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Re: [Xen-devel] [PATCH v2 5/6] x86/hvm: Forced Emulation Prefix for debug builds of Xen
On 09/23/2014 02:23 PM, Andrew Cooper wrote:
On 23/09/14 19:20, Boris Ostrovsky wrote:
On 09/23/2014 12:09 PM, Andrew Cooper wrote:
Analysis of XSAs 105 and 106 show that is possible to force a race
condition
which causes any arbitrary instruction to be emulated.
To aid testing, explicitly introduce the Forced Emulation Prefix for
debug
builds alone.
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Keir Fraser <keir@xxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
CC: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
CC: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@xxxxxxx>
CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
CC: Eddie Dong <eddie.dong@xxxxxxxxx>
CC: Kevin Tian <kevin.tian@xxxxxxxxx>
---
v2: (all suggested by Jan)
* Use hvm_fetch_from_guest_virt_nofault() in preference to
copy_from_guest()
* Vastly reduce use of #ifndef NDEBUG
---
docs/misc/xen-command-line.markdown | 11 +++++++++++
xen/arch/x86/hvm/hvm.c | 5 +++++
xen/arch/x86/hvm/svm/svm.c | 13 +++++++++++++
xen/arch/x86/hvm/vmx/vmx.c | 13 +++++++++++++
xen/include/asm-x86/hvm/hvm.h | 7 +++++++
5 files changed, 49 insertions(+)
diff --git a/docs/misc/xen-command-line.markdown
b/docs/misc/xen-command-line.markdown
index af93e17..389701a 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -682,6 +682,17 @@ Bit 11 - MSR operation logging
Recognized in debug builds of the hypervisor only.
+### hvm\_fep
+> `= <boolean>`
+
+> Default: `false`
+
+Allow use of the Forced Emulation Prefix in HVM guests, to allow
emulation of
+arbitrary instructions.
+
+This option is intended for development purposes, and is only
available in
+debug builds of the hypervisor.
+
### hvm\_port80
> `= <boolean>`
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 5c7e0a4..34f28d0 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -86,6 +86,11 @@ unsigned long __attribute__ ((__section__
(".bss.page_aligned")))
static bool_t __initdata opt_hap_enabled = 1;
boolean_param("hap", opt_hap_enabled);
+#ifndef opt_hvm_fep
+bool_t opt_hvm_fep;
+boolean_param("hvm_fep", opt_hvm_fep);
+#endif
+
static int cpu_callback(
struct notifier_block *nfb, unsigned long action, void *hcpu)
{
diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index b6beefc..cda968b 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -2118,6 +2118,19 @@ static void svm_vmexit_ud_intercept(struct
cpu_user_regs *regs)
struct hvm_emulate_ctxt ctxt;
int rc;
+ if ( opt_hvm_fep )
+ {
+ char sig[5]; /* ud2; .ascii "xen" */
+
+ if ( (hvm_fetch_from_guest_virt_nofault(
+ sig, regs->eip, sizeof(sig), 0) == HVMCOPY_okay) &&
+ (memcmp(sig, "\xf\xbxen", sizeof(sig)) == 0) )
+ {
+ regs->eip += sizeof(sig);
+ regs->eflags &= ~X86_EFLAGS_RF;
+ }
+ }
This code is exactly the same for SVM and VMX. Can it be factored out?
-boris
It can, and I considered that, but it would prevent optimising to
nothing for non-debug builds. Given that it was a single simple if()
statement, I chose not to.
What about an inline (or a macro)? It won't help with code size but is a
good thing from code maintainability point of vew.
-boris
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