Re: [PATCH 2/2] tracing: Add test for user space strings when filtering on string pointers

Hi Steven,

This patch passed my test. But I have some concern, please see comment inline. On Fri, Jan 07, 2022 at 05:56:57PM -0500, Steven Rostedt wrote:

From: Steven Rostedt rostedt@goodmis.org

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. If the string is not loaded into memory yet, it will trigger a fault in kernel space:

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

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

Link: https://lore.kernel.org/all/20220107044951.22080-1-kernelfans@gmail.com/

Cc: stable@vger.kernel.org Reported-by: Pingfan Liu kernelfans@gmail.com Fixes: 87a342f5db69d ("tracing/filters: Support filtering for char * strings") Signed-off-by: Steven Rostedt rostedt@goodmis.org

---

kernel/trace/trace_events_filter.c | 79 +++++++++++++++++++++++++++++- 1 file changed, 77 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c index 996920ed1812..cf0fa9a785c7 100644 --- a/kernel/trace/trace_events_filter.c +++ b/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> @@ -654,12 +655,50 @@ DEFINE_EQUALITY_PRED(32); DEFINE_EQUALITY_PRED(16); DEFINE_EQUALITY_PRED(8); +/* user space strings temp buffer */ +#define USTRING_BUF_SIZE 512

Should it be PATH_MAX(4096) in case of matching against a file path?

+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;
• 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))
  

Since no other trace_event_class except event_class_syscall_enter tries to uaccess, so the unreliable source only comes from event_class_syscall_enter.

In that case, the access to kernel address is forbidden. So here just return -EACCES ?

• 	return NULL;
  

• } else {

• /* user space address? */
  

• ustr = 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) { char *addr = (char *)(event + pred->offset); int cmp, match;

• addr = test_string(addr);

Among all of trace_event_class, only event_class_syscall_enter exposed to this fault (uprobe does not uaccess). So I think the strncpy_*() can be avoided based on class, which improves performance.

• if (!addr)

• return 0;
  

• cmp = pred->regex.match(addr, &pred->regex, pred->regex.field_len);

match = cmp ^ pred->not; @@ -671,10 +710,16 @@ 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;
• str = test_string(*addr);
• if (!str)

• return 0;
  

• cmp = pred->regex.match(*addr, &pred->regex, len);

• len = strlen(str) + 1; /* including tailing '\0' */
• cmp = pred->regex.match(str, &pred->regex, len);

match = cmp ^ pred->not; @@ -784,6 +829,10 @@ static int filter_pred_none(struct filter_pred *pred, void *event) static int regex_match_full(char *str, struct regex *r, int len) {

• str = test_string(str);

Since all regex_match_*() are called in filter_pred_*(), which have already protected codes from page fault. So no need to double check.

Thanks,

Pingfan

• if (!str)

• return 0;
  

• /* len of zero means str is dynamic and ends with '\0' */ if (!len) return strcmp(str, r->pattern) == 0;

@@ -793,6 +842,10 @@ static int regex_match_full(char *str, struct regex *r, int len) static int regex_match_front(char *str, struct regex *r, int len) {

• str = test_string(str);
• if (!str)

• return 0;
  

• if (len && len < r->len) return 0;

@@ -801,6 +854,10 @@ static int regex_match_front(char *str, struct regex *r, int len) static int regex_match_middle(char *str, struct regex *r, int len) {

• str = test_string(str);
• if (!str)

• return 0;
  

• if (!len) return strstr(str, r->pattern) != NULL;

@@ -811,6 +868,10 @@ static int regex_match_end(char *str, struct regex *r, int len) { int strlen = len - 1;

• str = test_string(str);
• if (!str)

• return 0;
  

• if (strlen >= r->len && memcmp(str + strlen - r->len, r->pattern, r->len) == 0) return 1;

@@ -819,6 +880,10 @@ static int regex_match_end(char *str, struct regex *r, int len) static int regex_match_glob(char *str, struct regex *r, int len __maybe_unused) {

• str = test_string(str);
• if (!str)

• return 0;
  

• if (glob_match(r->pattern, str)) return 1; return 0;

@@ -1335,6 +1400,13 @@ static int parse_pred(const char *str, void *data, strncpy(pred->regex.pattern, str + s, len); pred->regex.pattern[len] = 0;

• 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;
  

• }
  

• filter_build_regex(pred);

if (field->filter_type == FILTER_COMM) { @@ -1415,6 +1487,9 @@ static int parse_pred(const char *str, void *data, err_free: kfree(pred); return -EINVAL; +err_mem:

• kfree(pred);
• return -ENOMEM;

} enum { -- 2.33.0