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 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 | 14 ++++++++++++++ 1 file changed, 14 insertions(+)
diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c index 94405bb3829e..c7472ff27a93 100644 --- a/drivers/xen/xenbus/xenbus_probe.c +++ b/drivers/xen/xenbus/xenbus_probe.c @@ -951,6 +951,20 @@ static int __init xenbus_init(void) err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v); if (err) goto out_error; + /* Uninitialized. */ + if (v == 0 || v == ULLONG_MAX) { + 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,
On 11/19/21 3:29 PM, 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
Reviewed-by: Boris Ostrovsky boris.ostrvsky@oracle.com
On 19.11.2021 21:29, Stefano Stabellini wrote:
--- a/drivers/xen/xenbus/xenbus_probe.c +++ b/drivers/xen/xenbus/xenbus_probe.c @@ -951,6 +951,20 @@ static int __init xenbus_init(void) err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v); if (err) goto out_error;
/* Uninitialized. */if (v == 0 || v == ULLONG_MAX) {
Didn't you have a comment in v1 here regarding the check against 0? Or was that just like now only in the description? IOW I think there ought to be a code comment justifying the theoretically wrong check ...
Also, while I realize there are various other similar assumptions elsewhere, I would generally recommend to avoid such: There's no guarantee that now and forever unsigned long long and uint64_t are the same thing. And it's easy in cases like this one:
if (!v || !(v + 1)) {
Jan
On Mon, 22 Nov 2021, Jan Beulich wrote:
On 19.11.2021 21:29, Stefano Stabellini wrote:
--- a/drivers/xen/xenbus/xenbus_probe.c +++ b/drivers/xen/xenbus/xenbus_probe.c @@ -951,6 +951,20 @@ static int __init xenbus_init(void) err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v); if (err) goto out_error;
/* Uninitialized. */if (v == 0 || v == ULLONG_MAX) {Didn't you have a comment in v1 here regarding the check against 0? Or was that just like now only in the description? IOW I think there ought to be a code comment justifying the theoretically wrong check ...
Yeah, I added all the info in the commit message and shortened the in-code comment this time. I am also happy to keep the details in the in-code comment, e.g.:
/* * 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. */
Also, while I realize there are various other similar assumptions elsewhere, I would generally recommend to avoid such: There's no guarantee that now and forever unsigned long long and uint64_t are the same thing. And it's easy in cases like this one:
if (!v || !(v + 1)) {
I am happy to use this.
On Mon, 22 Nov 2021, Stefano Stabellini wrote:
On Mon, 22 Nov 2021, Jan Beulich wrote:
On 19.11.2021 21:29, Stefano Stabellini wrote:
--- a/drivers/xen/xenbus/xenbus_probe.c +++ b/drivers/xen/xenbus/xenbus_probe.c @@ -951,6 +951,20 @@ static int __init xenbus_init(void) err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v); if (err) goto out_error;
/* Uninitialized. */if (v == 0 || v == ULLONG_MAX) {Didn't you have a comment in v1 here regarding the check against 0? Or was that just like now only in the description? IOW I think there ought to be a code comment justifying the theoretically wrong check ...
Yeah, I added all the info in the commit message and shortened the in-code comment this time. I am also happy to keep the details in the in-code comment, e.g.:
/*
- 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.
*/
I sent a new version of the patch with the check below and slightly more concise version of this comment.
Also, while I realize there are various other similar assumptions elsewhere, I would generally recommend to avoid such: There's no guarantee that now and forever unsigned long long and uint64_t are the same thing. And it's easy in cases like this one:
if (!v || !(v + 1)) {I am happy to use this.
linux-stable-mirror@lists.linaro.org
-
Boris Ostrovsky -
Jan Beulich -
Stefano Stabellini