On Thu, Jan 25, 2024 at 01:19:19PM +0100, Ard Biesheuvel wrote: > On Wed, 24 Jan 2024 at 00:47, Kees Cook <[email protected]> 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 <[email protected]> > > Closes: https://github.com/ClangBuiltLinux/linux/issues/1966 [1] > > Link: https://github.com/llvm/llvm-project/issues/77813 [2] > > Cc: Steve French <[email protected]> > > Cc: Paulo Alcantara <[email protected]> > > Cc: Ronnie Sahlberg <[email protected]> > > Cc: Shyam Prasad N <[email protected]> > > Cc: Tom Talpey <[email protected]> > > Cc: [email protected] > > Cc: [email protected] > > Signed-off-by: Kees Cook <[email protected]> > > --- > > 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
