Re: [PATCH 3/3] tracing/filter: make filter_pred_pchar() survive the access to user space
From: Steven Rostedt
Date: Fri Jan 07 2022 - 17:25:37 EST
On Fri, 7 Jan 2022 12:49:51 +0800
Pingfan Liu <kernelfans@xxxxxxxxx> wrote:
> When
> echo "filename ~ \"cpu\"" > events/syscalls/sys_enter_openat/filter
> echo 1 > events/syscalls/sys_enter_at/enable
So this is definitely a bug. Thanks for reporting this.
>
> Then the following #PF is observed:
> kvm-03-guest16 login: [72198.026181] BUG: unable to handle page fault for address: 00007fffaae8ef60
> [72198.027625] #PF: supervisor read access in kernel mode
> [72198.028627] #PF: error_code(0x0001) - permissions violation
> [72198.029708] PGD 80000001008b7067 P4D 80000001008b7067 PUD 2393f1067 PMD 2393ec067 PTE 8000000108f47867
> [72198.031588] Oops: 0001 [#1] PREEMPT SMP PTI
> [72198.032410] CPU: 1 PID: 1 Comm: systemd Kdump: loaded Not tainted 5.14.0-32.el9.x86_64 #1
> [72198.034021] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
> [72198.035190] RIP: 0010:strlen+0x0/0x20
> [72198.035914] 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
> [72198.039576] RSP: 0018:ffffb5b900013e48 EFLAGS: 00010246
> [72198.040593] RAX: 0000000000000018 RBX: ffff8fc1c49ede00 RCX: 0000000000000000
> [72198.041991] RDX: 0000000000000020 RSI: ffff8fc1c02d601c RDI: 00007fffaae8ef60
> [72198.043419] RBP: 00007fffaae8ef60 R08: 0005034f4ddb8ea4 R09: 0000000000000000
> [72198.044800] R10: ffff8fc1c02d601c R11: 0000000000000000 R12: ffff8fc1c8a6e380
> [72198.046185] R13: 0000000000000000 R14: ffff8fc1c02d6010 R15: ffff8fc1c00453c0
> [72198.047610] FS: 00007fa86123db40(0000) GS:ffff8fc2ffd00000(0000) knlGS:0000000000000000
> [72198.049206] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [72198.050332] CR2: 00007fffaae8ef60 CR3: 0000000102880001 CR4: 00000000007706e0
> [72198.051760] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [72198.053168] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [72198.054550] PKRU: 55555554
> [72198.055114] Call Trace:
> [72198.055616] filter_pred_pchar+0x18/0x40
> [72198.056421] filter_match_preds+0x31/0x70
> [72198.057210] ftrace_syscall_enter+0x27a/0x2c0
> [72198.058088] syscall_trace_enter.constprop.0+0x1aa/0x1d0
> [72198.059163] do_syscall_64+0x16/0x90
> [72198.059898] entry_SYSCALL_64_after_hwframe+0x44/0xae
> [72198.060904] RIP: 0033:0x7fa861d88664
>
> Apparently, it is caused by supervisor read access in kernel mode.
>
> To tackle this issue caused by event_class_syscall_enter, using the pair
> of user_access_{begin/end}() may be an efficient method, but it means to
> stir up _ASM_EXTABLE. Hence this patch picks up the road of
> copy_from_user(). This is achieved by introducing a field 'uaccess' in
> ftrace_event_field, and run regex.match on the copied buffer.
>
> Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> To: linux-kernel@xxxxxxxxxxxxxxx
> ---
> kernel/trace/trace.h | 1 +
> kernel/trace/trace_events_filter.c | 29 ++++++++++++++++++++++++++---
> 2 files changed, 27 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 38715aa6cfdf..81a263a060e8 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -1281,6 +1281,7 @@ struct ftrace_event_field {
> int offset;
> int size;
> int is_signed;
> + int uaccess;
> };
>
> struct prog_entry;
> diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
> index 2a05315127f9..9af268b98c61 100644
> --- a/kernel/trace/trace_events_filter.c
> +++ b/kernel/trace/trace_events_filter.c
> @@ -10,6 +10,7 @@
> #include <linux/mutex.h>
> #include <linux/perf_event.h>
> #include <linux/slab.h>
> +#include <linux/syscalls.h>
>
> #include "trace.h"
> #include "trace_output.h"
> @@ -672,12 +673,30 @@ 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 *udata, *cmp_buff;
> int cmp, match;
> - int len = strlen(addr) + 1; /* including tailing '\0' */
> + int len, poffset;
> +
> + if (unlikely(pred->field->uaccess)) {
> + udata = kmalloc(PAGE_SIZE, GFP_KERNEL);
This is called within a tracepoint, thus you can not call kmalloc.
> + if (!udata)
> + return -ENOMEM;
> + poffset = (ulong)addr & (PAGE_SIZE - 1);
> + cmp_buff = udata + poffset;
> + if (copy_from_user(cmp_buff, addr, PAGE_SIZE - poffset)) {
> + kfree(udata);
> + return -EFAULT;
> + }
> + } else {
> + cmp_buff = addr;
> + }
> + len = strlen(cmp_buff) + 1; /* including tailing '\0' */
>
> - cmp = pred->regex.match(addr, &pred->regex, len);
> + cmp = pred->regex.match(cmp_buff, &pred->regex, len);
>
> match = cmp ^ pred->not;
> + if (unlikely(pred->field->uaccess))
> + kfree(udata);
>
> return match;
> }
> @@ -1220,6 +1239,7 @@ static int parse_pred(const char *str, struct trace_event_call *data,
> return -ENOMEM;
>
> pred->field = field;
> + field->uaccess = 0;
> pred->offset = field->offset;
> pred->op = op;
>
> @@ -1321,8 +1341,11 @@ static int parse_pred(const char *str, struct trace_event_call *data,
>
> } else if (field->filter_type == FILTER_DYN_STRING)
> pred->fn = filter_pred_strloc;
> - else
> + else {
> pred->fn = filter_pred_pchar;
> + if (data->class == &event_class_syscall_enter)
> + pred->field->uaccess = 1;
> + }
> /* go past the last quote */
> i++;
>
This is not the right way to fix it. Simply check if the string is user
space and copy it into a temp per cpu buffer. In fact, kernel space strings
should be tested as well.
I wrote this up. Can you make sure it works for you?
Thanks,
-- Steve
From: Steven Rostedt <rostedt@xxxxxxxxxxx>
Subject: [PATCH] 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. 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@xxxxxxxxx/
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Pingfan Liu <kernelfans@xxxxxxxxx>
Fixes: 87a342f5db69d ("tracing/filters: Support filtering for char * strings")
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
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@xxxxxxxxx>
*/
+#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
+
+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))
+ 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);
+ 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);
+ 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