| Message ID | 20170606174804.31124-13-Jason@zx2c4.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Tue, Jun 06, 2017 at 07:48:03PM +0200, Jason A. Donenfeld wrote: > This protocol uses lots of complex cryptography that relies on securely > generated random numbers. Thus, it's important that the RNG is actually > seeded before use. Fortuantely, it appears we're always operating in > process context (there are many GFP_KERNEL allocations and other > sleeping operations), and so we can simply demand that the RNG is seeded > before we use it. > > We take two strategies in this commit. The first is for the library code > that's called from other modules like hci or mgmt: here we just change > the call to get_random_bytes_wait, and return the result of the wait to > the caller, along with the other error codes of those functions like > usual. Then there's the SMP protocol handler itself, which makes many > many many calls to get_random_bytes during different phases. For this, > rather than have to change all the calls to get_random_bytes_wait and > propagate the error result, it's actually enough to just put a single > call to wait_for_random_bytes() at the beginning of the handler, to > ensure that all the subsequent invocations are safe, without having to > actually change them. Likewise, for the random address changing > function, we'd rather know early on in the function whether the RNG > initialization has been interrupted, rather than later, so we call > wait_for_random_bytes() at the top, so that later on the call to > get_random_bytes() is acceptable. Do we need to do all of this? Bluetooth folks, is it fair to assume that hci_power_on() has to be called before any bluetooth functions can be done? If so, adding a wait_for_random_bytes() in hci_power_on() might be all that is necessary. - Ted
Hi Ted, >> This protocol uses lots of complex cryptography that relies on securely >> generated random numbers. Thus, it's important that the RNG is actually >> seeded before use. Fortuantely, it appears we're always operating in >> process context (there are many GFP_KERNEL allocations and other >> sleeping operations), and so we can simply demand that the RNG is seeded >> before we use it. >> >> We take two strategies in this commit. The first is for the library code >> that's called from other modules like hci or mgmt: here we just change >> the call to get_random_bytes_wait, and return the result of the wait to >> the caller, along with the other error codes of those functions like >> usual. Then there's the SMP protocol handler itself, which makes many >> many many calls to get_random_bytes during different phases. For this, >> rather than have to change all the calls to get_random_bytes_wait and >> propagate the error result, it's actually enough to just put a single >> call to wait_for_random_bytes() at the beginning of the handler, to >> ensure that all the subsequent invocations are safe, without having to >> actually change them. Likewise, for the random address changing >> function, we'd rather know early on in the function whether the RNG >> initialization has been interrupted, rather than later, so we call >> wait_for_random_bytes() at the top, so that later on the call to >> get_random_bytes() is acceptable. > > Do we need to do all of this? Bluetooth folks, is it fair to assume > that hci_power_on() has to be called before any bluetooth functions > can be done? If so, adding a wait_for_random_bytes() in > hci_power_on() might be all that is necessary. yes, there are plenty of commands needed before a controller becomes usable. When plugging in new Bluetooth hardware, we have to power it up and read the initial settings and configuration out of. Also all the cryptographic features only apply to LE enabled controllers. The classic BR/EDR controllers have this all in hardware. So if you are not LE enabled, then there is not even a point in waiting for any seeding. However that said, also all LE controllers have an extra random number function we could call if we need extra seeding. We never bothered to hook this up since we thought that the kernel has enough sources. Regards Marcel
On Thu, Jun 8, 2017 at 7:04 AM, Marcel Holtmann <marcel@holtmann.org> wrote: >> Do we need to do all of this? Bluetooth folks, is it fair to assume >> that hci_power_on() has to be called before any bluetooth functions >> can be done? If so, adding a wait_for_random_bytes() in >> hci_power_on() might be all that is necessary. Maybe, but that could hassle bluetooth users who don't want to use any of the fancy bluetooth crypto and don't want to bother waiting for their RNG to initialize.
Hello Marcel,
On Thu, Jun 8, 2017 at 7:04 AM, Marcel Holtmann <marcel@holtmann.org> wrote:
> yes, there are plenty of commands needed before a controller becomes usable.
That doesn't clearly address with precision what Ted was wondering.
Specifically, the inquiry is: can you confirm with certainty whether
or not all calls to get_random_bytes() in the bluetooth directory are
*necessarily* going to come after a call to hci_power_on()?
Jason
Hi Jason, >> yes, there are plenty of commands needed before a controller becomes usable. > > That doesn't clearly address with precision what Ted was wondering. > Specifically, the inquiry is: can you confirm with certainty whether > or not all calls to get_random_bytes() in the bluetooth directory are > *necessarily* going to come after a call to hci_power_on()? on a powered down controller, you can not do any crypto. SMP is only during a connection and the RPAs are only generated when needed. So yes, doing this once in hci_power_on is plenty. However we might want to limit this to LE capable controllers since for BR/EDR only controllers this is not needed. For A2MP I need to check that we need the random numbers seeded there. However this hidden behind the high speed feature. Regards Marcel
On Thu, Jun 8, 2017 at 7:05 PM, Marcel Holtmann <marcel@holtmann.org> wrote:
> on a powered down controller, you can not do any crypto. SMP is only during a connection and the RPAs are only generated when needed. So yes, doing this once in hci_power_on is plenty. However we might want to limit this to LE capable controllers since for BR/EDR only controllers this is not needed. For A2MP I need to check that we need the random numbers seeded there. However this hidden behind the high speed feature.
Okay so it sounds like certain controllers will use this and certain
won't, and so it might be slightly complicated to follow the mouse
through the tube ideally. In that case, I'd recommend continuing with
this current patchset, which just adds the wait (which is a no-op if
it's already seeded) close to the actual call sites.
Can you review that patch and give your Signed-off-by if it looks good?
Jason
diff --git a/net/bluetooth/hci_request.c b/net/bluetooth/hci_request.c index b5faff458d8b..4078057c4fd7 100644 --- a/net/bluetooth/hci_request.c +++ b/net/bluetooth/hci_request.c @@ -1406,6 +1406,12 @@ int hci_update_random_address(struct hci_request *req, bool require_privacy, struct hci_dev *hdev = req->hdev; int err; + if (require_privacy) { + err = wait_for_random_bytes(); + if (unlikely(err)) + return err; + } + /* If privacy is enabled use a resolvable private address. If * current RPA has expired or there is something else than * the current RPA in use, then generate a new one. diff --git a/net/bluetooth/smp.c b/net/bluetooth/smp.c index 14585edc9439..5fef1bc96f42 100644 --- a/net/bluetooth/smp.c +++ b/net/bluetooth/smp.c @@ -537,7 +537,9 @@ int smp_generate_rpa(struct hci_dev *hdev, const u8 irk[16], bdaddr_t *rpa) smp = chan->data; - get_random_bytes(&rpa->b[3], 3); + err = get_random_bytes_wait(&rpa->b[3], 3); + if (unlikely(err)) + return err; rpa->b[5] &= 0x3f; /* Clear two most significant bits */ rpa->b[5] |= 0x40; /* Set second most significant bit */ @@ -570,7 +572,9 @@ int smp_generate_oob(struct hci_dev *hdev, u8 hash[16], u8 rand[16]) } else { while (true) { /* Seed private key with random number */ - get_random_bytes(smp->local_sk, 32); + err = get_random_bytes_wait(smp->local_sk, 32); + if (unlikely(err)) + return err; /* Generate local key pair for Secure Connections */ if (!generate_ecdh_keys(smp->local_pk, smp->local_sk)) @@ -589,7 +593,9 @@ int smp_generate_oob(struct hci_dev *hdev, u8 hash[16], u8 rand[16]) SMP_DBG("OOB Public Key Y: %32phN", smp->local_pk + 32); SMP_DBG("OOB Private Key: %32phN", smp->local_sk); - get_random_bytes(smp->local_rand, 16); + err = get_random_bytes_wait(smp->local_rand, 16); + if (unlikely(err)) + return err; err = smp_f4(smp->tfm_cmac, smp->local_pk, smp->local_pk, smp->local_rand, 0, hash); @@ -2831,7 +2837,11 @@ static int smp_sig_channel(struct l2cap_chan *chan, struct sk_buff *skb) struct hci_conn *hcon = conn->hcon; struct smp_chan *smp; __u8 code, reason; - int err = 0; + int err; + + err = wait_for_random_bytes(); + if (unlikely(err)) + return err; if (skb->len < 1) return -EILSEQ;
This protocol uses lots of complex cryptography that relies on securely generated random numbers. Thus, it's important that the RNG is actually seeded before use. Fortuantely, it appears we're always operating in process context (there are many GFP_KERNEL allocations and other sleeping operations), and so we can simply demand that the RNG is seeded before we use it. We take two strategies in this commit. The first is for the library code that's called from other modules like hci or mgmt: here we just change the call to get_random_bytes_wait, and return the result of the wait to the caller, along with the other error codes of those functions like usual. Then there's the SMP protocol handler itself, which makes many many many calls to get_random_bytes during different phases. For this, rather than have to change all the calls to get_random_bytes_wait and propagate the error result, it's actually enough to just put a single call to wait_for_random_bytes() at the beginning of the handler, to ensure that all the subsequent invocations are safe, without having to actually change them. Likewise, for the random address changing function, we'd rather know early on in the function whether the RNG initialization has been interrupted, rather than later, so we call wait_for_random_bytes() at the top, so that later on the call to get_random_bytes() is acceptable. Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> Cc: Marcel Holtmann <marcel@holtmann.org> Cc: Gustavo Padovan <gustavo@padovan.org> Cc: Johan Hedberg <johan.hedberg@gmail.com> --- net/bluetooth/hci_request.c | 6 ++++++ net/bluetooth/smp.c | 18 ++++++++++++++---- 2 files changed, 20 insertions(+), 4 deletions(-)