diff mbox series

crypto: af_alg - avoid undefined behavior accessing salg_name

Message ID 20201026200715.170261-1-ebiggers@kernel.org
State Accepted
Delegated to: Herbert Xu
Headers show
Series crypto: af_alg - avoid undefined behavior accessing salg_name | expand

Commit Message

Eric Biggers Oct. 26, 2020, 8:07 p.m. UTC
From: Eric Biggers <ebiggers@google.com>

Commit 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm
names") made the kernel start accepting arbitrarily long algorithm names
in sockaddr_alg.  However, the actual length of the salg_name field
stayed at the original 64 bytes.

This is broken because the kernel can access indices >= 64 in salg_name,
which is undefined behavior -- even though the memory that is accessed
is still located within the sockaddr structure.  It would only be
defined behavior if the array were properly marked as arbitrary-length
(either by making it a flexible array, which is the recommended way
these days, or by making it an array of length 0 or 1).

We can't simply change salg_name into a flexible array, since that would
break source compatibility with userspace programs that embed
sockaddr_alg into another struct, or (more commonly) declare a
sockaddr_alg like 'struct sockaddr_alg sa = { .salg_name = "foo" };'.

One solution would be to change salg_name into a flexible array only
when '#ifdef __KERNEL__'.  However, that would keep userspace without an
easy way to actually use the longer algorithm names.

Instead, add a new structure 'sockaddr_alg_new' that has the flexible
array field, and expose it to both userspace and the kernel.
Make the kernel use it correctly in alg_bind().

This addresses the syzbot report
"UBSAN: array-index-out-of-bounds in alg_bind"
(https://syzkaller.appspot.com/bug?extid=92ead4eb8e26a26d465e).

Reported-by: syzbot+92ead4eb8e26a26d465e@syzkaller.appspotmail.com
Fixes: 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm names")
Cc: <stable@vger.kernel.org> # v4.12+
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 crypto/af_alg.c             | 10 +++++++---
 include/uapi/linux/if_alg.h | 16 ++++++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)


base-commit: 3650b228f83adda7e5ee532e2b90429c03f7b9ec

Comments

Gustavo A. R. Silva Oct. 26, 2020, 9:21 p.m. UTC | #1
Hi,

On Mon, Oct 26, 2020 at 01:07:15PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> Commit 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm
> names") made the kernel start accepting arbitrarily long algorithm names
> in sockaddr_alg.  However, the actual length of the salg_name field
> stayed at the original 64 bytes.
> 
> This is broken because the kernel can access indices >= 64 in salg_name,
> which is undefined behavior -- even though the memory that is accessed
> is still located within the sockaddr structure.  It would only be
> defined behavior if the array were properly marked as arbitrary-length
> (either by making it a flexible array, which is the recommended way
> these days, or by making it an array of length 0 or 1).
> 
> We can't simply change salg_name into a flexible array, since that would
> break source compatibility with userspace programs that embed
> sockaddr_alg into another struct, or (more commonly) declare a
> sockaddr_alg like 'struct sockaddr_alg sa = { .salg_name = "foo" };'.
> 
> One solution would be to change salg_name into a flexible array only
> when '#ifdef __KERNEL__'.  However, that would keep userspace without an
> easy way to actually use the longer algorithm names.
> 
> Instead, add a new structure 'sockaddr_alg_new' that has the flexible
> array field, and expose it to both userspace and the kernel.
> Make the kernel use it correctly in alg_bind().
> 
> This addresses the syzbot report
> "UBSAN: array-index-out-of-bounds in alg_bind"
> (https://syzkaller.appspot.com/bug?extid=92ead4eb8e26a26d465e).
> 
> Reported-by: syzbot+92ead4eb8e26a26d465e@syzkaller.appspotmail.com
> Fixes: 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm names")
> Cc: <stable@vger.kernel.org> # v4.12+
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  crypto/af_alg.c             | 10 +++++++---
>  include/uapi/linux/if_alg.h | 16 ++++++++++++++++
>  2 files changed, 23 insertions(+), 3 deletions(-)
> 
> diff --git a/crypto/af_alg.c b/crypto/af_alg.c
> index d11db80d24cd1..9acb9d2c4bcf9 100644
> --- a/crypto/af_alg.c
> +++ b/crypto/af_alg.c
> @@ -147,7 +147,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
>  	const u32 allowed = CRYPTO_ALG_KERN_DRIVER_ONLY;
>  	struct sock *sk = sock->sk;
>  	struct alg_sock *ask = alg_sk(sk);
> -	struct sockaddr_alg *sa = (void *)uaddr;
> +	struct sockaddr_alg_new *sa = (void *)uaddr;
>  	const struct af_alg_type *type;
>  	void *private;
>  	int err;
> @@ -155,7 +155,11 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
>  	if (sock->state == SS_CONNECTED)
>  		return -EINVAL;
>  
> -	if (addr_len < sizeof(*sa))
> +	BUILD_BUG_ON(offsetof(struct sockaddr_alg_new, salg_name) !=
> +		     offsetof(struct sockaddr_alg, salg_name));
> +	BUILD_BUG_ON(offsetof(struct sockaddr_alg, salg_name) != sizeof(*sa));
> +
> +	if (addr_len < sizeof(*sa) + 1)
>  		return -EINVAL;
>  
>  	/* If caller uses non-allowed flag, return error. */
> @@ -163,7 +167,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
>  		return -EINVAL;
>  
>  	sa->salg_type[sizeof(sa->salg_type) - 1] = 0;
> -	sa->salg_name[sizeof(sa->salg_name) + addr_len - sizeof(*sa) - 1] = 0;
> +	sa->salg_name[addr_len - sizeof(*sa) - 1] = 0;
>  
>  	type = alg_get_type(sa->salg_type);
>  	if (PTR_ERR(type) == -ENOENT) {
> diff --git a/include/uapi/linux/if_alg.h b/include/uapi/linux/if_alg.h
> index 60b7c2efd921c..dc52a11ba6d15 100644
> --- a/include/uapi/linux/if_alg.h
> +++ b/include/uapi/linux/if_alg.h
> @@ -24,6 +24,22 @@ struct sockaddr_alg {
>  	__u8	salg_name[64];
>  };
>  
> +/*
> + * Linux v4.12 and later removed the 64-byte limit on salg_name[]; it's now an
> + * arbitrary-length field.  We had to keep the original struct above for source
> + * compatibility with existing userspace programs, though.  Use the new struct
> + * below if support for very long algorithm names is needed.  To do this,
> + * allocate 'sizeof(struct sockaddr_alg_new) + strlen(algname) + 1' bytes, and
> + * copy algname (including the null terminator) into salg_name.
> + */
> +struct sockaddr_alg_new {
> +	__u16	salg_family;
> +	__u8	salg_type[14];
> +	__u32	salg_feat;
> +	__u32	salg_mask;
> +	__u8	salg_name[];
> +};
> +

How something like this, instead:

 struct sockaddr_alg {
-	__u16	salg_family;
-	__u8	salg_type[14];
-	__u32	salg_feat;
-	__u32	salg_mask;
-	__u8	salg_name[64];
+	union {
+		struct {
+			__u16	salg_v1_family;
+			__u8	salg_v1_type[14];
+			__u32	salg_v1_feat;
+			__u32	salg_v1_mask;
+			__u8	salg_name[64];
+		};
+		struct {
+			__u16	salg_family;
+			__u8	salg_type[14];
+			__u32	salg_feat;
+			__u32	salg_mask;
+			__u8	salg_name_new[];
+		};
+	};
 };


--
Gustavo
Jann Horn Oct. 26, 2020, 9:23 p.m. UTC | #2
On Mon, Oct 26, 2020 at 9:08 PM Eric Biggers <ebiggers@kernel.org> wrote:
> Commit 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm
> names") made the kernel start accepting arbitrarily long algorithm names
> in sockaddr_alg.

That's not true; it's still limited by the size of struct
sockaddr_storage (128 bytes total for the entire address). If you make
it longer, __copy_msghdr_from_user() will silently truncate the size.

> This is broken because the kernel can access indices >= 64 in salg_name,
> which is undefined behavior -- even though the memory that is accessed
> is still located within the sockaddr structure.  It would only be
> defined behavior if the array were properly marked as arbitrary-length
> (either by making it a flexible array, which is the recommended way
> these days, or by making it an array of length 0 or 1).
>
> We can't simply change salg_name into a flexible array, since that would
> break source compatibility with userspace programs that embed
> sockaddr_alg into another struct, or (more commonly) declare a
> sockaddr_alg like 'struct sockaddr_alg sa = { .salg_name = "foo" };'.
>
> One solution would be to change salg_name into a flexible array only
> when '#ifdef __KERNEL__'.  However, that would keep userspace without an
> easy way to actually use the longer algorithm names.
>
> Instead, add a new structure 'sockaddr_alg_new' that has the flexible
> array field, and expose it to both userspace and the kernel.
> Make the kernel use it correctly in alg_bind().
[...]
> @@ -147,7 +147,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
>         const u32 allowed = CRYPTO_ALG_KERN_DRIVER_ONLY;
>         struct sock *sk = sock->sk;
>         struct alg_sock *ask = alg_sk(sk);
> -       struct sockaddr_alg *sa = (void *)uaddr;
> +       struct sockaddr_alg_new *sa = (void *)uaddr;
>         const struct af_alg_type *type;
>         void *private;
>         int err;
> @@ -155,7 +155,11 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
>         if (sock->state == SS_CONNECTED)
>                 return -EINVAL;
>
> -       if (addr_len < sizeof(*sa))
> +       BUILD_BUG_ON(offsetof(struct sockaddr_alg_new, salg_name) !=
> +                    offsetof(struct sockaddr_alg, salg_name));
> +       BUILD_BUG_ON(offsetof(struct sockaddr_alg, salg_name) != sizeof(*sa));
> +
> +       if (addr_len < sizeof(*sa) + 1)
>                 return -EINVAL;
>
>         /* If caller uses non-allowed flag, return error. */
> @@ -163,7 +167,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
>                 return -EINVAL;
>
>         sa->salg_type[sizeof(sa->salg_type) - 1] = 0;
> -       sa->salg_name[sizeof(sa->salg_name) + addr_len - sizeof(*sa) - 1] = 0;
> +       sa->salg_name[addr_len - sizeof(*sa) - 1] = 0;

This looks like an out-of-bounds write in the case `addr_len ==
sizeof(struct sockaddr_storage)`.
Eric Biggers Oct. 26, 2020, 9:56 p.m. UTC | #3
On Mon, Oct 26, 2020 at 10:23:35PM +0100, 'Jann Horn' via syzkaller-bugs wrote:
> On Mon, Oct 26, 2020 at 9:08 PM Eric Biggers <ebiggers@kernel.org> wrote:
> > Commit 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm
> > names") made the kernel start accepting arbitrarily long algorithm names
> > in sockaddr_alg.
> 
> That's not true; it's still limited by the size of struct
> sockaddr_storage (128 bytes total for the entire address).

Interesting, so the actual limit is 104 bytes.  It seems like the intent of that
commit was to make it unlimited, though...

> If you make it longer, __copy_msghdr_from_user() will silently truncate the
> size.

That's used for sys_sendmsg(), which AFAICT isn't relevant here.  sockaddr_alg
is used with sys_bind(), which fails with EINVAL if the address is longer than
sizeof(struct sockaddr_storage).

However, since sys_sendmsg() is truncating overly-long addresses, it's probably
the case that sizeof(struct sockaddr_storage) can never be increased in the
future...

> 
> > This is broken because the kernel can access indices >= 64 in salg_name,
> > which is undefined behavior -- even though the memory that is accessed
> > is still located within the sockaddr structure.  It would only be
> > defined behavior if the array were properly marked as arbitrary-length
> > (either by making it a flexible array, which is the recommended way
> > these days, or by making it an array of length 0 or 1).
> >
> > We can't simply change salg_name into a flexible array, since that would
> > break source compatibility with userspace programs that embed
> > sockaddr_alg into another struct, or (more commonly) declare a
> > sockaddr_alg like 'struct sockaddr_alg sa = { .salg_name = "foo" };'.
> >
> > One solution would be to change salg_name into a flexible array only
> > when '#ifdef __KERNEL__'.  However, that would keep userspace without an
> > easy way to actually use the longer algorithm names.
> >
> > Instead, add a new structure 'sockaddr_alg_new' that has the flexible
> > array field, and expose it to both userspace and the kernel.
> > Make the kernel use it correctly in alg_bind().
> [...]
> > @@ -147,7 +147,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
> >         const u32 allowed = CRYPTO_ALG_KERN_DRIVER_ONLY;
> >         struct sock *sk = sock->sk;
> >         struct alg_sock *ask = alg_sk(sk);
> > -       struct sockaddr_alg *sa = (void *)uaddr;
> > +       struct sockaddr_alg_new *sa = (void *)uaddr;
> >         const struct af_alg_type *type;
> >         void *private;
> >         int err;
> > @@ -155,7 +155,11 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
> >         if (sock->state == SS_CONNECTED)
> >                 return -EINVAL;
> >
> > -       if (addr_len < sizeof(*sa))
> > +       BUILD_BUG_ON(offsetof(struct sockaddr_alg_new, salg_name) !=
> > +                    offsetof(struct sockaddr_alg, salg_name));
> > +       BUILD_BUG_ON(offsetof(struct sockaddr_alg, salg_name) != sizeof(*sa));
> > +
> > +       if (addr_len < sizeof(*sa) + 1)
> >                 return -EINVAL;
> >
> >         /* If caller uses non-allowed flag, return error. */
> > @@ -163,7 +167,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
> >                 return -EINVAL;
> >
> >         sa->salg_type[sizeof(sa->salg_type) - 1] = 0;
> > -       sa->salg_name[sizeof(sa->salg_name) + addr_len - sizeof(*sa) - 1] = 0;
> > +       sa->salg_name[addr_len - sizeof(*sa) - 1] = 0;
> 
> This looks like an out-of-bounds write in the case `addr_len ==
> sizeof(struct sockaddr_storage)`.

I think you mean addr_len == sizeof(*sa)?  That's what the
'if (addr_len < sizeof(*sa) + 1) return -EINVAL' above is for.

- Eric
Jann Horn Oct. 26, 2020, 10:40 p.m. UTC | #4
On Mon, Oct 26, 2020 at 10:57 PM Eric Biggers <ebiggers@kernel.org> wrote:
> On Mon, Oct 26, 2020 at 10:23:35PM +0100, 'Jann Horn' via syzkaller-bugs wrote:
> > On Mon, Oct 26, 2020 at 9:08 PM Eric Biggers <ebiggers@kernel.org> wrote:
> > > Commit 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm
> > > names") made the kernel start accepting arbitrarily long algorithm names
> > > in sockaddr_alg.
> >
> > That's not true; it's still limited by the size of struct
> > sockaddr_storage (128 bytes total for the entire address).
>
> Interesting, so the actual limit is 104 bytes.  It seems like the intent of that
> commit was to make it unlimited, though...
>
> > If you make it longer, __copy_msghdr_from_user() will silently truncate the
> > size.
>
> That's used for sys_sendmsg(), which AFAICT isn't relevant here.  sockaddr_alg
> is used with sys_bind(), which fails with EINVAL if the address is longer than
> sizeof(struct sockaddr_storage).

Ugh, of course you're right, sorry.

> However, since sys_sendmsg() is truncating overly-long addresses, it's probably
> the case that sizeof(struct sockaddr_storage) can never be increased in the
> future...

Eh, I think there'd probably be bigger issues with that elsewhere.

> > > This is broken because the kernel can access indices >= 64 in salg_name,
> > > which is undefined behavior -- even though the memory that is accessed
> > > is still located within the sockaddr structure.  It would only be
> > > defined behavior if the array were properly marked as arbitrary-length
> > > (either by making it a flexible array, which is the recommended way
> > > these days, or by making it an array of length 0 or 1).
> > >
> > > We can't simply change salg_name into a flexible array, since that would
> > > break source compatibility with userspace programs that embed
> > > sockaddr_alg into another struct, or (more commonly) declare a
> > > sockaddr_alg like 'struct sockaddr_alg sa = { .salg_name = "foo" };'.
> > >
> > > One solution would be to change salg_name into a flexible array only
> > > when '#ifdef __KERNEL__'.  However, that would keep userspace without an
> > > easy way to actually use the longer algorithm names.
> > >
> > > Instead, add a new structure 'sockaddr_alg_new' that has the flexible
> > > array field, and expose it to both userspace and the kernel.
> > > Make the kernel use it correctly in alg_bind().
> > [...]
> > > @@ -147,7 +147,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
> > >         const u32 allowed = CRYPTO_ALG_KERN_DRIVER_ONLY;
> > >         struct sock *sk = sock->sk;
> > >         struct alg_sock *ask = alg_sk(sk);
> > > -       struct sockaddr_alg *sa = (void *)uaddr;
> > > +       struct sockaddr_alg_new *sa = (void *)uaddr;
> > >         const struct af_alg_type *type;
> > >         void *private;
> > >         int err;
> > > @@ -155,7 +155,11 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
> > >         if (sock->state == SS_CONNECTED)
> > >                 return -EINVAL;
> > >
> > > -       if (addr_len < sizeof(*sa))
> > > +       BUILD_BUG_ON(offsetof(struct sockaddr_alg_new, salg_name) !=
> > > +                    offsetof(struct sockaddr_alg, salg_name));
> > > +       BUILD_BUG_ON(offsetof(struct sockaddr_alg, salg_name) != sizeof(*sa));
> > > +
> > > +       if (addr_len < sizeof(*sa) + 1)
> > >                 return -EINVAL;
> > >
> > >         /* If caller uses non-allowed flag, return error. */
> > > @@ -163,7 +167,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
> > >                 return -EINVAL;
> > >
> > >         sa->salg_type[sizeof(sa->salg_type) - 1] = 0;
> > > -       sa->salg_name[sizeof(sa->salg_name) + addr_len - sizeof(*sa) - 1] = 0;
> > > +       sa->salg_name[addr_len - sizeof(*sa) - 1] = 0;
> >
> > This looks like an out-of-bounds write in the case `addr_len ==
> > sizeof(struct sockaddr_storage)`.

Sorry, I've been unusually unconcentrated today. Sorry about the
noise, ignore what I said.
Gustavo A. R. Silva Oct. 26, 2020, 11:10 p.m. UTC | #5
On Mon, Oct 26, 2020 at 04:21:48PM -0500, Gustavo A. R. Silva wrote:
> > +/*
> > + * Linux v4.12 and later removed the 64-byte limit on salg_name[]; it's now an
> > + * arbitrary-length field.  We had to keep the original struct above for source
> > + * compatibility with existing userspace programs, though.  Use the new struct
> > + * below if support for very long algorithm names is needed.  To do this,
> > + * allocate 'sizeof(struct sockaddr_alg_new) + strlen(algname) + 1' bytes, and
> > + * copy algname (including the null terminator) into salg_name.
> > + */
> > +struct sockaddr_alg_new {
> > +	__u16	salg_family;
> > +	__u8	salg_type[14];
> > +	__u32	salg_feat;
> > +	__u32	salg_mask;
> > +	__u8	salg_name[];
> > +};
> > +
> 
> How something like this, instead:
> 
>  struct sockaddr_alg {
> -	__u16	salg_family;
> -	__u8	salg_type[14];
> -	__u32	salg_feat;
> -	__u32	salg_mask;
> -	__u8	salg_name[64];
> +	union {
> +		struct {
> +			__u16	salg_v1_family;
> +			__u8	salg_v1_type[14];
> +			__u32	salg_v1_feat;
> +			__u32	salg_v1_mask;
> +			__u8	salg_name[64];
> +		};
> +		struct {
> +			__u16	salg_family;
> +			__u8	salg_type[14];
> +			__u32	salg_feat;
> +			__u32	salg_mask;
> +			__u8	salg_name_new[];
> +		};
> +	};
>  };
> 

Something similar to the following approach might work:

https://git.kernel.org/pub/scm/linux/kernel/git/gustavoars/linux.git/commit/?h=testing/uapi/gntalloc&id=db46c8aba41c436edb0b4ef2941bd7390b0e5d61

--
Gustavo
Eric Biggers Oct. 26, 2020, 11:40 p.m. UTC | #6
On Mon, Oct 26, 2020 at 06:10:59PM -0500, Gustavo A. R. Silva wrote:
> On Mon, Oct 26, 2020 at 04:21:48PM -0500, Gustavo A. R. Silva wrote:
> > > +/*
> > > + * Linux v4.12 and later removed the 64-byte limit on salg_name[]; it's now an
> > > + * arbitrary-length field.  We had to keep the original struct above for source
> > > + * compatibility with existing userspace programs, though.  Use the new struct
> > > + * below if support for very long algorithm names is needed.  To do this,
> > > + * allocate 'sizeof(struct sockaddr_alg_new) + strlen(algname) + 1' bytes, and
> > > + * copy algname (including the null terminator) into salg_name.
> > > + */
> > > +struct sockaddr_alg_new {
> > > +	__u16	salg_family;
> > > +	__u8	salg_type[14];
> > > +	__u32	salg_feat;
> > > +	__u32	salg_mask;
> > > +	__u8	salg_name[];
> > > +};
> > > +
> > 
> > How something like this, instead:
> > 
> >  struct sockaddr_alg {
> > -	__u16	salg_family;
> > -	__u8	salg_type[14];
> > -	__u32	salg_feat;
> > -	__u32	salg_mask;
> > -	__u8	salg_name[64];
> > +	union {
> > +		struct {
> > +			__u16	salg_v1_family;
> > +			__u8	salg_v1_type[14];
> > +			__u32	salg_v1_feat;
> > +			__u32	salg_v1_mask;
> > +			__u8	salg_name[64];
> > +		};
> > +		struct {
> > +			__u16	salg_family;
> > +			__u8	salg_type[14];
> > +			__u32	salg_feat;
> > +			__u32	salg_mask;
> > +			__u8	salg_name_new[];
> > +		};
> > +	};
> >  };
> > 
> 
> Something similar to the following approach might work:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/gustavoars/linux.git/commit/?h=testing/uapi/gntalloc&id=db46c8aba41c436edb0b4ef2941bd7390b0e5d61
> 

I suppose so.  It's very confusing to see a union like that at first glance,
though.  It definitely needs an explanatory comment...

- Eric
Herbert Xu Nov. 6, 2020, 7:01 a.m. UTC | #7
On Mon, Oct 26, 2020 at 01:07:15PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> Commit 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm
> names") made the kernel start accepting arbitrarily long algorithm names
> in sockaddr_alg.  However, the actual length of the salg_name field
> stayed at the original 64 bytes.
> 
> This is broken because the kernel can access indices >= 64 in salg_name,
> which is undefined behavior -- even though the memory that is accessed
> is still located within the sockaddr structure.  It would only be
> defined behavior if the array were properly marked as arbitrary-length
> (either by making it a flexible array, which is the recommended way
> these days, or by making it an array of length 0 or 1).
> 
> We can't simply change salg_name into a flexible array, since that would
> break source compatibility with userspace programs that embed
> sockaddr_alg into another struct, or (more commonly) declare a
> sockaddr_alg like 'struct sockaddr_alg sa = { .salg_name = "foo" };'.
> 
> One solution would be to change salg_name into a flexible array only
> when '#ifdef __KERNEL__'.  However, that would keep userspace without an
> easy way to actually use the longer algorithm names.
> 
> Instead, add a new structure 'sockaddr_alg_new' that has the flexible
> array field, and expose it to both userspace and the kernel.
> Make the kernel use it correctly in alg_bind().
> 
> This addresses the syzbot report
> "UBSAN: array-index-out-of-bounds in alg_bind"
> (https://syzkaller.appspot.com/bug?extid=92ead4eb8e26a26d465e).
> 
> Reported-by: syzbot+92ead4eb8e26a26d465e@syzkaller.appspotmail.com
> Fixes: 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm names")
> Cc: <stable@vger.kernel.org> # v4.12+
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  crypto/af_alg.c             | 10 +++++++---
>  include/uapi/linux/if_alg.h | 16 ++++++++++++++++
>  2 files changed, 23 insertions(+), 3 deletions(-)

Patch applied.  Thanks.
diff mbox series

Patch

diff --git a/crypto/af_alg.c b/crypto/af_alg.c
index d11db80d24cd1..9acb9d2c4bcf9 100644
--- a/crypto/af_alg.c
+++ b/crypto/af_alg.c
@@ -147,7 +147,7 @@  static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 	const u32 allowed = CRYPTO_ALG_KERN_DRIVER_ONLY;
 	struct sock *sk = sock->sk;
 	struct alg_sock *ask = alg_sk(sk);
-	struct sockaddr_alg *sa = (void *)uaddr;
+	struct sockaddr_alg_new *sa = (void *)uaddr;
 	const struct af_alg_type *type;
 	void *private;
 	int err;
@@ -155,7 +155,11 @@  static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 	if (sock->state == SS_CONNECTED)
 		return -EINVAL;
 
-	if (addr_len < sizeof(*sa))
+	BUILD_BUG_ON(offsetof(struct sockaddr_alg_new, salg_name) !=
+		     offsetof(struct sockaddr_alg, salg_name));
+	BUILD_BUG_ON(offsetof(struct sockaddr_alg, salg_name) != sizeof(*sa));
+
+	if (addr_len < sizeof(*sa) + 1)
 		return -EINVAL;
 
 	/* If caller uses non-allowed flag, return error. */
@@ -163,7 +167,7 @@  static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 		return -EINVAL;
 
 	sa->salg_type[sizeof(sa->salg_type) - 1] = 0;
-	sa->salg_name[sizeof(sa->salg_name) + addr_len - sizeof(*sa) - 1] = 0;
+	sa->salg_name[addr_len - sizeof(*sa) - 1] = 0;
 
 	type = alg_get_type(sa->salg_type);
 	if (PTR_ERR(type) == -ENOENT) {
diff --git a/include/uapi/linux/if_alg.h b/include/uapi/linux/if_alg.h
index 60b7c2efd921c..dc52a11ba6d15 100644
--- a/include/uapi/linux/if_alg.h
+++ b/include/uapi/linux/if_alg.h
@@ -24,6 +24,22 @@  struct sockaddr_alg {
 	__u8	salg_name[64];
 };
 
+/*
+ * Linux v4.12 and later removed the 64-byte limit on salg_name[]; it's now an
+ * arbitrary-length field.  We had to keep the original struct above for source
+ * compatibility with existing userspace programs, though.  Use the new struct
+ * below if support for very long algorithm names is needed.  To do this,
+ * allocate 'sizeof(struct sockaddr_alg_new) + strlen(algname) + 1' bytes, and
+ * copy algname (including the null terminator) into salg_name.
+ */
+struct sockaddr_alg_new {
+	__u16	salg_family;
+	__u8	salg_type[14];
+	__u32	salg_feat;
+	__u32	salg_mask;
+	__u8	salg_name[];
+};
+
 struct af_alg_iv {
 	__u32	ivlen;
 	__u8	iv[0];