From ebebe30794d38c51f71fe4951ba6af4159d9837d Mon Sep 17 00:00:00 2001 From: Pawan Gupta Date: Sat, 3 May 2025 09:46:31 -0700 Subject: [PATCH] x86/ibt: Keep IBT disabled during alternative patching 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: : push %rbp mov %rsp,%rbp mov %esi,%eax mov %rsi,%rdx shr $0x20,%rdx mov %edi,%edi mov %rax,%rsi call *0x21e65d0(%rip) # ^^^^^^^^^^^^^^^^^^^^^^^ 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. Signed-off-by: Pawan Gupta Signed-off-by: Dave Hansen Reviewed-by: Alexandre Chartre --- arch/x86/kernel/alternative.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index 3e67e72bffdc..6e0b09b2c972 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -31,6 +31,7 @@ #include #include #include +#include int __read_mostly alternatives_patched; @@ -2085,6 +2086,8 @@ static noinline void __init alt_reloc_selftest(void) void __init alternative_instructions(void) { + u64 ibt; + int3_selftest(); /* @@ -2111,6 +2114,9 @@ void __init alternative_instructions(void) */ paravirt_set_cap(); + /* Keep CET-IBT disabled until caller/callee are patched */ + ibt = ibt_save(/*disable*/ true); + __apply_fineibt(__retpoline_sites, __retpoline_sites_end, __cfi_sites, __cfi_sites_end, true); @@ -2134,6 +2140,8 @@ 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)) {