> 2024年10月14日 下午10:40，Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> 写道：
>
> On Mon, 14 Oct 2024 14:14:05 +0800
> Ma Qiao <mqaio@xxxxxxxxxxxxxxxxx> wrote:
>
>> From: Qiao Ma <mqaio@xxxxxxxxxxxxxxxxx>
>>
>> Uprobe needs to fetch args into a percpu buffer, and then copy to ring
>> buffer to avoid non-atomic context problem.
>>
>> Sometimes user-space strings, arrays can be very large, but the size of
>> percpu buffer is only page size. And store_trace_args() won't check
>> whether these data exceeds a single page or not, caused out-of-bounds
>> memory access.
>>
>> It could be reproduced by following steps:
>> 1. build kernel with CONFIG_KASAN enabled
>> 2. save follow program as test.c
>>
>> ```
>> \#include <stdio.h>
>> \#include <stdlib.h>
>> \#include <string.h>
>>
>> // If string length large than MAX_STRING_SIZE, the fetch_store_strlen()
>> // will return 0, cause __get_data_size() return shorter size, and
>> // store_trace_args() will not trigger out-of-bounds access.
>> // So make string length less than 4096.
>> \#define STRLEN 4093
>>
>> void generate_string(char *str, int n)
>> {
>> int i;
>> for (i = 0; i < n; ++i)
>> {
>> char c = i % 26 + 'a';
>> str[i] = c;
>> }
>> str[n-1] = '\0';
>> }
>>
>> void print_string(char *str)
>> {
>> printf("%s\n", str);
>> }
>>
>> int main()
>> {
>> char tmp[STRLEN];
>>
>> generate_string(tmp, STRLEN);
>> print_string(tmp);
>>
>> return 0;
>> }
>> ```
>> 3. compile program
>> `gcc -o test test.c`
>>
>> 4. get the offset of `print_string()`
>> ```
>> objdump -t test | grep -w print_string
>> 0000000000401199 g F .text 000000000000001b print_string
>> ```
>>
>> 5. configure uprobe with offset 0x1199
>> ```
>> off=0x1199
>>
>> cd /sys/kernel/debug/tracing/
>> echo "p /root/test:${off} arg1=+0(%di):ustring arg2=\$comm arg3=+0(%di):ustring"
>>> uprobe_events
>> echo 1 > events/uprobes/enable
>> echo 1 > tracing_on
>> ```
>>
>> 6. run `test`, and kasan will report error.
>> ==================================================================
>> BUG: KASAN: use-after-free in strncpy_from_user+0x1d6/0x1f0
>> Write of size 8 at addr ffff88812311c004 by task test/499CPU: 0 UID: 0 PID: 499 Comm: test Not tainted 6.12.0-rc3+ #18
>> Hardware name: Red Hat KVM, BIOS 1.16.0-4.al8 04/01/2014
>> Call Trace:
>> <TASK>
>> dump_stack_lvl+0x55/0x70
>> print_address_description.constprop.0+0x27/0x310
>> kasan_report+0x10f/0x120
>> ? strncpy_from_user+0x1d6/0x1f0
>> strncpy_from_user+0x1d6/0x1f0
>> ? rmqueue.constprop.0+0x70d/0x2ad0
>> process_fetch_insn+0xb26/0x1470
>> ? __pfx_process_fetch_insn+0x10/0x10
>> ? _raw_spin_lock+0x85/0xe0
>> ? __pfx__raw_spin_lock+0x10/0x10
>> ? __pte_offset_map+0x1f/0x2d0
>> ? unwind_next_frame+0xc5f/0x1f80
>> ? arch_stack_walk+0x68/0xf0
>> ? is_bpf_text_address+0x23/0x30
>> ? kernel_text_address.part.0+0xbb/0xd0
>> ? __kernel_text_address+0x66/0xb0
>> ? unwind_get_return_address+0x5e/0xa0
>> ? __pfx_stack_trace_consume_entry+0x10/0x10
>> ? arch_stack_walk+0xa2/0xf0
>> ? _raw_spin_lock_irqsave+0x8b/0xf0
>> ? __pfx__raw_spin_lock_irqsave+0x10/0x10
>> ? depot_alloc_stack+0x4c/0x1f0
>> ? _raw_spin_unlock_irqrestore+0xe/0x30
>> ? stack_depot_save_flags+0x35d/0x4f0
>> ? kasan_save_stack+0x34/0x50
>> ? kasan_save_stack+0x24/0x50
>> ? mutex_lock+0x91/0xe0
>> ? __pfx_mutex_lock+0x10/0x10
>> prepare_uprobe_buffer.part.0+0x2cd/0x500
>> uprobe_dispatcher+0x2c3/0x6a0
>> ? __pfx_uprobe_dispatcher+0x10/0x10
>> ? __kasan_slab_alloc+0x4d/0x90
>> handler_chain+0xdd/0x3e0
>> handle_swbp+0x26e/0x3d0
>> ? __pfx_handle_swbp+0x10/0x10
>> ? uprobe_pre_sstep_notifier+0x151/0x1b0
>> irqentry_exit_to_user_mode+0xe2/0x1b0
>> asm_exc_int3+0x39/0x40
>> RIP: 0033:0x401199
>> Code: 01 c2 0f b6 45 fb 88 02 83 45 fc 01 8b 45 fc 3b 45 e4 7c b7 8b 45 e4 48 98 48 8d 50 ff 48 8b 45 e8 48 01 d0 ce
>> RSP: 002b:00007ffdf00576a8 EFLAGS: 00000206
>> RAX: 00007ffdf00576b0 RBX: 0000000000000000 RCX: 0000000000000ff2
>> RDX: 0000000000000ffc RSI: 0000000000000ffd RDI: 00007ffdf00576b0
>> RBP: 00007ffdf00586b0 R08: 00007feb2f9c0d20 R09: 00007feb2f9c0d20
>> R10: 0000000000000001 R11: 0000000000000202 R12: 0000000000401040
>> R13: 00007ffdf0058780 R14: 0000000000000000 R15: 0000000000000000
>> </TASK>
>>
>> This commit enforces the buffer's maxlen less than a page-size to avoid
>> store_trace_args() out-of-memory access.
>>
>> Fixes: dcad1a204f72 ("tracing/uprobes: Fetch args before reserving a ring buffer")
>> Signed-off-by: Qiao Ma <mqaio@xxxxxxxxxxxxxxxxx>
>> ---
>> kernel/trace/trace_probe_tmpl.h | 2 +-
>> kernel/trace/trace_uprobe.c | 6 ++++++
>> 2 files changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/trace/trace_probe_tmpl.h b/kernel/trace/trace_probe_tmpl.h
>> index 2caf0d2afb322..0338d9468bb4d 100644
>> --- a/kernel/trace/trace_probe_tmpl.h
>> +++ b/kernel/trace/trace_probe_tmpl.h
>> @@ -269,7 +269,7 @@ store_trace_args(void *data, struct trace_probe *tp, void *rec, void *edata,
>> ret = process_fetch_insn(arg->code, rec, edata, dl, base);
>> if (arg->dynamic && likely(ret > 0)) {
>> dyndata += ret;
>> - maxlen -= ret;
>> + maxlen = max(maxlen - ret, 0);
>
> Hmm, do you see this part does something wrong?
> If this exceed maxlen here, that means a buffer overflow. Please make it WARN_ON_ONCE().
Hmmm, I was wrong, maxlen can never be negative, even this patch set ucb->dsize less than the real size of args.
And even if some weird bugs really cause maxlen to be negative, it is too late here to WARN(),
because out-of-memory access has been occured.
So maybe the best way is not modify here?
>
>> }
>> }
>> }
>> diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
>> index c40531d2cbadd..e972855a5a6bf 100644
>> --- a/kernel/trace/trace_uprobe.c
>> +++ b/kernel/trace/trace_uprobe.c
>> @@ -875,6 +875,7 @@ struct uprobe_cpu_buffer {
>> };
>> static struct uprobe_cpu_buffer __percpu *uprobe_cpu_buffer;
>> static int uprobe_buffer_refcnt;
>> +#define MAX_UCB_BUFFER_SIZE PAGE_SIZE
>>
>> static int uprobe_buffer_init(void)
>> {
>> @@ -979,6 +980,11 @@ static struct uprobe_cpu_buffer *prepare_uprobe_buffer(struct trace_uprobe *tu,
>> ucb = uprobe_buffer_get();
>> ucb->dsize = tu->tp.size + dsize;
>>
>> + if (WARN_ON_ONCE(ucb->dsize > MAX_UCB_BUFFER_SIZE)) {
>> + ucb->dsize = MAX_UCB_BUFFER_SIZE;
>> + dsize = MAX_UCB_BUFFER_SIZE - tu->tp.size;
>> + }
>> +
>
> This part looks good to me.
>
> Thank you!
>
>> store_trace_args(ucb->buf, &tu->tp, regs, NULL, esize, dsize);
>>
>> *ucbp = ucb;
>> --
>> 2.39.3
>>
>
>
> --
> Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
