| Message ID | 20211122221625.1473164-1-sstabellini@kernel.org (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | [v3] xen: detect uninitialized xenbus in xenbus_init | expand |
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
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
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,