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 <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index ecf105f37de79271d444c096e9e2b175dbe158fa..a179cfff3903205c1b02682be3716408eb5ef0b0 100644 (file)
--- a/arch/x86/kernel/alternative.c
+++ b/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;
 
@@ -1629,6 +1630,8 @@ static noinline void __init alt_reloc_selftest(void)
 
 void __init alternative_instructions(void)
 {
+       u64 ibt;
+
        int3_selftest();
 
        /*
@@ -1666,6 +1669,9 @@ void __init alternative_instructions(void)
         */
        paravirt_set_cap();
 
+       /* Keep CET-IBT disabled until caller/callee are patched */
+       ibt = ibt_save(/*disable*/ true);
+
        /*
         * First patch paravirt functions, such that we overwrite the indirect
         * call with the direct call.
@@ -1699,6 +1705,8 @@ void __init alternative_instructions(void)
         */
        apply_seal_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)) {