Bluetooth: Remove VLA usage in aes_cmac

PATCH In review Gustavo A. R. Silva 2 participants 7 messages Mar 21, 2018 – Apr 5, 2018

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Networking Network drivers Bluetooth drivers Bluetooth

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Gustavo A. R. Silva

author

2018-03-21 01:05 UTC #0 lore raw

PATCH v2 1/1 Bluetooth: Remove VLA usage in aes_cmac

In preparation to enabling -Wvla, remove VLA and replace it with dynamic memory allocation instead.

The use of stack Variable Length Arrays needs to be avoided, as they can be a vector for stack exhaustion, which can be both a runtime bug or a security flaw. Also, in general, as code evolves it is easy to lose track of how big a VLA can get. Thus, we can end up having runtime failures that are hard to debug.

Also, fixed as part of the directive to remove all VLAs from the kernel: https://lkml.org/lkml/2018/3/7/621

Signed-off-by Gustavo A. R. Silva <[email protected]>

---
Changes in v2:
- Fix memory leak in previous patch.

 net/bluetooth/smp.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

net/bluetooth/smp.c +12 -5

index a2ddae2..0fa7035 100644
@@ -173,7 +173,7 @@ static int aes_cmac(struct crypto_shash *tfm, const u8 k[16], const u8 *m,
 		    size_t len, u8 mac[16])
 {
 	uint8_t tmp[16], mac_msb[16], msg_msb[CMAC_MSG_MAX];
-	SHASH_DESC_ON_STACK(desc, tfm);
+	struct shash_desc *shash;
 	int err;
 
 	if (len > CMAC_MSG_MAX)

@@ -184,8 +184,13 @@ static int aes_cmac(struct crypto_shash *tfm, const u8 k[16], const u8 *m,
 		return -EINVAL;
 	}
 
-	desc->tfm = tfm;
-	desc->flags = 0;
+	shash = kzalloc(sizeof(*shash) + crypto_shash_descsize(tfm),
+			GFP_KERNEL);
+	if (!shash)
+		return -ENOMEM;
+
+	shash->tfm = tfm;
+	shash->flags = 0;
 
 	/* Swap key and message from LSB to MSB */
 	swap_buf(k, tmp, 16);

@@ -197,11 +202,13 @@ static int aes_cmac(struct crypto_shash *tfm, const u8 k[16], const u8 *m,
 	err = crypto_shash_setkey(tfm, tmp, 16);
 	if (err) {
 		BT_ERR("cipher setkey failed: %d", err);
+		kfree(shash);
 		return err;
 	}
 
-	err = crypto_shash_digest(desc, msg_msb, len, mac_msb);
-	shash_desc_zero(desc);
+	err = crypto_shash_digest(shash, msg_msb, len, mac_msb);
+	shash_desc_zero(shash);
+	kfree(shash);
 	if (err) {
 		BT_ERR("Hash computation error %d", err);
 		return err;

Signature · 1 line

2.7.4

Marcel Holtmann

maintainer

2018-03-21 13:45 UTC #1 lore raw

Hi Gustavo,

Quoted 12 lines

In preparation to enabling -Wvla, remove VLA and replace it with dynamic memory allocation instead.
The use of stack Variable Length Arrays needs to be avoided, as they can be a vector for stack exhaustion, which can be both a runtime bug or a security flaw. Also, in general, as code evolves it is easy to lose track of how big a VLA can get. Thus, we can end up having runtime failures that are hard to debug.
Also, fixed as part of the directive to remove all VLAs from the kernel: https://lkml.org/lkml/2018/3/7/621

Quoted 7 lines

Signed-off-by Gustavo A. R. Silva <[email protected]>
---
Changes in v2:
- Fix memory leak in previous patch.
net/bluetooth/smp.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)

patch has been applied to bluetooth-next tree.

Regards

Marcel

Gustavo A. R. Silva

author

2018-03-21 13:49 UTC #2 lore raw

Quoted 19 lines · Marcel Holtmann

Hi Gustavo,
Quoted 17 lines
In preparation to enabling -Wvla, remove VLA and replace it with dynamic memory allocation instead.
The use of stack Variable Length Arrays needs to be avoided, as they can be a vector for stack exhaustion, which can be both a runtime bug or a security flaw. Also, in general, as code evolves it is easy to lose track of how big a VLA can get. Thus, we can end up having runtime failures that are hard to debug.
Also, fixed as part of the directive to remove all VLAs from the kernel: https://lkml.org/lkml/2018/3/7/621
Signed-off-by Gustavo A. R. Silva <[email protected]>
---
Changes in v2:
- Fix memory leak in previous patch.

Quoted 9 lines

Quoted 2 lines
net/bluetooth/smp.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)
patch has been applied to bluetooth-next tree.
Regards
Marcel

Awesome.

Thanks, Marcel.
--
Gustavo

Marcel Holtmann

maintainer

2018-04-05 07:23 UTC #3 lore raw

In reply to Gustavo A. R. Silva

Hi Gustavo,

Quoted 25 lines

In preparation to enabling -Wvla, remove VLA and replace it with dynamic memory allocation instead.
The use of stack Variable Length Arrays needs to be avoided, as they can be a vector for stack exhaustion, which can be both a runtime bug or a security flaw. Also, in general, as code evolves it is easy to lose track of how big a VLA can get. Thus, we can end up having runtime failures that are hard to debug.
Also, fixed as part of the directive to remove all VLAs from the kernel: https://lkml.org/lkml/2018/3/7/621
Signed-off-by Gustavo A. R. Silva <[email protected]>
---
Changes in v2:
- Fix memory leak in previous patch.
net/bluetooth/smp.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)
net/bluetooth/smp.c +0 -0
index a2ddae2..0fa7035 100644

net/bluetooth/smp.c static int aes_cmac(struct crypto_shash *tfm, const u8 k[16], const u8 *m, · line 176

Quoted 5 lines

net/bluetooth/smp.c +1 -1

@@ -173,7 +173,7 @@ static int aes_cmac(struct crypto_shash *tfm, const u8 k[16], const u8 *m,
 		    size_t len, u8 mac[16])
 {
 	uint8_t tmp[16], mac_msb[16], msg_msb[CMAC_MSG_MAX];
-	SHASH_DESC_ON_STACK(desc, tfm);
+	struct shash_desc *shash;

so I took this patch back out of bluetooth-next before sending the pull request. I think the discussion on how to fix SHASH_DESC_ON_STACK macro needs to complete first. Once that has concluded we can revisit if this patch is still needed or if another solution has been found. Same as with WiFi, these are not just one-shot calls where a memory allocation doesn’t matter. We need this for random address resolution and thus there can be many of the aes_cmac calls when seeing neighboring devices.

Regards

Marcel

Gustavo A. R. Silva

author

2018-04-05 08:35 UTC #4 lore raw

Hi Marcel,

Quoted 2 lines · Marcel Holtmann

so I took this patch back out of bluetooth-next before sending the pull request. I think the discussion on how to fix SHASH_DESC_ON_STACK macro needs to complete first. Once that has concluded we can revisit if this patch is still needed or if another solution has been found. Same as with WiFi, these are not just one-shot calls where a memory allocation doesn’t matter. We need this for random address resolution and thus there can be many of the aes_cmac calls when seeing neighboring devices.

Yeah. I agree.

Based on Herbert's response to the discussion about SHASH_DESC_ON_STACK https://lkml.org/lkml/2018/3/27/300

it seems it is feasible to fix that macro very easily. I will follow up on this.

By the way, what is you opinion on replacing crypto_shash_descsize(ctx) with PAGE_SIZE / 8 in SHASH_DESC_ON_STACK?

Does it work for you?

Thanks
--
Gustavo

Marcel Holtmann

maintainer

2018-04-05 08:46 UTC #5 lore raw

Hi Gustavo,

Quoted 11 lines

Quoted 1 line
so I took this patch back out of bluetooth-next before sending the pull request. I think the discussion on how to fix SHASH_DESC_ON_STACK macro needs to complete first. Once that has concluded we can revisit if this patch is still needed or if another solution has been found. Same as with WiFi, these are not just one-shot calls where a memory allocation doesn’t matter. We need this for random address resolution and thus there can be many of the aes_cmac calls when seeing neighboring devices.
Yeah. I agree.
Based on Herbert's response to the discussion about SHASH_DESC_ON_STACK https://lkml.org/lkml/2018/3/27/300
it seems it is feasible to fix that macro very easily. I will follow up on this.
By the way, what is you opinion on replacing crypto_shash_descsize(ctx) with PAGE_SIZE / 8 in SHASH_DESC_ON_STACK?
Does it work for you?

isn’t that just waste?

The macro itself is this.

#define SHASH_DESC_ON_STACK(shash, ctx)                           \              
        char __##shash##_desc[sizeof(struct shash_desc) +         \              
                crypto_shash_descsize(ctx)] CRYPTO_MINALIGN_ATTR; \              
        struct shash_desc *shash = (struct shash_desc *)__##shash##_desc

For AES-CMAC, we could always do this with a manual macro that gives us the right size. However that is error prone if any internals change. I think what has to happened that crypto_shash_decsize becomes something the compiler can evaluate at compile time.

Regards

Marcel

Gustavo A. R. Silva

author

2018-04-05 09:51 UTC #6 lore raw

Quoted 6 lines · Marcel Holtmann

Quoted 3 lines
By the way, what is you opinion on replacing crypto_shash_descsize(ctx) with PAGE_SIZE / 8 in SHASH_DESC_ON_STACK?
Does it work for you?
isn’t that just waste?

Agree.

Quoted 9 lines

The macro itself is this.
#define SHASH_DESC_ON_STACK(shash, ctx)                           \
         char __##shash##_desc[sizeof(struct shash_desc) +         \
                 crypto_shash_descsize(ctx)] CRYPTO_MINALIGN_ATTR; \
         struct shash_desc *shash = (struct shash_desc *)__##shash##_desc
For AES-CMAC, we could always do this with a manual macro that gives us the right size. However that is error prone if any internals change. I think what has to happened that crypto_shash_decsize becomes something the compiler can evaluate at compile time.

Yeah. That would imply an analysis of the algorithm each of the callers use. In the case of AES-CMAC, what is the maximum digest size?

I tried to find a fixed-length value for AES-CMAC but I didn't get any output with git grep -n _DIGEST_SIZE | grep AES

Thanks
--
Gustavo