Mailing List Archive

commit 9fd732d18d564001227c98ca8fa5da8eb348ede5
Author: Andrew Cooper <andrew.cooper3@citrix.com>
AuthorDate: Fri Dec 22 17:44:48 2023 +0000
Commit: Andrew Cooper <andrew.cooper3@citrix.com>
CommitDate: Tue Apr 9 16:45:01 2024 +0100

x86: Use indirect calls in reset-stack infrastructure

Mixing up JMP and CALL indirect targets leads a very fun form of speculative
type confusion. A target which is expecting to be called CALLed needs a
return address on the stack, and an indirect JMP doesn't place one there.

An indirect JMP which predicts to a target intending to be CALLed can end up
with a RET speculatively executing with a value from the JMPers stack frame.

There are several ways get indirect JMPs in Xen.

* From tailcall optimisations. These are safe because the compiler has
arranged the stack to point at the callee's return address.

* From jump tables. These are unsafe, but Xen is built with -fno-jump-tables
to work around several compiler issues.

* From reset_stack_and_jump_ind(), which is particularly unsafe. Because of
the additional stack adjustment made, the value picked up off the stack is
regs->r15 of the next vCPU to run.

In order to mitigate this type confusion, we want to make all indirect targets
be CALL targets, and remove the use of indirect JMP except via tailcall
optimisation.

Luckily due to XSA-348, all C target functions of reset_stack_and_jump_ind()
are noreturn. {svm,vmx}_do_resume() exits via reset_stack_and_jump(); a
direct JMP with entirely different prediction properties. idle_loop() is an
infinite loop which eventually exits via reset_stack_and_jump_ind() from a new
schedule. i.e. These paths are all fine having one extra return address on
the stack.

This leaves continue_pv_domain(), which is expecting to be a JMP target.
Alter it to strip the return address off the stack, which is safe because
there isn't actually a RET expecting to return to its caller.

This allows us change reset_stack_and_jump_ind() to reset_stack_and_call_ind()
in order to mitigate the speculative type confusion.

This is part of XSA-456 / CVE-2024-2201.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 8e186f98ce0e35d1754ec9299da41ec98873b65c)
---
xen/arch/x86/domain.c | 4 ++--
xen/arch/x86/include/asm/current.h | 4 ++--
xen/arch/x86/x86_64/entry.S | 8 ++++++++
3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 13912f907b..307446273a 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -2125,12 +2125,12 @@ void context_switch(struct vcpu *prev, struct vcpu *next)
/* Ensure that the vcpu has an up-to-date time base. */
update_vcpu_system_time(next);

- reset_stack_and_jump_ind(nextd->arch.ctxt_switch->tail);
+ reset_stack_and_call_ind(nextd->arch.ctxt_switch->tail);
}

void continue_running(struct vcpu *same)
{
- reset_stack_and_jump_ind(same->domain->arch.ctxt_switch->tail);
+ reset_stack_and_call_ind(same->domain->arch.ctxt_switch->tail);
}

int __sync_local_execstate(void)
diff --git a/xen/arch/x86/include/asm/current.h b/xen/arch/x86/include/asm/current.h
index ba82b413e2..2f723bcf1b 100644
--- a/xen/arch/x86/include/asm/current.h
+++ b/xen/arch/x86/include/asm/current.h
@@ -196,10 +196,10 @@ unsigned long get_stack_dump_bottom (unsigned long sp);
switch_stack_and_jump(fn, "jmp %c", "i")

/* The constraint may only specify non-call-clobbered registers. */
-#define reset_stack_and_jump_ind(fn) \
+#define reset_stack_and_call_ind(fn) \
({ \
(void)((fn) == (void (*)(void))NULL); \
- switch_stack_and_jump(fn, "INDIRECT_JMP %", "b"); \
+ switch_stack_and_jump(fn, "INDIRECT_CALL %", "b"); \
})

/*
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 1d7344117b..ad7dd3b23b 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -637,6 +637,14 @@ ENTRY(dom_crash_sync_extable)
#ifdef CONFIG_PV
ENTRY(continue_pv_domain)
ENDBR64
+
+ /*
+ * For speculative type confusion reasons, we're CALLed rather than
+ * JMPed to. Drop the return address.
+ */
+ add $8, %rsp
+ ALTERNATIVE "", "mov $2, %eax; incsspd %eax", X86_FEATURE_XEN_SHSTK
+
call check_wakeup_from_wait
ret_from_intr:
GET_CURRENT(bx)
--
generated by git-patchbot for /home/xen/git/xen.git#staging-4.18