Re: [PATCH] uprobe: avoid out-of-bounds memory access of fetching args

From: Google
Date: Mon Oct 14 2024 - 10:40:39 EST

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().

> }
> }
> }
> 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>