linux-hardening - Re: [PATCH] smb: Work around Clang _

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Message-ID: <CAMj1kXGcKBPnHOm6PtsrxePdv5a6AokB=qvMrwvGmPh_Uk6vsA@mail.gmail.com>
Date: Thu, 25 Jan 2024 13:19:19 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Kees Cook <keescook@...omium.org>
Cc: Steve French <sfrench@...ba.org>, Nathan Chancellor <nathan@...nel.org>, 
	Paulo Alcantara <pc@...guebit.com>, Ronnie Sahlberg <ronniesahlberg@...il.com>, 
	Shyam Prasad N <sprasad@...rosoft.com>, Tom Talpey <tom@...pey.com>, linux-cifs@...r.kernel.org, 
	llvm@...ts.linux.dev, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] smb: Work around Clang __bdos() type confusion

On Wed, 24 Jan 2024 at 00:47, Kees Cook <keescook@...omium.org> wrote:
>
> Recent versions of Clang gets confused about the possible size of the
> "user" allocation, and CONFIG_FORTIFY_SOURCE ends up emitting a
> warning[1]:
>
> repro.c:126:4: warning: call to '__write_overflow_field' declared with 'warning' attribute: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
>   126 |                         __write_overflow_field(p_size_field, size);
>       |                         ^
>
> for this memset():
>
>         int len;
>         __le16 *user;
>         ...
>         len = ses->user_name ? strlen(ses->user_name) : 0;
>         user = kmalloc(2 + (len * 2), GFP_KERNEL);
>         ...
>         if (len) {
>                 ...
>         } else {
>                 memset(user, '\0', 2);
>         }
>
> While Clang works on this bug[2], switch to using a direct assignment,
> which avoids memset() entirely which both simplifies the code and silences
> the false positive warning. (Making "len" size_t also silences the
> warning, but the direct assignment seems better.)
>
> Reported-by: Nathan Chancellor <nathan@...nel.org>
> Closes: https://github.com/ClangBuiltLinux/linux/issues/1966 [1]
> Link: https://github.com/llvm/llvm-project/issues/77813 [2]
> Cc: Steve French <sfrench@...ba.org>
> Cc: Paulo Alcantara <pc@...guebit.com>
> Cc: Ronnie Sahlberg <ronniesahlberg@...il.com>
> Cc: Shyam Prasad N <sprasad@...rosoft.com>
> Cc: Tom Talpey <tom@...pey.com>
> Cc: linux-cifs@...r.kernel.org
> Cc: llvm@...ts.linux.dev
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  fs/smb/client/cifsencrypt.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/smb/client/cifsencrypt.c b/fs/smb/client/cifsencrypt.c
> index ef4c2e3c9fa6..6322f0f68a17 100644
> --- a/fs/smb/client/cifsencrypt.c
> +++ b/fs/smb/client/cifsencrypt.c
> @@ -572,7 +572,7 @@ static int calc_ntlmv2_hash(struct cifs_ses *ses, char *ntlmv2_hash,
>                 len = cifs_strtoUTF16(user, ses->user_name, len, nls_cp);
>                 UniStrupr(user);
>         } else {
> -               memset(user, '\0', 2);
> +               *(u16 *)user = 0;

Is 'user' guaranteed to be 16-bit aligned?

>         }
>
>         rc = crypto_shash_update(ses->server->secmech.hmacmd5,
> --
> 2.34.1
>
>