[PATCH nft] evaluate: fix double free on dtype release

[PATCH nft] evaluate: fix double free on dtype release

[Florian Westphal]

We release ->dtype twice, will either segfault or assert
on dtype->refcount != 0 check in datatype_free().

Signed-off-by: Florian Westphal <fw@strlen.de>
---
 src/evaluate.c                                              | 2 +-
 .../bogons/nft-f/double-free-on-binop-dtype_assert          | 6 ++++++
 2 files changed, 7 insertions(+), 1 deletion(-)
 create mode 100644 tests/shell/testcases/bogons/nft-f/double-free-on-binop-dtype_assert

diff --git a/src/evaluate.c b/src/evaluate.c
index 16ad6473db1a..58cc811aca9a 100644
--- a/src/evaluate.c
+++ b/src/evaluate.c
@@ -1171,7 +1171,7 @@ static int expr_evaluate_prefix(struct eval_ctx *ctx, struct expr **expr)
 	base = prefix->prefix;
 	assert(expr_is_constant(base));
 
-	prefix->dtype	  = base->dtype;
+	prefix->dtype	  = datatype_get(base->dtype);
 	prefix->byteorder = base->byteorder;
 	prefix->len	  = base->len;
 	prefix->flags	 |= EXPR_F_CONSTANT;
diff --git a/tests/shell/testcases/bogons/nft-f/double-free-on-binop-dtype_assert b/tests/shell/testcases/bogons/nft-f/double-free-on-binop-dtype_assert
new file mode 100644
index 000000000000..b7a9a1cc7e8b
--- /dev/null
+++ b/tests/shell/testcases/bogons/nft-f/double-free-on-binop-dtype_assert
@@ -0,0 +1,6 @@
+table inet t {
+	chain c {
+		udp length . @th,160,118 vmap { 47-63 . 0xe3731353631303331313037353532/3 : accept }
+		jump noexist # only here so this fails to load after patch.
+	}
+}
-- 
2.41.0

Re: [PATCH nft] evaluate: fix double free on dtype release

[Pablo Neira Ayuso]

On Tue, Dec 05, 2023 at 01:08:17PM +0100, Florian Westphal wrote:
> We release ->dtype twice, will either segfault or assert
> on dtype->refcount != 0 check in datatype_free().
> 
> Signed-off-by: Florian Westphal <fw@strlen.de>
> ---
>  src/evaluate.c                                              | 2 +-
>  .../bogons/nft-f/double-free-on-binop-dtype_assert          | 6 ++++++
>  2 files changed, 7 insertions(+), 1 deletion(-)
>  create mode 100644 tests/shell/testcases/bogons/nft-f/double-free-on-binop-dtype_assert
> 
> diff --git a/src/evaluate.c b/src/evaluate.c
> index 16ad6473db1a..58cc811aca9a 100644
> --- a/src/evaluate.c
> +++ b/src/evaluate.c
> @@ -1171,7 +1171,7 @@ static int expr_evaluate_prefix(struct eval_ctx *ctx, struct expr **expr)
>  	base = prefix->prefix;
>  	assert(expr_is_constant(base));
>  
> -	prefix->dtype	  = base->dtype;
> +	prefix->dtype	  = datatype_get(base->dtype);

I prefer datatype_clone() just in case base->dtype gets updated for
whatever reason.

>  	prefix->byteorder = base->byteorder;
>  	prefix->len	  = base->len;
>  	prefix->flags	 |= EXPR_F_CONSTANT;
> diff --git a/tests/shell/testcases/bogons/nft-f/double-free-on-binop-dtype_assert b/tests/shell/testcases/bogons/nft-f/double-free-on-binop-dtype_assert
> new file mode 100644
> index 000000000000..b7a9a1cc7e8b
> --- /dev/null
> +++ b/tests/shell/testcases/bogons/nft-f/double-free-on-binop-dtype_assert
> @@ -0,0 +1,6 @@
> +table inet t {
> +	chain c {
> +		udp length . @th,160,118 vmap { 47-63 . 0xe3731353631303331313037353532/3 : accept }
> +		jump noexist # only here so this fails to load after patch.
> +	}
> +}
> -- 
> 2.41.0
> 
> 

Re: [PATCH nft] evaluate: fix double free on dtype release

[Thomas Haller]

On Tue, 2023-12-05 at 15:19 +0100, Pablo Neira Ayuso wrote:
> On Tue, Dec 05, 2023 at 01:08:17PM +0100, Florian Westphal wrote:
> >  
> > -	prefix->dtype	  = base->dtype;
> > +	prefix->dtype	  = datatype_get(base->dtype);
> 
> I prefer datatype_clone() just in case base->dtype gets updated for
> whatever reason.

Hi,

That seems unnecessary.

`struct datatype` is a ref-counted, immutable data structure. That is a
great feature and callers should rely on it.

In "[PATCH nft 0/5] more various cleanups related to struct datatype"
all modifications move inside "datatype.c". This makes it clearer that
modifications happen during initialization only. Regardless, also on
`master` the instance is never mutated, after passing around the
pointer.


Thomas

Re: [PATCH nft] evaluate: fix double free on dtype release

[Pablo Neira Ayuso]

On Tue, Dec 05, 2023 at 05:53:06PM +0100, Thomas Haller wrote:
> On Tue, 2023-12-05 at 15:19 +0100, Pablo Neira Ayuso wrote:
> > On Tue, Dec 05, 2023 at 01:08:17PM +0100, Florian Westphal wrote:
> > >  
> > > -	prefix->dtype	  = base->dtype;
> > > +	prefix->dtype	  = datatype_get(base->dtype);
> > 
> > I prefer datatype_clone() just in case base->dtype gets updated for
> > whatever reason.
> 
> Hi,
> 
> That seems unnecessary.
> 
> `struct datatype` is a ref-counted, immutable data structure. That is a
> great feature and callers should rely on it.
> 
> In "[PATCH nft 0/5] more various cleanups related to struct datatype"
> all modifications move inside "datatype.c". This makes it clearer that
> modifications happen during initialization only. Regardless, also on
> `master` the instance is never mutated, after passing around the
> pointer.

datatype_get() is perfectly fine for this case as you point out.
No update of prefix->prefix datatype is done indeed.

Thanks.