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x86/ibt: Keep IBT disabled during alternative patching

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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>
This commit is contained in:
Pawan Gupta
2025-05-03 09:46:31 -07:00
committed by Greg Kroah-Hartman
parent 9e7364c32c
commit 6699bf27a4
1 changed files with 8 additions and 0 deletions
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8
arch/x86/kernel/alternative.c
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@@ -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)) {