000000000027 c000000002a47e50
GPR12: 0000000000000000 c000000e87bf7300 0000000000000000 0000000000000000
GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR20: c0000004a43ec590 0000000000400cc0 0000000000000003 c0000000012c3e65
GPR24: c000000a58e4fa18 0000000000000025 0000000000000020 000000000001ff97
GPR28: c0000001168a00dd c0000001168c0074 c000000a58e4f920 000000000000f804
NIP [c000000000f57c34] set_precision+0x68/0xa4
LR [c000000000f57c30] set_precision+0x64/0xa4
Call Trace:
[c000000a58e4f890] [c000000000f57c30] set_precision+0x64/0xa4 (unreliable)
[c000000a58e4f900] [c000000000f5ccc4] vsnprintf+0x198/0x4c8
[c000000a58e4f980] [c000000000f53228] seq_buf_vprintf+0x50/0xa0
[c000000a58e4f9b0] [c00000000031cec0] trace_seq_printf+0x60/0xe0
[c000000a58e4f9e0] [c00000000031b5f0] trace_print_print+0x78/0xa4
[c000000a58e4fa60] [c0000000003133a4] print_trace_line+0x2ac/0x6d8
[c000000a58e4fb20] [c0000000003145c0] s_show+0x58/0x2c0
[c000000a58e4fba0] [c0000000005dfb2c] seq_read_iter+0x448/0x618
[c000000a58e4fc70] [c0000000005dfe08] seq_read+0x10c/0x174
[c000000a58e4fd10] [c00000000059a7e0] vfs_read+0xe0/0x39c
[c000000a58e4fdc0] [c00000000059b59c] ksys_read+0x7c/0x140
[c000000a58e4fe10] [c000000000035d74] system_call_exception+0x134/0x330
[c000000a58e4fe50] [c00000000000d6a0] system_call_common+0x160/0x2e4

The problem was that in trace_print_print() that reads the trace_marker
write data had the following code:

	int max = iter->ent_size - offsetof(struct print_entry, buf);

	[..]
	trace_seq_printf(s, ": %.*s", max, field->buf);

Where "max" was the size of the entry. Now that the write to trace_marker
can be as big as what the sub-buffer can hold, and the sub-buffer for
powerpc is 64K in size, the "max" value was: 63492, and that was passed to
trace_seq_printf() which eventually calls vsnprintf() with the same format
and parameters.

The max "precision" that "%.*s" can be is max signed short (32767) where
63492 happens to be greater than.

Prevent the max size written by trace_marker to be greater than what a
signed short can hold.

Link: https://lore.kernel.org/all/C7E7AF1A-D30F-4D18-B8E5-AF1EF58004F5@linux.ibm.com/

Reported-by: Sachin Sant <sachinp@linux.ibm.com>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Fixes: 8ec90be7f15f ("tracing: Allow for max buffer data size trace_marker writes")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/trace/trace.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 8198bfc54b58..1606fa99367b 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7310,7 +7310,9 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
 /* Used in tracing_mark_raw_write() as well */
 #define FAULTED_STR "<faulted>"
 #define FAULTED_SIZE (sizeof(FAULTED_STR) - 1) /* '\0' is already accounted for */
-
+#ifndef SHORT_MAX
+#define SHORT_MAX	((1<<15) - 1)
+#endif
 	if (tracing_disabled)
 		return -EINVAL;
 
@@ -7328,6 +7330,16 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
 	if (cnt < FAULTED_SIZE)
 		size += FAULTED_SIZE - cnt;
 
+	/*
+	 * trace_print_print() uses vsprintf() to determine the size via
+	 * the precision format "%.*s" which can not be greater than
+	 * a signed short.
+	 */
+	if (size > SHORT_MAX) {
+		cnt -= size - SHORT_MAX;
+		goto again;
+	}
+
 	if (size > TRACE_SEQ_BUFFER_SIZE) {
 		cnt -= size - TRACE_SEQ_BUFFER_SIZE;
 		goto again;
-- 
2.43.0