x86/traps: Initialize DR7 by writing its architectural reset value

commit fa7d0f83c5c4223a01598876352473cb3d3bd4d7 upstream.

Initialize DR7 by writing its architectural reset value to always set
bit 10, which is reserved to '1', when "clearing" DR7 so as not to
trigger unanticipated behavior if said bit is ever unreserved, e.g. as
a feature enabling flag with inverted polarity.

Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Sean Christopherson <seanjc@google.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Cc:stable@vger.kernel.org
Link: https://lore.kernel.org/all/20250620231504.2676902-3-xin%40zytor.com
[ context adjusted: no KVM_DEBUGREG_AUTO_SWITCH flag test" ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
index fdbbbfec745aa58855641ae59e17a3a50fc2696a..820b4aeabd0c24b2a79c2612285951d8caf54471 100644 (file)
--- a/arch/x86/include/asm/debugreg.h
+++ b/arch/x86/include/asm/debugreg.h
@@ -9,6 +9,14 @@
 #include <asm/cpufeature.h>
 #include <asm/msr.h>
 
+/*
+ * Define bits that are always set to 1 in DR7, only bit 10 is
+ * architecturally reserved to '1'.
+ *
+ * This is also the init/reset value for DR7.
+ */
+#define DR7_FIXED_1    0x00000400
+
 DECLARE_PER_CPU(unsigned long, cpu_dr7);
 
 #ifndef CONFIG_PARAVIRT_XXL
@@ -100,8 +108,8 @@ static __always_inline void native_set_debugreg(int regno, unsigned long value)
 
 static inline void hw_breakpoint_disable(void)
 {
-       /* Zero the control register for HW Breakpoint */
-       set_debugreg(0UL, 7);
+       /* Reset the control register for HW Breakpoint */
+       set_debugreg(DR7_FIXED_1, 7);
 
        /* Zero-out the individual HW breakpoint address registers */
        set_debugreg(0UL, 0);
@@ -125,9 +133,12 @@ static __always_inline unsigned long local_db_save(void)
                return 0;
 
        get_debugreg(dr7, 7);
-       dr7 &= ~0x400; /* architecturally set bit */
+
+       /* Architecturally set bit */
+       dr7 &= ~DR7_FIXED_1;
        if (dr7)
-               set_debugreg(0, 7);
+               set_debugreg(DR7_FIXED_1, 7);
+
        /*
         * Ensure the compiler doesn't lower the above statements into
         * the critical section; disabling breakpoints late would not

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index f094493232b6456dcdf85e765a872c71a4072a87..8980786686bffe4109095c83401ec7d4c741128d 100644 (file)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -31,6 +31,7 @@
 
 #include <asm/apic.h>
 #include <asm/pvclock-abi.h>
+#include <asm/debugreg.h>
 #include <asm/desc.h>
 #include <asm/mtrr.h>
 #include <asm/msr-index.h>
@@ -247,7 +248,6 @@ enum x86_intercept_stage;
 #define DR7_BP_EN_MASK 0x000000ff
 #define DR7_GE         (1 << 9)
 #define DR7_GD         (1 << 13)
-#define DR7_FIXED_1    0x00000400
 #define DR7_VOLATILE   0xffff2bff
 
 #define KVM_GUESTDBG_VALID_MASK \

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index c39c5c37f4e825de0db2ab2de5de98fc02ea1044..bab44900b937cb9e7432c75d47ba5ce46537da07 100644 (file)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2220,7 +2220,7 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
 static void initialize_debug_regs(void)
 {
        /* Control register first -- to make sure everything is disabled. */
-       set_debugreg(0, 7);
+       set_debugreg(DR7_FIXED_1, 7);
        set_debugreg(DR6_RESERVED, 6);
        /* dr5 and dr4 don't exist */
        set_debugreg(0, 3);

diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
index 102641fd217284c0fbdfd8d06d53e00beebee8d1..8b1a9733d13e3f69a12a8cf4a9dcc2062cec3336 100644 (file)
--- a/arch/x86/kernel/kgdb.c
+++ b/arch/x86/kernel/kgdb.c
@@ -385,7 +385,7 @@ static void kgdb_disable_hw_debug(struct pt_regs *regs)
        struct perf_event *bp;
 
        /* Disable hardware debugging while we are in kgdb: */
-       set_debugreg(0UL, 7);
+       set_debugreg(DR7_FIXED_1, 7);
        for (i = 0; i < HBP_NUM; i++) {
                if (!breakinfo[i].enabled)
                        continue;

diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 4636ef3599737c1e810ee421d87c575305f690ce..3c7621663800b338320d1ea2fe7de5789743731c 100644 (file)
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -93,7 +93,7 @@ void __show_regs(struct pt_regs *regs, enum show_regs_mode mode,
 
        /* Only print out debug registers if they are in their non-default state. */
        if ((d0 == 0) && (d1 == 0) && (d2 == 0) && (d3 == 0) &&
-           (d6 == DR6_RESERVED) && (d7 == 0x400))
+           (d6 == DR6_RESERVED) && (d7 == DR7_FIXED_1))
                return;
 
        printk("%sDR0: %08lx DR1: %08lx DR2: %08lx DR3: %08lx\n",

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 7196ca7048be0cfbb57d13f0f142ea4f90b48634..8565aa31afafe11ad676db96f58733538704c913 100644 (file)
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -132,7 +132,7 @@ void __show_regs(struct pt_regs *regs, enum show_regs_mode mode,
 
        /* Only print out debug registers if they are in their non-default state. */
        if (!((d0 == 0) && (d1 == 0) && (d2 == 0) && (d3 == 0) &&
-           (d6 == DR6_RESERVED) && (d7 == 0x400))) {
+           (d6 == DR6_RESERVED) && (d7 == DR7_FIXED_1))) {
                printk("%sDR0: %016lx DR1: %016lx DR2: %016lx\n",
                       log_lvl, d0, d1, d2);
                printk("%sDR3: %016lx DR6: %016lx DR7: %016lx\n",

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index be7bb6d20129dba93eb9ec8a0c5afe2ecefa37c1..7bae91eb7b2345b16caf74283262469a7fb7a175 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10979,7 +10979,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
                wrmsrl(MSR_IA32_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
 
        if (unlikely(vcpu->arch.switch_db_regs)) {
-               set_debugreg(0, 7);
+               set_debugreg(DR7_FIXED_1, 7);
                set_debugreg(vcpu->arch.eff_db[0], 0);
                set_debugreg(vcpu->arch.eff_db[1], 1);
                set_debugreg(vcpu->arch.eff_db[2], 2);
@@ -10988,7 +10988,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
                if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT))
                        kvm_x86_call(set_dr6)(vcpu, vcpu->arch.dr6);
        } else if (unlikely(hw_breakpoint_active())) {
-               set_debugreg(0, 7);
+               set_debugreg(DR7_FIXED_1, 7);
        }
 
        vcpu->arch.host_debugctl = get_debugctlmsr();