[PATCH v3] xen: detect uninitialized xenbus in xenbus_init

[PATCH v3] xen: detect uninitialized xenbus in xenbus_init

[Stefano Stabellini]

From: Stefano Stabellini <stefano.stabellini@xilinx.com>

If the xenstore page hasn't been allocated properly, reading the value
of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually return
error. Instead, it will succeed and return zero. Instead of attempting
to xen_remap a bad guest physical address, detect this condition and
return early.

Note that although a guest physical address of zero for
HVM_PARAM_STORE_PFN is theoretically possible, it is not a good choice
and zero has never been validly used in that capacity.

Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX.

For 32-bit Linux, any pfn above ULONG_MAX would get truncated. Pfns
above ULONG_MAX should never be passed by the Xen tools to HVM guests
anyway, so check for this condition and return early.

Cc: stable@vger.kernel.org
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
---
Changes in v3:
- improve in-code comment
- improve check

Changes in v2:
- add check for ULLONG_MAX (unitialized)
- add check for ULONG_MAX #if BITS_PER_LONG == 32 (actual error)
- add pr_err error message
---
 drivers/xen/xenbus/xenbus_probe.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
index 94405bb3829e..d3ca57d48a73 100644
--- a/drivers/xen/xenbus/xenbus_probe.c
+++ b/drivers/xen/xenbus/xenbus_probe.c
@@ -951,6 +951,30 @@ static int __init xenbus_init(void)
 		err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
 		if (err)
 			goto out_error;
+		/*
+		 * Uninitialized hvm_params are zero and return no error.
+		 * Although it is theoretically possible to have
+		 * HVM_PARAM_STORE_PFN set to zero on purpose, in reality it is
+		 * not zero when valid. If zero, it means that Xenstore hasn't
+		 * been properly initialized. Instead of attempting to map a
+		 * wrong guest physical address return error.
+		 *
+		 * Also recognize the invalid value of INVALID_PFN which is
+		 * ULLONG_MAX.
+		 */
+		if (!v || !(v + 1)) {
+			err = -ENOENT;
+			goto out_error;
+		}
+		/* Avoid truncation on 32-bit. */
+#if BITS_PER_LONG == 32
+		if (v > ULONG_MAX) {
+			pr_err("%s: cannot handle HVM_PARAM_STORE_PFN=%llx > ULONG_MAX\n",
+			       __func__, v);
+			err = -EINVAL;
+			goto out_error;
+		}
+#endif
 		xen_store_gfn = (unsigned long)v;
 		xen_store_interface =
 			xen_remap(xen_store_gfn << XEN_PAGE_SHIFT,
-- 
2.25.1

Re: [PATCH v3] xen: detect uninitialized xenbus in xenbus_init

[Juergen Gross]

[-- Attachment #1.1.1: Type: text/plain, Size: 2437 bytes --]

On 22.11.21 23:16, Stefano Stabellini wrote:
> From: Stefano Stabellini <stefano.stabellini@xilinx.com>
> 
> If the xenstore page hasn't been allocated properly, reading the value
> of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually return
> error. Instead, it will succeed and return zero. Instead of attempting
> to xen_remap a bad guest physical address, detect this condition and
> return early.
> 
> Note that although a guest physical address of zero for
> HVM_PARAM_STORE_PFN is theoretically possible, it is not a good choice
> and zero has never been validly used in that capacity.
> 
> Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX.
> 
> For 32-bit Linux, any pfn above ULONG_MAX would get truncated. Pfns
> above ULONG_MAX should never be passed by the Xen tools to HVM guests
> anyway, so check for this condition and return early.
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
> ---
> Changes in v3:
> - improve in-code comment
> - improve check
> 
> Changes in v2:
> - add check for ULLONG_MAX (unitialized)
> - add check for ULONG_MAX #if BITS_PER_LONG == 32 (actual error)
> - add pr_err error message
> ---
>   drivers/xen/xenbus/xenbus_probe.c | 24 ++++++++++++++++++++++++
>   1 file changed, 24 insertions(+)
> 
> diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
> index 94405bb3829e..d3ca57d48a73 100644
> --- a/drivers/xen/xenbus/xenbus_probe.c
> +++ b/drivers/xen/xenbus/xenbus_probe.c
> @@ -951,6 +951,30 @@ static int __init xenbus_init(void)
>   		err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
>   		if (err)
>   			goto out_error;
> +		/*
> +		 * Uninitialized hvm_params are zero and return no error.
> +		 * Although it is theoretically possible to have
> +		 * HVM_PARAM_STORE_PFN set to zero on purpose, in reality it is
> +		 * not zero when valid. If zero, it means that Xenstore hasn't
> +		 * been properly initialized. Instead of attempting to map a
> +		 * wrong guest physical address return error.
> +		 *
> +		 * Also recognize the invalid value of INVALID_PFN which is
> +		 * ULLONG_MAX.

Adjust the comment, e.g. s/ULLONG_MAX/all bits set/ (in the commit
message, too)?

> +		 */
> +		if (!v || !(v + 1)) {

For me "if (!v || !~v)" would be more readable, but I don't really feel
strong here.


Juergen


[-- Attachment #1.1.2: OpenPGP public key --]
[-- Type: application/pgp-keys, Size: 3135 bytes --]

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 495 bytes --]

Re: [PATCH v3] xen: detect uninitialized xenbus in xenbus_init

[Jan Beulich]

On 23.11.2021 06:42, Juergen Gross wrote:
> On 22.11.21 23:16, Stefano Stabellini wrote:
>> From: Stefano Stabellini <stefano.stabellini@xilinx.com>
>>
>> If the xenstore page hasn't been allocated properly, reading the value
>> of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually return
>> error. Instead, it will succeed and return zero. Instead of attempting
>> to xen_remap a bad guest physical address, detect this condition and
>> return early.
>>
>> Note that although a guest physical address of zero for
>> HVM_PARAM_STORE_PFN is theoretically possible, it is not a good choice
>> and zero has never been validly used in that capacity.
>>
>> Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX.
>>
>> For 32-bit Linux, any pfn above ULONG_MAX would get truncated. Pfns
>> above ULONG_MAX should never be passed by the Xen tools to HVM guests
>> anyway, so check for this condition and return early.
>>
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
>> ---
>> Changes in v3:
>> - improve in-code comment
>> - improve check
>>
>> Changes in v2:
>> - add check for ULLONG_MAX (unitialized)
>> - add check for ULONG_MAX #if BITS_PER_LONG == 32 (actual error)
>> - add pr_err error message
>> ---
>>   drivers/xen/xenbus/xenbus_probe.c | 24 ++++++++++++++++++++++++
>>   1 file changed, 24 insertions(+)
>>
>> diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
>> index 94405bb3829e..d3ca57d48a73 100644
>> --- a/drivers/xen/xenbus/xenbus_probe.c
>> +++ b/drivers/xen/xenbus/xenbus_probe.c
>> @@ -951,6 +951,30 @@ static int __init xenbus_init(void)
>>   		err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
>>   		if (err)
>>   			goto out_error;
>> +		/*
>> +		 * Uninitialized hvm_params are zero and return no error.
>> +		 * Although it is theoretically possible to have
>> +		 * HVM_PARAM_STORE_PFN set to zero on purpose, in reality it is
>> +		 * not zero when valid. If zero, it means that Xenstore hasn't
>> +		 * been properly initialized. Instead of attempting to map a
>> +		 * wrong guest physical address return error.
>> +		 *
>> +		 * Also recognize the invalid value of INVALID_PFN which is
>> +		 * ULLONG_MAX.
> 
> Adjust the comment, e.g. s/ULLONG_MAX/all bits set/ (in the commit
> message, too)?

I also don't think the reference to INVALID_PFN is appropriate here. Afaict
the two aren't the same on 32-bit. Plus I can't even find a constant named
this way in Linux'es include/.

>> +		 */
>> +		if (!v || !(v + 1)) {
> 
> For me "if (!v || !~v)" would be more readable, but I don't really feel
> strong here.

Oh, indeed.

Jan