x86/ibt: Keep IBT disabled during alternative patching

commit ebebe30794d38c51f71fe4951ba6af4159d9837d upstream.

cfi_rewrite_callers() updates the fineIBT hash matching at the caller side,
but except for paranoid-mode it relies on apply_retpoline() and friends for
any ENDBR relocation. This could temporarily cause an indirect branch to
land on a poisoned ENDBR.

For instance, with para-virtualization enabled, a simple wrmsrl() could
have an indirect branch pointing to native_write_msr() who's ENDBR has been
relocated due to fineIBT:

<wrmsrl>:
       push   %rbp
       mov    %rsp,%rbp
       mov    %esi,%eax
       mov    %rsi,%rdx
       shr    $0x20,%rdx
       mov    %edi,%edi
       mov    %rax,%rsi
       call   *0x21e65d0(%rip)        # <pv_ops+0xb8>
       ^^^^^^^^^^^^^^^^^^^^^^^

Such an indirect call during the alternative patching could #CP if the
caller is not *yet* adjusted for the new target ENDBR. To prevent a false
 #CP, keep CET-IBT disabled until all callers are patched.

Patching during the module load does not need to be guarded by IBT-disable
because the module code is not executed until the patching is complete.

  [ pawan: Since apply_paravirt() happens before __apply_fineibt()
	   relocates the ENDBR, pv_ops in the example above is not relevant.
	   It is still safer to keep this commit because missing an ENDBR
	   means an oops. ]

Signed-off-by: Pawan Gupta <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Alexandre Chartre <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: pa1gupta

arch/x86/kernel/alternative.c

@@ -30,6 +30,7 @@
 #include <asm/fixmap.h>
 #include <asm/paravirt.h>
 #include <asm/asm-prototypes.h>
+#include <asm/cfi.h>
 
 int __read_mostly alternatives_patched;
 
@@ -1006,6 +1007,8 @@ static noinline void __init int3_selftest(void)
 
 void __init alternative_instructions(void)
 {
+	u64 ibt;
+
 	int3_selftest();
 
 	/*
@@ -1043,6 +1046,9 @@ void __init alternative_instructions(void)
 	 */
 	paravirt_set_cap();
 
+	/* Keep CET-IBT disabled until caller/callee are patched */
+	ibt = ibt_save();
+
 	/*
 	 * First patch paravirt functions, such that we overwrite the indirect
 	 * call with the direct call.
@@ -1064,6 +1070,8 @@ void __init alternative_instructions(void)
 
 	apply_ibt_endbr(__ibt_endbr_seal, __ibt_endbr_seal_end);
 
+	ibt_restore(ibt);
+
 #ifdef CONFIG_SMP
 	/* Patch to UP if other cpus not imminent. */
 	if (!noreplace_smp && (num_present_cpus() == 1 || setup_max_cpus <= 1)) {