crypto: ccp - Use kzalloc for sev ioctl interfaces to prevent kernel memory leak

For some sev ioctl interfaces, input may be passed that is less than or equal to SEV_FW_BLOB_MAX_SIZE, but larger than the data that PSP firmware returns. In this case, kmalloc will allocate memory that is the size of the input rather than the size of the data. Since PSP firmware doesn't fully overwrite the buffer, the sev ioctl interfaces with the issue may return uninitialized slab memory.

Currently, all of the ioctl interfaces in the ccp driver are safe, but to prevent future problems, change all ioctl interfaces that allocate memory with kmalloc to use kzalloc.

Fixes e799035609e15 ("crypto: ccp: Implement SEV_PEK_CSR ioctl command")

Fixes 76a2b524a4b1d ("crypto: ccp: Implement SEV_PDH_CERT_EXPORT ioctl command")

Fixes d6112ea0cb344 ("crypto: ccp - introduce SEV_GET_ID2 command")

Cc [email protected]

Reported-by Andy Nguyen <[email protected]>

Suggested-by David Rientjes <[email protected]>

Suggested-by Peter Gonda <[email protected]>

Signed-off-by John Allen <[email protected]>

---
v2:
- Add fixes tags and CC [email protected]
---

For some sev ioctl interfaces, input may be passed that is less than or equal to SEV_FW_BLOB_MAX_SIZE, but larger than the data that PSP firmware returns. In this case, kmalloc will allocate memory that is the size of the input rather than the size of the data. Since PSP firmware doesn't fully overwrite the buffer, the sev ioctl interfaces with the issue may return uninitialized slab memory.

Currently, all of the ioctl interfaces in the ccp driver are safe, but to prevent future problems, change all ioctl interfaces that allocate memory with kmalloc to use kzalloc.

Fixes e799035609e15 ("crypto: ccp: Implement SEV_PEK_CSR ioctl

command")

Fixes 76a2b524a4b1d ("crypto: ccp: Implement SEV_PDH_CERT_EXPORT

ioctl command")

Fixes d6112ea0cb344 ("crypto: ccp - introduce SEV_GET_ID2 command")

Cc [email protected]

Reported-by Andy Nguyen <[email protected]>

Suggested-by David Rientjes <[email protected]>

Suggested-by Peter Gonda <[email protected]>

Signed-off-by John Allen <[email protected]>

---
v2:
- Add fixes tags and CC [email protected]
---

static int sev_ioctl_do_platform_status(struct sev_issue_cmd *argp) { struct sev_user_data_status data; int ret;

ret = __sev_do_cmd_locked(SEV_CMD_PLATFORM_STATUS, &data, &argp->error); if (ret) return ret;

if (copy_to_user((void __user *)argp->data, &data, sizeof(data))) ret = -EFAULT;

return ret;
}

Would it be safer to memset @data here to all zeros too?

[AMD Official Use Only - General]

Hello Peter,

-----Original Message-----
From: Peter Gonda <[email protected]>
Sent: Monday, May 16, 2022 10:53 AM
To: Allen, John <[email protected]>
Cc: Herbert Xu <[email protected]>; Linux Crypto Mailing List <[email protected]>; Sean Christopherson <[email protected]>; Lendacky, Thomas <[email protected]>; Kalra, Ashish <[email protected]>; LKML <[email protected]>; Andy Nguyen <[email protected]>; David Rientjes <[email protected]>; [email protected]
Subject: Re: [PATCH v2] crypto: ccp - Use kzalloc for sev ioctl interfaces to prevent kernel memory leak

Quoted 31 lines · John Allen

For some sev ioctl interfaces, input may be passed that is less than or equal to SEV_FW_BLOB_MAX_SIZE, but larger than the data that PSP firmware returns. In this case, kmalloc will allocate memory that is the size of the input rather than the size of the data. Since PSP firmware doesn't fully overwrite the buffer, the sev ioctl interfaces with the issue may return uninitialized slab memory.

Currently, all of the ioctl interfaces in the ccp driver are safe, but to prevent future problems, change all ioctl interfaces that allocate memory with kmalloc to use kzalloc.

Fixes e799035609e15 ("crypto: ccp: Implement SEV_PEK_CSR ioctl

command")

Fixes 76a2b524a4b1d ("crypto: ccp: Implement SEV_PDH_CERT_EXPORT

ioctl command")

Fixes d6112ea0cb344 ("crypto: ccp - introduce SEV_GET_ID2 command")

Cc [email protected]

Reported-by Andy Nguyen <[email protected]>

Suggested-by David Rientjes <[email protected]>

Suggested-by Peter Gonda <[email protected]>

Signed-off-by John Allen <[email protected]>

---
v2:
- Add fixes tags and CC [email protected]
---

static int sev_ioctl_do_platform_status(struct sev_issue_cmd *argp) { struct sev_user_data_status data; int ret;

ret = __sev_do_cmd_locked(SEV_CMD_PLATFORM_STATUS, &data, &argp->error); if (ret) return ret;

Quoted 6 lines

if (copy_to_user((void __user *)argp->data, &data, sizeof(data))) ret = -EFAULT;

return ret;
}

Would it be safer to memset @data here to all zeros too?

It will be, but this command/function is safe as firmware will fill in the whole buffer here with the PLATFORM STATUS data retuned to the user.

Thanks,
Ashish

[AMD Official Use Only - General]

Hello Peter,

-----Original Message-----
From: Peter Gonda <[email protected]>
Sent: Monday, May 16, 2022 10:53 AM
To: Allen, John <[email protected]>
Cc: Herbert Xu <[email protected]>; Linux Crypto Mailing List <[email protected]>; Sean Christopherson <[email protected]>; Lendacky, Thomas <[email protected]>; Kalra, Ashish <[email protected]>; LKML <[email protected]>; Andy Nguyen <[email protected]>; David Rientjes <[email protected]>; [email protected]
Subject: Re: [PATCH v2] crypto: ccp - Use kzalloc for sev ioctl interfaces to prevent kernel memory leak

Quoted 36 lines · John Allen

For some sev ioctl interfaces, input may be passed that is less than or equal to SEV_FW_BLOB_MAX_SIZE, but larger than the data that PSP firmware returns. In this case, kmalloc will allocate memory that is the size of the input rather than the size of the data. Since PSP firmware doesn't fully overwrite the buffer, the sev ioctl interfaces with the issue may return uninitialized slab memory.

Currently, all of the ioctl interfaces in the ccp driver are safe, but to prevent future problems, change all ioctl interfaces that allocate memory with kmalloc to use kzalloc.

Fixes e799035609e15 ("crypto: ccp: Implement SEV_PEK_CSR ioctl

command")

Fixes 76a2b524a4b1d ("crypto: ccp: Implement SEV_PDH_CERT_EXPORT

ioctl command")

Fixes d6112ea0cb344 ("crypto: ccp - introduce SEV_GET_ID2 command")

Cc [email protected]

Reported-by Andy Nguyen <[email protected]>

Suggested-by David Rientjes <[email protected]>

Suggested-by Peter Gonda <[email protected]>

Signed-off-by John Allen <[email protected]>

---
v2:
- Add fixes tags and CC [email protected]
---

static int sev_ioctl_do_platform_status(struct sev_issue_cmd *argp) { struct sev_user_data_status data; int ret;

ret = __sev_do_cmd_locked(SEV_CMD_PLATFORM_STATUS, &data, &argp->error); if (ret) return ret;

if (copy_to_user((void __user *)argp->data, &data, sizeof(data))) ret = -EFAULT;

Quoted 4 lines

return ret;
}

Would it be safer to memset @data here to all zeros too?

It will be, but this command/function is safe as firmware will fill in the whole buffer here with the PLATFORM STATUS data retuned to the user.

That does seem safe for now but I thought we decided it would be prudent to not trust the PSPs implementation here and clear all the buffers that eventually get sent to userspace?

Quoted 8 lines

Quoted 1 line

Would it be safer to memset @data here to all zeros too?

It will be, but this command/function is safe as firmware will fill in the whole buffer here with the PLATFORM STATUS data retuned to the user.

That does seem safe for now but I thought we decided it would be prudent to not trust the PSPs implementation here and clear all the buffers that eventually get sent to userspace?

Yes, but the issue is when the user programs a buffer size larger the one filled in by the firmware. In this case firmware is always going to fill up the whole buffer with PLATFORM_STATUS data, so it will be always be safe. The issue is mainly with the kernel side doing a copy_to_user() based on user programmed length instead of the firmware returned buffer length.

Quoted 8 lines

Quoted 1 line

Would it be safer to memset @data here to all zeros too?

It will be, but this command/function is safe as firmware will fill in the whole buffer here with the PLATFORM STATUS data retuned to the user.

That does seem safe for now but I thought we decided it would be prudent to not trust the PSPs implementation here and clear all the buffers that eventually get sent to userspace?

Yes, but the issue is when the user programs a buffer size larger the
one filled in by the firmware. In this case firmware is always going
to fill up the whole buffer with PLATFORM_STATUS data, so it will be
always be safe. The issue is mainly with the kernel side doing a
copy_to_user() based on user programmed length instead of the firmware returned buffer length.

Peter's point is that it costs the kernel very little to be paranoid and not make assumptions about whether or not the PSP will fill an entire struct as expected.

I agree it feels a bit silly since all fields are output, but on the other hand the PSP spec just says:

The following data structure is written to memory at STATUS_PADDR

and the data structure has several reserved fields. I don't love assuming that the PSP will always write zeros for the reserved fields and not do something like:

	if (rmp_initialized)
		data[3] |= IS_RMP_INIT;
	else
		data[3] &= ~IS_RMP_INIT;

Given that zeroing @data in the kernel is easy and this is not a hot patch, I prefer the paranoid approach unless the PSP spec is much more explicit in saying that it writes all bits and bytes on success.