tracing: Add test for user space strings when filtering on string pointers

Pingfan reported that the following causes a fault:

  echo "filename ~ \"cpu\"" > events/syscalls/sys_enter_openat/filter
  echo 1 > events/syscalls/sys_enter_at/enable

The reason is that trace event filter treats the user space pointer
defined by "filename" as a normal pointer to compare against the "cpu"
string. The following bug happened:

 kvm-03-guest16 login: [72198.026181] BUG: unable to handle page fault for address: 00007fffaae8ef60
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0001) - permissions violation
 PGD 80000001008b7067 P4D 80000001008b7067 PUD 2393f1067 PMD 2393ec067 PTE 8000000108f47867
 Oops: 0001 [#1] PREEMPT SMP PTI
 CPU: 1 PID: 1 Comm: systemd Kdump: loaded Not tainted 5.14.0-32.el9.x86_64 #1
 Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
 RIP: 0010:strlen+0x0/0x20
 Code: 48 89 f9 74 09 48 83 c1 01 80 39 00 75 f7 31 d2 44 0f b6 04 16 44 88 04 11
       48 83 c2 01 45 84 c0 75 ee c3 0f 1f 80 00 00 00 00 <80> 3f 00 74 10 48 89 f8
       48 83 c0 01 80 38 00 75 f7 48 29 f8 c3 31
 RSP: 0018:ffffb5b900013e48 EFLAGS: 00010246
 RAX: 0000000000000018 RBX: ffff8fc1c49ede00 RCX: 0000000000000000
 RDX: 0000000000000020 RSI: ffff8fc1c02d601c RDI: 00007fffaae8ef60
 RBP: 00007fffaae8ef60 R08: 0005034f4ddb8ea4 R09: 0000000000000000
 R10: ffff8fc1c02d601c R11: 0000000000000000 R12: ffff8fc1c8a6e380
 R13: 0000000000000000 R14: ffff8fc1c02d6010 R15: ffff8fc1c00453c0
 FS:  00007fa86123db40(0000) GS:ffff8fc2ffd00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00007fffaae8ef60 CR3: 0000000102880001 CR4: 00000000007706e0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 PKRU: 55555554
 Call Trace:
  filter_pred_pchar+0x18/0x40
  filter_match_preds+0x31/0x70
  ftrace_syscall_enter+0x27a/0x2c0
  syscall_trace_enter.constprop.0+0x1aa/0x1d0
  do_syscall_64+0x16/0x90
  entry_SYSCALL_64_after_hwframe+0x44/0xae
 RIP: 0033:0x7fa861d88664

The above happened because the kernel tried to access user space directly
and triggered a "supervisor read access in kernel mode" fault. Worse yet,
the memory could not even be loaded yet, and a SEGFAULT could happen as
well. This could be true for kernel space accessing as well.

To be even more robust, test both kernel and user space strings. If the
string fails to read, then simply have the filter fail.

Note, TASK_SIZE is used to determine if the pointer is user or kernel space
and the appropriate strncpy_from_kernel/user_nofault() function is used to
copy the memory. For some architectures, the compare to TASK_SIZE may always
pick user space or kernel space. If it gets it wrong, the only thing is that
the filter will fail to match. In the future, this needs to be fixed to have
the event denote which should be used. But failing a filter is much better
than panicing the machine, and that can be solved later.

Link: https://lore.kernel.org/all/20220107044951.22080-1-kernelfans@gmail.com/
Link: https://lkml.kernel.org/r/20220110115532.536088fd@gandalf.local.home

Cc: stable@vger.kernel.org
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Tom Zanussi <zanussi@kernel.org>
Reported-by: Pingfan Liu <kernelfans@gmail.com>
Tested-by: Pingfan Liu <kernelfans@gmail.com>
Fixes: 87a342f5 ("tracing/filters: Support filtering for char * strings")
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Documentation/trace/events.rst

@@ -230,6 +230,16 @@ Currently the caret ('^') for an error always appears at the beginning of
 the filter string; the error message should still be useful though
 even without more accurate position info.
 
+5.2.1 Filter limitations
+------------------------
+
+If a filter is placed on a string pointer ``(char *)`` that does not point
+to a string on the ring buffer, but instead points to kernel or user space
+memory, then, for safety reasons, at most 1024 bytes of the content is
+copied onto a temporary buffer to do the compare. If the copy of the memory
+faults (the pointer points to memory that should not be accessed), then the
+string compare will be treated as not matching.
+
 5.3 Clearing filters
 --------------------
 

kernel/trace/trace_events_filter.c

@@ -5,6 +5,7 @@
  * Copyright (C) 2009 Tom Zanussi <tzanussi@gmail.com>
  */
 
+#include <linux/uaccess.h>
 #include <linux/module.h>
 #include <linux/ctype.h>
 #include <linux/mutex.h>
@@ -654,6 +655,47 @@ DEFINE_EQUALITY_PRED(32);
 DEFINE_EQUALITY_PRED(16);
 DEFINE_EQUALITY_PRED(8);
 
+/* user space strings temp buffer */
+#define USTRING_BUF_SIZE	1024
+
+struct ustring_buffer {
+	char		buffer[USTRING_BUF_SIZE];
+};
+
+static __percpu struct ustring_buffer *ustring_per_cpu;
+
+static __always_inline char *test_string(char *str)
+{
+	struct ustring_buffer *ubuf;
+	char __user *ustr;
+	char *kstr;
+
+	if (!ustring_per_cpu)
+		return NULL;
+
+	ubuf = this_cpu_ptr(ustring_per_cpu);
+	kstr = ubuf->buffer;
+
+	/*
+	 * We use TASK_SIZE to denote user or kernel space, but this will
+	 * not work for all architectures. If it picks the wrong one, it may
+	 * just fail the filter (but will not bug).
+	 *
+	 * TODO: Have a way to properly denote which one this is for.
+	 */
+	if (likely((unsigned long)str >= TASK_SIZE)) {
+		/* For safety, do not trust the string pointer */
+		if (!strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE))
+			return NULL;
+	} else {
+		/* user space address? */
+		ustr = (char __user *)str;
+		if (!strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE))
+			return NULL;
+	}
+	return kstr;
+}
+
 /* Filter predicate for fixed sized arrays of characters */
 static int filter_pred_string(struct filter_pred *pred, void *event)
 {
@@ -671,10 +713,16 @@ static int filter_pred_string(struct filter_pred *pred, void *event)
 static int filter_pred_pchar(struct filter_pred *pred, void *event)
 {
 	char **addr = (char **)(event + pred->offset);
+	char *str;
 	int cmp, match;
-	int len = strlen(*addr) + 1;	/* including tailing '\0' */
+	int len;
 
-	cmp = pred->regex.match(*addr, &pred->regex, len);
+	str = test_string(*addr);
+	if (!str)
+		return 0;
+
+	len = strlen(str) + 1;	/* including tailing '\0' */
+	cmp = pred->regex.match(str, &pred->regex, len);
 
 	match = cmp ^ pred->not;
 
@@ -1348,8 +1396,17 @@ static int parse_pred(const char *str, void *data,
 			pred->fn = filter_pred_strloc;
 		} else if (field->filter_type == FILTER_RDYN_STRING)
 			pred->fn = filter_pred_strrelloc;
-		else
+		else {
+
+			if (!ustring_per_cpu) {
+				/* Once allocated, keep it around for good */
+				ustring_per_cpu = alloc_percpu(struct ustring_buffer);
+				if (!ustring_per_cpu)
+					goto err_mem;
+			}
+
 			pred->fn = filter_pred_pchar;
+		}
 		/* go past the last quote */
 		i++;
 
@@ -1415,6 +1472,9 @@ static int parse_pred(const char *str, void *data,
 err_free:
 	kfree(pred);
 	return -EINVAL;
+err_mem:
+	kfree(pred);
+	return -ENOMEM;
 }
 
 enum {