Re: [PATCH] smb: Work around Clang __bdos() type confusion

[Kees Cook <keescook@chromium.org>]

On Thu, Jan 25, 2024 at 01:19:19PM +0100, Ard Biesheuvel wrote:
> On Wed, 24 Jan 2024 at 00:47, Kees Cook <keescook@chromium.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@kernel.org>
> > Closes: https://github.com/ClangBuiltLinux/linux/issues/1966 [1]
> > Link: https://github.com/llvm/llvm-project/issues/77813 [2]
> > Cc: Steve French <sfrench@samba.org>
> > Cc: Paulo Alcantara <pc@manguebit.com>
> > Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
> > Cc: Shyam Prasad N <sprasad@microsoft.com>
> > Cc: Tom Talpey <tom@talpey.com>
> > Cc: linux-cifs@vger.kernel.org
> > Cc: llvm@lists.linux.dev
> > Signed-off-by: Kees Cook <keescook@chromium.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?

It's the first two bytes of a kmalloced address range, which I'm nearly
certain will be sanely aligned, as those allocs are commonly used for
holding structs, etc.

-Kees

-- 
Kees Cook