[PATCH bpf-next] bpf: use strncpy_from_unsafe_strict() in bpf_seq_printf() helper

[Yonghong Song <yhs@fb.com>]

In bpf_seq_printf() helper, when user specified a "%s" in the
format string, strncpy_from_unsafe() is used to read the actual string
to a buffer. The string could be a format string or a string in
the kernel data structure. It is really unlikely that the string
will reside in the user memory.

This is different from Commit b2a5212fb634 ("bpf: Restrict bpf_trace_printk()'s %s
usage and add %pks, %pus specifier") which still used
strncpy_from_unsafe() for "%s" to preserve the old behavior.

If in the future, bpf_seq_printf() indeed needs to read user
memory, we can implement "%pus" format string.

Based on discussion in [1], if the intent is to read kernel memory,
strncpy_from_unsafe_strict() should be used. So this patch
changed to use strncpy_from_unsafe_strict().

[1]: https://lore.kernel.org/bpf/20200521152301.2587579-1-hch@lst.de/T/

Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Yonghong Song <yhs@fb.com>
---
 kernel/trace/bpf_trace.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 187cd6995bbb..3a4afbc7f0bc 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -585,9 +585,9 @@ BPF_CALL_5(bpf_seq_printf, struct seq_file *, m, char *, fmt, u32, fmt_size,
 				goto out;
 			}
 
-			err = strncpy_from_unsafe(bufs->buf[memcpy_cnt],
-						  (void *) (long) args[fmt_cnt],
-						  MAX_SEQ_PRINTF_STR_LEN);
+			err = strncpy_from_unsafe_strict(bufs->buf[memcpy_cnt],
+							 (void *) (long) args[fmt_cnt],
+							 MAX_SEQ_PRINTF_STR_LEN);
 			if (err < 0)
 				bufs->buf[memcpy_cnt][0] = '\0';
 			params[fmt_cnt] = (u64)(long)bufs->buf[memcpy_cnt];
-- 
2.24.1