Commit b36e62eb authored by Yonghong Song's avatar Yonghong Song Committed by Alexei Starovoitov

bpf: Use strncpy_from_unsafe_strict() in bpf_seq_printf() helper

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 b2a5212f ("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/

Fixes: 492e639f ("bpf: Add bpf_seq_printf and bpf_seq_write helpers")
Signed-off-by: default avatarYonghong Song <yhs@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarSong Liu <songliubraving@fb.com>
Cc: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/bpf/20200529004810.3352219-1-yhs@fb.comSigned-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent c3c16f2e
...@@ -585,7 +585,7 @@ BPF_CALL_5(bpf_seq_printf, struct seq_file *, m, char *, fmt, u32, fmt_size, ...@@ -585,7 +585,7 @@ BPF_CALL_5(bpf_seq_printf, struct seq_file *, m, char *, fmt, u32, fmt_size,
goto out; goto out;
} }
err = strncpy_from_unsafe(bufs->buf[memcpy_cnt], err = strncpy_from_unsafe_strict(bufs->buf[memcpy_cnt],
(void *) (long) args[fmt_cnt], (void *) (long) args[fmt_cnt],
MAX_SEQ_PRINTF_STR_LEN); MAX_SEQ_PRINTF_STR_LEN);
if (err < 0) if (err < 0)
......
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