[PATCH 6.1 2/3] bpf: Do cleanup in bpf_bprintf_cleanup only when needed

Thadeu Lima de Souza Cascardo
2024-02-17T12:13:42Z

commit f19a4050455aad847fb93f18dc1fe502eb60f989 upstream.

Currently we always cleanup/decrement bpf_bprintf_nest_level variable
in bpf_bprintf_cleanup if it's > 0.

There's possible scenario where this could cause a problem, when
bpf_bprintf_prepare does not get bin_args buffer (because num_args is 0)
and following bpf_bprintf_cleanup call decrements bpf_bprintf_nest_level
variable, like:

in task context:
bpf_bprintf_prepare(num_args != 0) increments 'bpf_bprintf_nest_level = 1'
-> first irq :
bpf_bprintf_prepare(num_args == 0)
bpf_bprintf_cleanup decrements 'bpf_bprintf_nest_level = 0'
-> second irq:
bpf_bprintf_prepare(num_args != 0) bpf_bprintf_nest_level = 1
gets same buffer as task context above

Adding check to bpf_bprintf_cleanup and doing the real cleanup only if we
got bin_args data in the first place.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20221215214430.1336195-3-jolsa@kernel.org
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
---
include/linux/bpf.h | 2 +-
kernel/bpf/helpers.c | 16 +++++++++-------
kernel/trace/bpf_trace.c | 6 +++---
3 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 0e4fb16cd5c1..bc4e6969899a 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2747,7 +2747,7 @@ struct bpf_bprintf_data {

int bpf_bprintf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
u32 num_args, struct bpf_bprintf_data *data);
-void bpf_bprintf_cleanup(void);
+void bpf_bprintf_cleanup(struct bpf_bprintf_data *data);

/* the implementation of the opaque uapi struct bpf_dynptr */
struct bpf_dynptr_kern {
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 42141d59e9c6..64f41f58b37b 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -781,12 +781,14 @@ static int try_get_fmt_tmp_buf(char **tmp_buf)
return 0;
}

-void bpf_bprintf_cleanup(void)
+void bpf_bprintf_cleanup(struct bpf_bprintf_data *data)
{
- if (this_cpu_read(bpf_bprintf_nest_level)) {
- this_cpu_dec(bpf_bprintf_nest_level);
- preempt_enable();
- }
+ if (!data->bin_args)
+ return;
+ if (WARN_ON_ONCE(this_cpu_read(bpf_bprintf_nest_level) == 0))
+ return;
+ this_cpu_dec(bpf_bprintf_nest_level);
+ preempt_enable();
}

/*
@@ -1018,7 +1020,7 @@ int bpf_bprintf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
err = 0;
out:
if (err)
- bpf_bprintf_cleanup();
+ bpf_bprintf_cleanup(data);
return err;
}

@@ -1044,7 +1046,7 @@ BPF_CALL_5(bpf_snprintf, char *, str, u32, str_size, char *, fmt,

err = bstr_printf(str, str_size, fmt, data.bin_args);

- bpf_bprintf_cleanup();
+ bpf_bprintf_cleanup(&data);

return err + 1;
}
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index bbf44095999f..263a54d0d9ee 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -395,7 +395,7 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
trace_bpf_trace_printk(buf);
raw_spin_unlock_irqrestore(&trace_printk_lock, flags);

- bpf_bprintf_cleanup();
+ bpf_bprintf_cleanup(&data);

return ret;
}
@@ -453,7 +453,7 @@ BPF_CALL_4(bpf_trace_vprintk, char *, fmt, u32, fmt_size, const void *, args,
trace_bpf_trace_printk(buf);
raw_spin_unlock_irqrestore(&trace_printk_lock, flags);

- bpf_bprintf_cleanup();
+ bpf_bprintf_cleanup(&data);

return ret;
}
@@ -493,7 +493,7 @@ BPF_CALL_5(bpf_seq_printf, struct seq_file *, m, char *, fmt, u32, fmt_size,

seq_bprintf(m, fmt, data.bin_args);

- bpf_bprintf_cleanup();
+ bpf_bprintf_cleanup(&data);

return seq_has_overflowed(m) ? -EOVERFLOW : 0;
}
--
2.34.1