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

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Message-Id: <20240123234731.work.358-kees@kernel.org>
Date: Tue, 23 Jan 2024 15:47:34 -0800
From: Kees Cook <keescook@...omium.org>
To: Steve French <sfrench@...ba.org>
Cc: Kees Cook <keescook@...omium.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: [PATCH] smb: Work around Clang __bdos() type confusion

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