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Subject[PATCH 4.4 234/266] x86/speculation/mds: Clear CPU buffers on exit to user
Date
From: Thomas Gleixner <tglx@linutronix.de>

commit 04dcbdb8057827b043b3c71aa397c4c63e67d086 upstream.

Add a static key which controls the invocation of the CPU buffer clear
mechanism on exit to user space and add the call into
prepare_exit_to_usermode() and do_nmi() right before actually returning.

Add documentation which kernel to user space transition this covers and
explain why some corner cases are not mitigated.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Jon Masters <jcm@redhat.com>
Tested-by: Jon Masters <jcm@redhat.com>
[bwh: Backported to 4.4: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
Documentation/x86/mds.rst | 52 +++++++++++++++++++++++++++++++++++
arch/x86/entry/common.c | 3 ++
arch/x86/include/asm/nospec-branch.h | 13 ++++++++
arch/x86/kernel/cpu/bugs.c | 3 ++
arch/x86/kernel/nmi.c | 4 ++
arch/x86/kernel/traps.c | 8 +++++
6 files changed, 83 insertions(+)

--- a/Documentation/x86/mds.rst
+++ b/Documentation/x86/mds.rst
@@ -97,3 +97,55 @@ According to current knowledge additiona
itself are not required because the necessary gadgets to expose the leaked
data cannot be controlled in a way which allows exploitation from malicious
user space or VM guests.
+
+Mitigation points
+-----------------
+
+1. Return to user space
+^^^^^^^^^^^^^^^^^^^^^^^
+
+ When transitioning from kernel to user space the CPU buffers are flushed
+ on affected CPUs when the mitigation is not disabled on the kernel
+ command line. The migitation is enabled through the static key
+ mds_user_clear.
+
+ The mitigation is invoked in prepare_exit_to_usermode() which covers
+ most of the kernel to user space transitions. There are a few exceptions
+ which are not invoking prepare_exit_to_usermode() on return to user
+ space. These exceptions use the paranoid exit code.
+
+ - Non Maskable Interrupt (NMI):
+
+ Access to sensible data like keys, credentials in the NMI context is
+ mostly theoretical: The CPU can do prefetching or execute a
+ misspeculated code path and thereby fetching data which might end up
+ leaking through a buffer.
+
+ But for mounting other attacks the kernel stack address of the task is
+ already valuable information. So in full mitigation mode, the NMI is
+ mitigated on the return from do_nmi() to provide almost complete
+ coverage.
+
+ - Double fault (#DF):
+
+ A double fault is usually fatal, but the ESPFIX workaround, which can
+ be triggered from user space through modify_ldt(2) is a recoverable
+ double fault. #DF uses the paranoid exit path, so explicit mitigation
+ in the double fault handler is required.
+
+ - Machine Check Exception (#MC):
+
+ Another corner case is a #MC which hits between the CPU buffer clear
+ invocation and the actual return to user. As this still is in kernel
+ space it takes the paranoid exit path which does not clear the CPU
+ buffers. So the #MC handler repopulates the buffers to some
+ extent. Machine checks are not reliably controllable and the window is
+ extremly small so mitigation would just tick a checkbox that this
+ theoretical corner case is covered. To keep the amount of special
+ cases small, ignore #MC.
+
+ - Debug Exception (#DB):
+
+ This takes the paranoid exit path only when the INT1 breakpoint is in
+ kernel space. #DB on a user space address takes the regular exit path,
+ so no extra mitigation required.
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -28,6 +28,7 @@
#include <asm/vdso.h>
#include <asm/uaccess.h>
#include <asm/cpufeature.h>
+#include <asm/nospec-branch.h>

#define CREATE_TRACE_POINTS
#include <trace/events/syscalls.h>
@@ -295,6 +296,8 @@ __visible inline void prepare_exit_to_us
#endif

user_enter();
+
+ mds_user_clear_cpu_buffers();
}

#define SYSCALL_EXIT_WORK_FLAGS \
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -262,6 +262,8 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_
DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);

+DECLARE_STATIC_KEY_FALSE(mds_user_clear);
+
#include <asm/segment.h>

/**
@@ -287,6 +289,17 @@ static inline void mds_clear_cpu_buffers
asm volatile("verw %[ds]" : : [ds] "m" (ds) : "cc");
}

+/**
+ * mds_user_clear_cpu_buffers - Mitigation for MDS vulnerability
+ *
+ * Clear CPU buffers if the corresponding static key is enabled
+ */
+static inline void mds_user_clear_cpu_buffers(void)
+{
+ if (static_branch_likely(&mds_user_clear))
+ mds_clear_cpu_buffers();
+}
+
#endif /* __ASSEMBLY__ */

/*
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -58,6 +58,9 @@ DEFINE_STATIC_KEY_FALSE(switch_mm_cond_i
/* Control unconditional IBPB in switch_mm() */
DEFINE_STATIC_KEY_FALSE(switch_mm_always_ibpb);

+/* Control MDS CPU buffer clear before returning to user space */
+DEFINE_STATIC_KEY_FALSE(mds_user_clear);
+
void __init check_bugs(void)
{
identify_boot_cpu();
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -29,6 +29,7 @@
#include <asm/mach_traps.h>
#include <asm/nmi.h>
#include <asm/x86_init.h>
+#include <asm/nospec-branch.h>

#define CREATE_TRACE_POINTS
#include <trace/events/nmi.h>
@@ -522,6 +523,9 @@ nmi_restart:
write_cr2(this_cpu_read(nmi_cr2));
if (this_cpu_dec_return(nmi_state))
goto nmi_restart;
+
+ if (user_mode(regs))
+ mds_user_clear_cpu_buffers();
}
NOKPROBE_SYMBOL(do_nmi);

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -61,6 +61,7 @@
#include <asm/alternative.h>
#include <asm/fpu/xstate.h>
#include <asm/trace/mpx.h>
+#include <asm/nospec-branch.h>
#include <asm/mpx.h>
#include <asm/vm86.h>

@@ -337,6 +338,13 @@ dotraplinkage void do_double_fault(struc
regs->ip = (unsigned long)general_protection;
regs->sp = (unsigned long)&normal_regs->orig_ax;

+ /*
+ * This situation can be triggered by userspace via
+ * modify_ldt(2) and the return does not take the regular
+ * user space exit, so a CPU buffer clear is required when
+ * MDS mitigation is enabled.
+ */
+ mds_user_clear_cpu_buffers();
return;
}
#endif

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