2024-01-23 23:47:50

by Kees Cook

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 <[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;
}

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

2024-01-25 12:19:42

by Ard Biesheuvel

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

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?

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

2024-01-25 18:00:29

by Kees Cook

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

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

2024-01-25 18:32:11

by Ard Biesheuvel

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

On Thu, 25 Jan 2024 at 19:00, Kees Cook <[email protected]> wrote:
>
> 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.
>

Ah yes, this kmalloc() was carefully hidden in the commit log :-)