From: YiFei Zhu <zhuyifei@google.com>

do_smm_enter and helper_rsm sets the env->dr, but does not sync the
values with cpu_x86_update_dr7. A malicious kernel may control the
instruction pointer in SMM by setting a breakpoint on the SMI
entry point, and after do_smm_enter cpu->breakpoints contains the
stale breakpoint; and because IDT is not reloaded upon SMI entry,
the debug exception handler controlled by the malicious kernel
is invoked.

Fixes: 01df040b5247 ("x86: Debug register emulation (Jan Kiszka)")
Reported-by: unvariant.winter@gmail.com
Signed-off-by: YiFei Zhu <zhuyifei@google.com>
Link: https://lore.kernel.org/r/2bacb9b24e9d337dbe48791aa25d349eb9c52c3a.1758794468.git.zhuyifei@google.com
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/tcg/system/smm_helper.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/target/i386/tcg/system/smm_helper.c b/target/i386/tcg/system/smm_helper.c
index 251eb7856ce..fb028a8272f 100644
--- a/target/i386/tcg/system/smm_helper.c
+++ b/target/i386/tcg/system/smm_helper.c
@@ -168,7 +168,7 @@ void do_smm_enter(X86CPU *cpu)
                        env->cr[0] & ~(CR0_PE_MASK | CR0_EM_MASK | CR0_TS_MASK |
                                       CR0_PG_MASK));
     cpu_x86_update_cr4(env, 0);
-    env->dr[7] = 0x00000400;
+    helper_set_dr(env, 7, 0x00000400);
 
     cpu_x86_load_seg_cache(env, R_CS, (env->smbase >> 4) & 0xffff, env->smbase,
                            0xffffffff,
@@ -233,8 +233,8 @@ void helper_rsm(CPUX86State *env)
     env->eip = x86_ldq_phys(cs, sm_state + 0x7f78);
     cpu_load_eflags(env, x86_ldl_phys(cs, sm_state + 0x7f70),
                     ~(CC_O | CC_S | CC_Z | CC_A | CC_P | CC_C | DF_MASK));
-    env->dr[6] = x86_ldl_phys(cs, sm_state + 0x7f68);
-    env->dr[7] = x86_ldl_phys(cs, sm_state + 0x7f60);
+    helper_set_dr(env, 6, x86_ldl_phys(cs, sm_state + 0x7f68));
+    helper_set_dr(env, 7, x86_ldl_phys(cs, sm_state + 0x7f60));
 
     cpu_x86_update_cr4(env, x86_ldl_phys(cs, sm_state + 0x7f48));
     cpu_x86_update_cr3(env, x86_ldq_phys(cs, sm_state + 0x7f50));
@@ -268,8 +268,8 @@ void helper_rsm(CPUX86State *env)
     env->regs[R_EDX] = x86_ldl_phys(cs, sm_state + 0x7fd8);
     env->regs[R_ECX] = x86_ldl_phys(cs, sm_state + 0x7fd4);
     env->regs[R_EAX] = x86_ldl_phys(cs, sm_state + 0x7fd0);
-    env->dr[6] = x86_ldl_phys(cs, sm_state + 0x7fcc);
-    env->dr[7] = x86_ldl_phys(cs, sm_state + 0x7fc8);
+    helper_set_dr(env, 6, x86_ldl_phys(cs, sm_state + 0x7fcc));
+    helper_set_dr(env, 7, x86_ldl_phys(cs, sm_state + 0x7fc8));
 
     env->tr.selector = x86_ldl_phys(cs, sm_state + 0x7fc4) & 0xffff;
     env->tr.base = x86_ldl_phys(cs, sm_state + 0x7f64);
-- 
2.51.0