| Message ID | 20211119202951.403525-1-sstabellini@kernel.org (mailing list archive) |
|---|---|
| State | Superseded |
| Commit | 239d2bf20906e6788db46d51c0c634814caad6d0 |
| Headers | show |
| Series | [v2] xen: detect uninitialized xenbus in xenbus_init | expand |
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.
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,