Re: [PATCH v2 1/5] crypto: dh - Fix double free of ctx->p

[Tudor Ambarus <tudor.ambarus@xxxxxxxxxxxxx>]

On 11/06/2017 04:30 AM, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
>
> When setting the secret with the software Diffie-Hellman implementation,
> if allocating 'g' failed (e.g. if it was longer than
> MAX_EXTERN_MPI_BITS), then 'p' was freed twice: once immediately, and
> once later when the crypto_kpp tfm was destroyed.
>
> Fix it by using dh_free_ctx() (renamed to dh_clear_ctx()) in the error
> paths, as that correctly sets the pointers to NULL.
>
> KASAN report:
>
>      MPI: mpi too large (32760 bits)
>      ==================================================================
>      BUG: KASAN: use-after-free in mpi_free+0x131/0x170
>      Read of size 4 at addr ffff88006c7cdf90 by task reproduce_doubl/367
>
>      CPU: 1 PID: 367 Comm: reproduce_doubl Not tainted 4.14.0-rc7-00040-g05298abde6fe #7
>      Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>      Call Trace:
>       dump_stack+0xb3/0x10b
>       ? mpi_free+0x131/0x170
>       print_address_description+0x79/0x2a0
>       ? mpi_free+0x131/0x170
>       kasan_report+0x236/0x340
>       ? akcipher_register_instance+0x90/0x90
>       __asan_report_load4_noabort+0x14/0x20
>       mpi_free+0x131/0x170
>       ? akcipher_register_instance+0x90/0x90
>       dh_exit_tfm+0x3d/0x140
>       crypto_kpp_exit_tfm+0x52/0x70
>       crypto_destroy_tfm+0xb3/0x250
>       __keyctl_dh_compute+0x640/0xe90
>       ? kasan_slab_free+0x12f/0x180
>       ? dh_data_from_key+0x240/0x240
>       ? key_create_or_update+0x1ee/0xb20
>       ? key_instantiate_and_link+0x440/0x440
>       ? lock_contended+0xee0/0xee0
>       ? kfree+0xcf/0x210
>       ? SyS_add_key+0x268/0x340
>       keyctl_dh_compute+0xb3/0xf1
>       ? __keyctl_dh_compute+0xe90/0xe90
>       ? SyS_add_key+0x26d/0x340
>       ? entry_SYSCALL_64_fastpath+0x5/0xbe
>       ? trace_hardirqs_on_caller+0x3f4/0x560
>       SyS_keyctl+0x72/0x2c0
>       entry_SYSCALL_64_fastpath+0x1f/0xbe
>      RIP: 0033:0x43ccf9
>      RSP: 002b:00007ffeeec96158 EFLAGS: 00000246 ORIG_RAX: 00000000000000fa
>      RAX: ffffffffffffffda RBX: 000000000248b9b9 RCX: 000000000043ccf9
>      RDX: 00007ffeeec96170 RSI: 00007ffeeec96160 RDI: 0000000000000017
>      RBP: 0000000000000046 R08: 0000000000000000 R09: 0248b9b9143dc936
>      R10: 0000000000001000 R11: 0000000000000246 R12: 0000000000000000
>      R13: 0000000000409670 R14: 0000000000409700 R15: 0000000000000000
>
>      Allocated by task 367:
>       save_stack_trace+0x16/0x20
>       kasan_kmalloc+0xeb/0x180
>       kmem_cache_alloc_trace+0x114/0x300
>       mpi_alloc+0x4b/0x230
>       mpi_read_raw_data+0xbe/0x360
>       dh_set_secret+0x1dc/0x460
>       __keyctl_dh_compute+0x623/0xe90
>       keyctl_dh_compute+0xb3/0xf1
>       SyS_keyctl+0x72/0x2c0
>       entry_SYSCALL_64_fastpath+0x1f/0xbe
>
>      Freed by task 367:
>       save_stack_trace+0x16/0x20
>       kasan_slab_free+0xab/0x180
>       kfree+0xb5/0x210
>       mpi_free+0xcb/0x170
>       dh_set_secret+0x2d7/0x460
>       __keyctl_dh_compute+0x623/0xe90
>       keyctl_dh_compute+0xb3/0xf1
>       SyS_keyctl+0x72/0x2c0
>       entry_SYSCALL_64_fastpath+0x1f/0xbe
>
> Fixes: 802c7f1c84e4 ("crypto: dh - Add DH software implementation")
> Cc: <stable@xxxxxxxxxxxxxxx> # v4.8+
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>

Reviewed-by: Tudor Ambarus <tudor.ambarus@xxxxxxxxxxxxx>

> ---
>   crypto/dh.c | 33 +++++++++++++--------------------
>   1 file changed, 13 insertions(+), 20 deletions(-)
>
> diff --git a/crypto/dh.c b/crypto/dh.c
> index b1032a5c1bfa..aadaf36fb56f 100644
> --- a/crypto/dh.c
> +++ b/crypto/dh.c
> @@ -21,19 +21,12 @@ struct dh_ctx {
>   	MPI xa;
>   };
>   
> -static inline void dh_clear_params(struct dh_ctx *ctx)
> +static void dh_clear_ctx(struct dh_ctx *ctx)
>   {
>   	mpi_free(ctx->p);
>   	mpi_free(ctx->g);
> -	ctx->p = NULL;
> -	ctx->g = NULL;
> -}
> -
> -static void dh_free_ctx(struct dh_ctx *ctx)
> -{
> -	dh_clear_params(ctx);
>   	mpi_free(ctx->xa);
> -	ctx->xa = NULL;
> +	memset(ctx, 0, sizeof(*ctx));
>   }
>   
>   /*
> @@ -71,10 +64,8 @@ static int dh_set_params(struct dh_ctx *ctx, struct dh *params)
>   		return -EINVAL;
>   
>   	ctx->g = mpi_read_raw_data(params->g, params->g_size);
> -	if (!ctx->g) {
> -		mpi_free(ctx->p);
> +	if (!ctx->g)
>   		return -EINVAL;
> -	}
>   
>   	return 0;
>   }
> @@ -86,21 +77,23 @@ static int dh_set_secret(struct crypto_kpp *tfm, const void *buf,
>   	struct dh params;
>   
>   	/* Free the old MPI key if any */
> -	dh_free_ctx(ctx);
> +	dh_clear_ctx(ctx);
>   
>   	if (crypto_dh_decode_key(buf, len, &params) < 0)
> -		return -EINVAL;
> +		goto err_clear_ctx;
>   
>   	if (dh_set_params(ctx, &params) < 0)
> -		return -EINVAL;
> +		goto err_clear_ctx;
>   
>   	ctx->xa = mpi_read_raw_data(params.key, params.key_size);
> -	if (!ctx->xa) {
> -		dh_clear_params(ctx);
> -		return -EINVAL;
> -	}
> +	if (!ctx->xa)
> +		goto err_clear_ctx;
>   
>   	return 0;
> +
> +err_clear_ctx:
> +	dh_clear_ctx(ctx);
> +	return -EINVAL;
>   }
>   
>   static int dh_compute_value(struct kpp_request *req)
> @@ -158,7 +151,7 @@ static void dh_exit_tfm(struct crypto_kpp *tfm)
>   {
>   	struct dh_ctx *ctx = dh_get_ctx(tfm);
>   
> -	dh_free_ctx(ctx);
> +	dh_clear_ctx(ctx);
>   }
>   
>   static struct kpp_alg dh = {