linux-hardening - Re: [PATCH] bpf: replace deprecated strncpy with strscpy

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Date: Wed, 3 Apr 2024 16:55:00 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: Ratheesh Kannoth <rkannoth@...vell.com>,
 Justin Stitt <justinstitt@...gle.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
 John Fastabend <john.fastabend@...il.com>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
 KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>,
 Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
 Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling
 <morbo@...gle.com>, bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
 llvm@...ts.linux.dev, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] bpf: replace deprecated strncpy with strscpy

On 4/3/24 5:06 AM, Ratheesh Kannoth wrote:
> On 2024-04-03 at 05:22:50, Justin Stitt (justinstitt@...gle.com) wrote:
>> strncpy() is deprecated for use on NUL-terminated destination strings
>> [1] and as such we should prefer more robust and less ambiguous string
>> interfaces.
>>
>> bpf sym names get looked up and compared/cleaned with various string
>> apis. This suggests they need to be NUL-terminated (strncpy() suggests
>> this but does not guarantee it).
>>
>> |	static int compare_symbol_name(const char *name, char *namebuf)
>> |	{
>> |		cleanup_symbol_name(namebuf);
>> |		return strcmp(name, namebuf);
>> |	}
>>
>> |	static void cleanup_symbol_name(char *s)
>> |	{
>> |		...
>> |		res = strstr(s, ".llvm.");
>> |		...
>> |	}
>>
>> Use strscpy() as this method guarantees NUL-termination on the
>> destination buffer.
>>
>> This patch also replaces two uses of strncpy() used in log.c. These are
>> simple replacements as postfix has been zero-initialized on the stack
>> and has source arguments with a size less than the destination's size.
>>
>> Note that this patch uses the new 2-argument version of strscpy
>> introduced in Commit e6584c3964f2f ("string: Allow 2-argument
>> strscpy()").
>>
>> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
>> Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
>> Link: https://github.com/KSPP/linux/issues/90
>> Cc: linux-hardening@...r.kernel.org
>> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
>> ---
>> Note: build-tested only.
>>
>> Found with: $ rg "strncpy\("
>> ---
>>   kernel/bpf/core.c | 4 ++--
>>   kernel/bpf/log.c  | 4 ++--
>>   2 files changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
>> index 696bc55de8e8..8c9078f4549c 100644
>> --- a/kernel/bpf/core.c
>> +++ b/kernel/bpf/core.c
>> @@ -747,7 +747,7 @@ const char *__bpf_address_lookup(unsigned long addr, unsigned long *size,
>>   		unsigned long symbol_start = ksym->start;
>>   		unsigned long symbol_end = ksym->end;
>>
>> -		strncpy(sym, ksym->name, KSYM_NAME_LEN);
>> +		strscpy(sym, ksym->name, KSYM_NAME_LEN);
> You dont have to check return value of strscpy for errors ?

That would be overkill, it can be easily audited that both pointers are
of size KSYM_NAME_LEN, as is the count arg.

Thanks,
Daniel